The Keto–Inflammatory Network: From Systems Biology to Biological Code

Abstract

The transition from energy sufficiency to deficiency triggers complex metabolic and immune adaptations that have traditionally been viewed through a reductionist pathological lens. During early lactation, coordinated mobilization of adipose tissue, muscle protein, and bone minerals supports milk synthesis, with ketogenesis specifically arising from hepatic oxidation of non–esterified fatty acids. This review introduces the Keto–Inflammatory Network (KIN), a novel framework positioning ketonemia as an evolutionarily conserved adaptive response rather than inherent metabolic dysfunction. The KIN integrates β–hydroxybutyrate (BHB) signaling with immune modulation, epigenetic regulation, circadian rhythms, and microbiota interactions. Through mechanisms including NLRP3 inflammasome inhibition, HDAC–mediated epigenetic modifications, and HCAR2 receptor activation, ketone bodies orchestrate anti–inflammatory responses while maintaining metabolic flexibility. Building upon important precedent work recognizing beneficial roles of ketones in ruminant metabolism, this review synthesizes recent advances in immunometabolism and systems biology into an integrated framework. The KIN encompasses calcium–ketone integration through the Calci–Keto–Inflammatory Code (CKIC), temporal regulation via the Ketoinflammatory Clock, and trans–kingdom signaling through microbiota interactions. In dairy cattle, this perspective reframes periparturient ketonemia as existing on a continuum from adaptive to pathological, with biological meaning determined by integrated metabolic–inflammatory patterns rather than absolute ketone concentrations. The CKIC paradigm, while requiring prospective validation, suggests novel therapeutic approaches leveraging ketone signaling for inflammatory diseases, autoimmune conditions, and metabolic disorders while challenging traditional threshold–based ketosis management strategies. This systems–level understanding opens new avenues for precision interventions that work with, rather than against, evolved adaptive mechanisms refined through millions of years of mammalian evolution. By distinguishing ketonemia (measurable ketone elevation) from pathological ketosis (dysregulated ketone accumulation), and by integrating evidence from both ruminant and monogastric models, this review provides a comprehensive framework for next–generation metabolic medicine.

1. Introduction

Multicellular organisms require communication systems capable of integrating metabolism, immunity, and environmental sensing into coherent organismal responses. Among the most ancient and conserved of these biological information processing systems is the capacity for ketogenesis—the hepatic production of ketone bodies from fatty acid precursors during periods of altered energy availability [1,2,3,4]. While ketosis has been traditionally conceptualized through the reductionist framework of metabolic compensation for glucose deficiency—a pathological state requiring suppression—recent systems–level investigations have revealed ketone bodies, particularly β–hydroxybutyrate (BHB), as pleiotropic signaling molecules that encode physiological information and orchestrate adaptive programs extending far beyond simple energy provision [5,6,7].

The conservation of ketogenic pathways across vertebrate evolution—from arctic mammals enduring months of food scarcity to migratory birds completing transcontinental flights without feeding [8]. This evolutionary persistence implies that ketone bodies function as carriers of survival–relevant information about nutritional status, immune challenges, temporal phase, and environmental conditions—information that must be reliably encoded, transmitted, and decoded to generate appropriate adaptive responses [5,7].

1.1. Terminology and Conceptual Framework

Before proceeding, we must establish critical terminological distinctions that have historically obscured understanding of ketone metabolism. The term “ketosis” has been used indiscriminately to describe both physiological ketone elevation and pathological metabolic dysfunction, conflating adaptive responses with disease states [9,10].

Throughout this review, we distinguish between:

Ketonemia: Elevated blood concentrations of ketone bodies (BHB, acetoacetate, and acetone), which may occur across a spectrum from physiological to pathological contexts. Ketonemia represents a measurable biochemical state without inherent value judgment.

Physiological ketosis: Regulated ketone elevation occurring during fasting, lactation, pregnancy, or other metabolic transitions, where ketone production represents coordinated metabolic–immune adaptation serving beneficial functions including energy provision to peripheral tissues, immune modulation, and cellular signaling. Physiological ketosis typically develops gradually over days, maintains metabolic stability (preserved calcium homeostasis, controlled inflammation, adequate feed intake), and stabilizes or resolves as the animal adapts to the new physiological state.

Pathological ketosis: Dysregulated ketone accumulation exceeding adaptive capacity, characterized by rapid onset or acceleration (typically over 24–48 h), concurrent metabolic dysfunction (hypocalcemia, severe negative energy balance, escalating inflammation, hepatic lipidosis), progressive clinical deterioration (inappetence, depression, reduced milk production), and failure to stabilize without intervention. This includes both subclinical ketosis (elevated blood BHB without overt clinical signs but associated with reduced performance and increased disease risk) and clinical ketosis (elevated BHB with observable clinical manifestations and metabolic decompensation or loss of metabolic stability).

Critical distinction from ketoacidosis: Bovine ketosis (hyperketonemia) is fundamentally distinct from ketoacidosis as observed in diabetic humans. While ketoacidosis is characterized by severe metabolic acidosis with marked blood pH depression, bovine ketosis—even at severe levels (BHB > 3.0 mmol/L)—does not cause significant blood pH changes. The rumen buffering system and metabolic differences between ruminants and monogastrics prevent development of ketoacidosis during typical bovine ketosis. While systematic characterization of acid–base status during hyperketonemia in dairy cattle remains an area requiring investigation, available evidence does not support blood pH depression as a feature of bovine ketosis.

Clinical practice appropriately employs concentration–based thresholds as pragmatic diagnostic cutpoints: blood BHB ≥ 1.2 mmol/L commonly defines subclinical ketosis, while blood BHB ≥ 3.0 mmol/L indicates clinical ketosis [11,12,13]. However, these conventional threshold definitions vary considerably across studies due to differences in analytical methodology (enzymatic assays vs. Fourier–transform infrared spectroscopy vs. handheld meters), sample matrix (whole blood vs. serum vs. plasma vs. milk), and temporal sampling protocols (postpartum day, time of day, feeding status), creating diagnostic inconsistencies that complicate cross–study comparisons [12]. These thresholds provide valuable epidemiological benchmarks, enable standardized communication across research and practice, and facilitate herd–level monitoring programs [14,15]. However, the critical insight this review advances is that static thresholds represent starting points for interpretation, not biological endpoints defining adaptive versus pathological states. What distinguishes adaptive from pathological ketosis is not whether a cow crosses 1.2 mmol/L BHB, but rather the temporal dynamics surrounding that crossing: the rate of BHB elevation, whether the trajectory is accelerating or stabilizing, and critically, the concurrent metabolic and inflammatory context [16,17]. Two cows with identical BHB concentrations may be on entirely different biological trajectories—one adapting successfully with metabolic stabilization, one progressing toward dysfunction with escalating dysregulation—information that single–timepoint measurements cannot capture but temporal patterns and integrated metabolic–immune signatures reveal.

The critical challenge facing metabolic medicine is not whether ketonemia exceeds an arbitrary threshold, but rather understanding the biological information encoded in ketone patterns—their magnitude relative to baseline, temporal dynamics (velocity and acceleration of change), relationships to concurrent metabolic and inflammatory signals, and ultimately whether these patterns reflect adaptive programming or pathological dysfunction. This review advances the emerging recognition that not all deviations from baseline ketone concentrations represent pathology, instead proposing frameworks for distinguishing adaptive from pathological responses based on systems–level integration and temporal pattern recognition rather than relying exclusively on static concentration thresholds.

1.2. Epidemiology and Predictive Approaches

The scope and impact of periparturient ketosis have been extensively documented through large–scale epidemiological investigations. McCart et al. reported that 43.2% of cows exceeded the threshold deemed subclinically ketotic (≥1.2 mmol/L), demonstrating the high prevalence of hyperketonemia in modern dairy systems [11]. Critically, their work revealed that timing of onset and blood BHB concentration at first detection are important indicators of individual cow performance, with early–onset ketosis (3–5 DIM) associated with 6.1–fold increased risk of displaced abomasum and substantially reduced milk production and reproductive performance [11]. The temporal relationship between hyperketonemia and displaced abomasum may be bidirectional, with metabolic disruption potentially predisposing to abomasal displacement, while abomasal displacement reduces feed intake and exacerbates negative energy balance, creating reciprocal pathological interactions.

Recent advances in predictive modeling have demonstrated that pattern–based approaches using test–day milk and performance variables can identify hyperketonemic cows with exceptional accuracy (82.6–97.3%) through logistic and linear regression models [14]. Analysis of over 240,000 lactations across 335 farms confirmed previously identified risk factors while revealing novel associations between predicted hyperketonemia and milk fatty acid profiles, differential somatic cell counts, and production parameters [15]. These cross–sectional associations demonstrate predictive relationships at the time of disease detection, distinguishing them from the anticipatory prediction achieved weeks prepartum through multi–platform metabolomics [5,15], during the critical window when adaptive mechanisms remain modifiable. This temporal distinction between concurrent detection and anticipatory forecasting is fundamental to precision prevention strategies. These high–throughput prediction strategies exemplify the value of pattern recognition over simple threshold–based detection, demonstrating that multiple integrated variables provide superior diagnostic power compared to single biomarker measurements.

The consistent associations between ketosis and negative outcomes across diverse populations and production systems establish ketosis as a significant health and economic challenge warranting deeper mechanistic understanding. However, these epidemiological observations, while documenting associations, cannot distinguish whether ketosis represents primary pathology or an adaptive response that becomes pathological when dysregulated or prolonged beyond its beneficial phase.

1.3. Challenging the Pathological Paradigm

Important precedent work has begun to challenge traditional pathological framings of periparturient metabolism. Bradford’s comprehensive review explicitly questioned “which physiological processes are adaptive and which are pathological” during the transition period, recognizing that essentially all cows experience systemic inflammation postpartum with magnitude varying widely among individuals [18]. His group demonstrated that inflammation–associated pathways are involved in homeorhetic adaptations to lactation [19] and that BHB alters immune responses by promoting tolerance rather than resistance to bacterial challenge via HCAR2–dependent mechanisms [20].

Baumgard and colleagues revealed that inflammatory biomarkers are elevated before ketosis development, challenging simple cause–effect assumptions and proposing that lipopolysaccharide infiltration may play a key etiological role [21]. Their work quantifying glucose requirements of activated immune systems demonstrated profound metabolic–immune integration during inflammatory challenges [22].

Research collaboration between Loor, Drackley, Bertoni and Trevisi introduced the concept of “inflammometabolic indices,” explicitly recognizing inflammation and metabolism as integrated systems rather than independent processes [23], and documented that immune activation often precedes rather than follows metabolic dysfunction during the periparturient period [24,25]. Experimental evidence from Bruckmaier’s group demonstrated that elevated BHB directly alters mammary immune responses and cytokine synthesis, providing mechanistic support for the association between hyperketonemia and infectious disease susceptibility [26,27].

Despite these advances recognizing inflammation–metabolism integration and questioning the adaptive–versus–pathological distinction, the dominant paradigm continues to treat ketone elevation as inherently problematic, focusing therapeutic efforts on suppression rather than supporting adaptive regulatory mechanisms. The current review builds upon this foundation by introducing comprehensive frameworks—the Keto–Inflammatory Network (KIN) and Calci–Keto–Inflammatory Code (CKIC)—that synthesize recent discoveries in immunometabolism, epigenetics, and systems biology to reconceptualize ketosis as a complex biological information processing system, extending these concepts through integration with calcium homeostasis and development of a code–based interpretive framework.

1.4. Ketosis Across Species and Physiological Contexts

In humans, physiological ketosis occurs naturally during multiple life stages and conditions including prolonged fasting, intensive exercise, pregnancy, lactation, and early postnatal development, each representing distinct informational contexts requiring coordinated metabolic–immune adjustments [7,28]. Yet conventional medical paradigms have systematically pathologized these natural states, largely based on extrapolation from extreme pathological conditions such as diabetic ketoacidosis—a conceptual error equivalent to condemning fever because hyperthermia is dangerous [29]. The recognition that metabolic responses exist on a continuum from adaptive to pathological, with context–dependent meaning, represents a critical conceptual evolution.

The periparturient dairy cow represents perhaps the most intensive natural model of integrated metabolic–immune information processing, where the metabolic demands of late pregnancy, parturition, and early lactation trigger coordinated systemic transitions involving energy mobilization, hormonal restructuring, immune system modulation, and fundamental alterations in microbial ecology [24,30]. During this critical transition period, plasma BHB concentrations can increase 5– to 10–fold within days of calving, coinciding with profound alterations in circulating immune cell populations, systemic inflammatory mediator profiles, and calcium homeostasis [24,31]. Veterinary medicine has conventionally treated this ketone elevation as primary pathology requiring aggressive suppression, despite limited evidence that such interventions improve long–term outcomes and growing recognition that early ketone elevation may represent adaptive programming [16,32].

The temporal sequence and pattern of these metabolic changes—not merely their magnitude—appears to encode diagnostic information that distinguishes adaptive from pathological trajectories, suggesting that biological systems “read” complex metabolic signatures to coordinate appropriate responses [16,32].

1.5. Molecular Discoveries and Paradigm Shifts

Recent paradigm–shifting discoveries in molecular immunology and metabolic biochemistry have fundamentally challenged reductionist conceptual boundaries separating metabolism and immune function [16,32]. The recognition that immune cells undergo systematic metabolic reprogramming during activation states, that small molecule metabolites function as potent cellular signaling mediators with context–dependent meanings, and that mitochondria serve as critical immune sensing organelles has created entirely new theoretical frameworks for understanding health, disease, and therapeutic intervention [33].

These discoveries reveal that metabolites are not merely biochemical intermediates in linear pathways but information carriers—molecular symbols in biological languages that enable cellular communication and coordinate distributed decision–making across tissues and organ systems. Yet these insights have been slow to penetrate clinical thinking about ketosis, which remains dominated by threshold–based pathology models focused on metabolite normalization rather than understanding the biological information encoded in metabolic patterns.

1.6. Framework and Scope of Review

This comprehensive review challenges the reductionist pathological paradigm by introducing a systems–level framework for understanding ketone metabolism. We begin by characterizing the KIN—the interconnected system of metabolic, inflammatory, immune, microbial, and circadian pathways through which ketone bodies exert their pleiotropic effects. This network perspective reveals connections invisible to reductionist approaches: bidirectional metabolic–immune crosstalk, trans–kingdom signaling between host and microbiome, temporal coordination through circadian mechanisms, and multi–scale integration from molecular interactions to whole–organism physiology.

As our analysis deepens, we recognize that network connectivity alone inadequately captures the information processing properties exhibited by this system. The evidence compels us toward a more fundamental reconceptualization: the CKIC—a biological information processing system with true code–like properties including syntax (molecular grammar governing signal patterns), semantics (biological meaning emerging from metabolic signatures), and pragmatics (context–dependent interpretation generating appropriate adaptive responses). This framework positions ketosis not as metabolic failure but as an evolutionarily conserved adaptive program—an encoded biological language through which organisms assess their state, communicate between distributed cellular systems, and coordinate situation–appropriate responses that optimize survival and reproductive success under diverse environmental challenges.

By progressing from network architecture to code comprehension, we provide both deeper mechanistic insights into how ketone signaling operates and more advanced therapeutic strategies that support rather than suppress evolved regulatory intelligence. This review synthesizes evidence from molecular biology, immunology, microbiology, chronobiology, and clinical investigation to demonstrate that ketosis represents refined biological adaptation warranting recognition and, when necessary, informed modulation—not reflexive pathologization and suppression. The framework presented directly challenges century–old paradigms while providing practical tools for predictive diagnostics and therapeutic innovation in both human and veterinary medicine.

2. The Reductionist Paradigm—Ketosis as Pathology

The dominant medical and veterinary paradigm characterizing ketosis has been shaped by over a century of observations made primarily within pathological contexts, creating a deeply embedded conceptual framework that views any elevation in ketone body concentrations as inherently problematic and requiring therapeutic intervention [9,10]. This reductionist perspective emerged from early clinical observations of diabetic ketoacidosis, starvation ketosis in malnourished populations, and periparturient ketosis in high–producing dairy cattle, where extreme ketone elevations were consistently associated with morbidity, reduced performance, and increased mortality risk [5,10,34]. The pathological framing became so entrenched that Kronfeld [35] characterized ketone production as analogous to ‘exhaust of a dirty engine’—metabolic waste requiring elimination through dietary manipulation and fat supplementation to suppress ketogenesis. Critically, this pathological framing has systematically ignored the possibility that ketone bodies might function as signaling molecules encoding biological information.

Within the traditional veterinary framework, ketosis in dairy cattle has been conceptualized as a direct consequence of insufficient glucose precursor availability relative to the enormous metabolic demands of early lactation milk synthesis (Figure 1) [30,36]. This glucose–deficit model proposes that when dietary intake of gluconeogenic precursors fails to meet the 40–60% increase in requirements associated with peak milk production, oxaloacetate becomes limiting for both TCA cycle function and gluconeogenesis. Stored adipose tissue lipids are mobilized through lipolysis; the resulting NEFAs undergo hepatic β–oxidation, but oxaloacetate depletion prevents complete acetyl–CoA oxidation, diverting excess acetyl–CoA into ketogenesis and resulting in ketone accumulation [36,37]. This framework treats ketone elevation as simple metabolic overflow—analogous to water spilling from an overfilled container—with no consideration that ketone production might be actively regulated to communicate metabolic state to distant tissues. However, recent immunometabolomics investigations have revealed that the pathophysiology of ketosis is far more complex than simple glucose deficiency, with alterations in immune function, inflammatory mediators, and metabolic pathways detectable weeks before clinical disease manifestation [16]. The predictive power of these early metabolic signatures suggests coordinated biological programming rather than passive metabolic failure—a finding fundamentally incompatible with the overflow model [16].

The clinical classification system distinguishing between subclinical ketosis (elevated blood BHB concentrations without overt clinical signs) and clinical ketosis exemplifies the reductionist threshold–based paradigm that has dominated metabolic disease diagnosis [12,13,36]. This approach treats BHB concentration as a binary signal—normal versus pathological—determined by crossing arbitrary numerical boundaries (>1.0, >1.2, >1.4, >3.0 mmol/L) that vary across studies, lack biological justification, and critically, strip away the temporal, inflammatory, and metabolic context determining biological meaning.

A BHB concentration of 1.3 mmol/L at 3 days postpartum in a cow with controlled inflammation represents fundamentally different biological information than the identical concentration at 14 days postpartum with elevated acute phase proteins and hypocalcemia—yet threshold–based classification treats these as equivalent pathological states requiring identical intervention. The predictive power of multi–metabolite signatures weeks before clinical disease manifestation [16] demonstrates that biological systems encode information through coordinated metabolic patterns—information that threshold–based classification systematically discards.

Epidemiological investigations document that 43% of periparturient dairy cows experience ketosis within three weeks postpartum, with significant economic impacts including reduced milk yield, impaired reproduction, and increased culling [11]. Importantly, cows with high somatic cell counts prior to dry–off show increased ketosis risk [38], suggesting interconnected disease processes rather than isolated metabolic dysfunction—a possibility the reductionist paradigm has not adequately explored.

These epidemiological associations, while traditionally interpreted as ketosis causing secondary pathologies, are equally consistent with ketosis representing an integrated response to underlying inflammatory or immune dysregulation—a possibility the reductionist paradigm has not adequately explored.

The pathophysiological model underlying traditional ketosis management focuses on the complex endocrine changes accompanying negative energy balance (NEB), including decreased insulin concentrations, elevated glucagon, cortisol, and growth hormone levels that collectively stimulate non–esterified fatty acid (NEFA) mobilization from adipose tissue stores [39,40]. These mobilized fatty acids undergo hepatic β–oxidation through the carnitine palmitoyltransferase system, and when the rate of acetyl–CoA production exceeds the oxidative capacity of the tricarboxylic acid cycle (TCA), excess acetyl–CoA is diverted into ketogenesis through the mitochondrial enzymes acetyl–CoA acetyltransferase, 3–hydroxy–3–methylglutaryl–CoA synthase 2 (HMGCS2), and HMG–CoA lyase [4,41]. While biochemically accurate, this mechanistic description implicitly frames ketogenesis as passive consequence of substrate availability rather than actively regulated biological process—ignoring substantial evidence for transcriptional, post–translational, and allosteric regulation of ketogenic enzymes that would enable ketone production to function as controlled signaling rather than uncontrolled overflow.

While negative energy balance—the state where dietary energy intake fails to meet the demands of milk production—represents the primary driver of periparturient ketogenesis [30,36], the biochemical execution of this response involves complex nutrient–dependent pathways. Glucogenic amino acid availability modulates hepatic glucose production and thereby ketogenic flux [42]. Lipotropic nutrients including choline and methionine facilitate hepatic lipoprotein assembly and export, influencing hepatic lipid accumulation. While cobalt availability may influence vitamin B12–dependent propionate metabolism, recent evidence indicates that neither dietary cobalt nor vitamin B12 supplementation improves performance or ketosis indicators in periparturient dairy cows as reported by [43]. The interplay between energy status and these nutrient–dependent pathways creates the integrated metabolic response observed during the transition period [30,37].

Traditional therapeutic approaches to ketosis employ gluconeogenic precursors (propylene glycol, propionate) that replenish TCA cycle intermediates, energy–dense bypass fats, and feed additives improving propionate flux [44,45]. While these interventions operate through restoration of hepatic oxidative capacity rather than crude ketone suppression—balancing substrate flux to enable acetyl–CoA oxidation and prevent lipid accumulation—they fundamentally assume ketone elevation represents dysfunction requiring correction [44,45]. This paradigm emerged before recognition that ketones function as signaling molecules mediating inflammasome regulation, epigenetic modifications, and protein sparing [46], raising the possibility that metabolite normalization strategies may disrupt adaptive biological information processing.

The limitations of this reductionist paradigm have become increasingly apparent as molecular and systems biology reveal complex regulatory networks connecting ketone metabolism with immune function, circadian rhythms, epigenetic regulation, and microbial ecology. The growing recognition that ketone bodies, particularly BHB, function as signaling molecules through G–protein coupled receptors, as histone deacetylase inhibitors influencing gene expression, and as inflammasome regulators modulating immune responses demands fundamental reconceptualization of what ketosis represents biologically [6,46]. The remainder of this review presents an alternative framework—grounded in network analysis and information theory—that repositions ketosis from metabolic pathology to an adaptive program, with profound implications for both mechanistic understanding and therapeutic strategy.

3. The Keto–Inflammatory Network—A Systems Level Framework

The classical reductionist view of ketosis as passive metabolic overflow fundamentally fails to account for the intricate regulatory capabilities and evolutionary conservation characterizing ketone body metabolism across diverse species and physiological contexts [32]. Contemporary systems biology approaches reveal that biological networks operate through interconnected, multi–layered integration systems capable of generating context–appropriate responses that optimize organismal fitness under varying environmental conditions [32,47,48].

The KIN represents a theoretical framework that reconceptualizes ketosis as a multilayered, evolutionarily refined biological program in which ketone bodies coordinate metabolic flexibility, immune modulation, stress adaptation, and cellular communication across multiple organ systems (Figure 2) [5,6,7]. This paradigm shift moves beyond viewing ketones merely as alternative energy substrates or toxic metabolic by–products to recognizing their roles as regulatory molecules capable of orchestrating intricate physiological adaptations that enhance survival and reproductive success under diverse environmental challenges.

3.1. Network Architecture: Molecular Integration and Distributed Processing

The fundamental architecture of the KIN operates through multiple interconnected regulatory nodes that sense, integrate, and respond to diverse physiological signals including energy availability, inflammatory status, microbial ecology, circadian timing, environmental stressors, and developmental stage. Central to this architecture is BHB, which serves simultaneously as energy substrate, G–protein coupled receptor ligand (GPR109A/HCAR2), histone deacetylase inhibitor, post–translational modifier (β–hydroxybutyrylation), and inflammasome regulator (Figure 2) [49]. This multiplexed functionality creates potential for coordinated responses across multiple biological systems.

The network exhibits architectural features characteristic of evolved biological systems. Redundancy provides fault tolerance through multiple pathways for ketone production (hepatic ketogenesis, astrocytic synthesis) and sensing (receptor–mediated, metabolic, epigenetic). Distributed processing generates emergent behavior from local decisions by hepatocytes, immune cells, neurons, and other cell types, without requiring centralized control. Scale–free topology employs hub nodes such as hepatic ketogenesis and inflammatory mediators to coordinate activity across widely distributed peripheral components [50]. These properties enable the network to maintain function despite perturbations and to coordinate responses across vastly different biological scales—from molecular interactions within individual cells to whole–organism behavioral and metabolic adjustments.

Critically, the network generates qualitatively different responses depending on the specific combination of input signals, duration of activation, and physiological context [50]. Identical ketone concentrations produce different biological outcomes based on concurrent inflammatory signals, nutritional status, or circadian phase—a functional plasticity that distinguishes the KIN from simple linear metabolic pathways.

Ruminant–Specific Considerations and Evidence Sources

The application of the KIN framework to ruminant physiology requires explicit acknowledgment of metabolic and immunological features that distinguish ruminants from the monogastric species (primarily rodents and humans) in which many KIN mechanisms were initially characterized. These species–specific differences do not invalidate the core principles of metabolic–immune integration but rather necessitate careful interpretation of how evolutionarily conserved pathways operate within ruminant–specific physiological contexts.

Fermentation–based metabolism and HCAR2 signaling: Unlike monogastric species where ketone production represents an adaptive response to fasting or carbohydrate restriction, ruminants maintain continuous production of short–chain fatty acids (SCFAs) through ruminal fermentation regardless of nutritional state. Ruminal butyrate exerts local effects on rumen epithelial gene expression, functioning as an HDAC inhibitor and modulating transcriptional programs related to epithelial development and barrier function [51,52,53,54]. Critically, bovine leukocytes express functional HCAR2 receptors that respond to both BHBA and SCFAs, with receptor expression increasing from early to mid–lactation and mediating calcium mobilization and immune cell activation [55]. This demonstrates that the molecular machinery for ketone–immune integration described in monogastric models is conserved in ruminants, though operating within a metabolic context characterized by continuous microbial fermentation rather than episodic ketogenic responses to nutritional restriction.

Obligate hepatic gluconeogenesis and TCA cycle competition: Ruminants exhibit near–complete dependence on hepatic gluconeogenesis for glucose supply due to minimal dietary glucose absorption and extensive ruminal fermentation of ingested carbohydrates to volatile fatty acids [30,36,37]. This obligate gluconeogenic metabolism creates fundamental competition between glucose synthesis (which requires oxaloacetate and TCA cycle intermediates) and fatty acid oxidation/ketogenesis (which generates acetyl–CoA but depletes TCA cycle capacity when oxaloacetate availability becomes limiting) [30]. This metabolic architecture differs markedly from omnivorous monogastrics that can modulate between dietary glucose absorption and endogenous production. The implications for KIN function include the reality that any condition increasing fatty acid oxidation (including inflammation–induced lipolysis) will more readily precipitate ketogenesis in ruminants due to the constant demand for gluconeogenic precursors competing with acetyl–CoA disposal through the TCA cycle.

Microbiota–host interactions: Ruminant microbiota produce SCFAs, lipopolysaccharide (LPS), and other microbial–derived metabolites that continuously challenge host immune surveillance [56]. Chronic low–level immune activation from microbial exposure may fundamentally alter the interpretation of ketone–immune interactions compared to monogastrics with simpler gut architecture. The substantial microbial biomass within the rumen and potential for microbial product translocation during periods of barrier dysfunction create inflammatory challenges largely absent in monogastric models. Indeed, evidence suggests that inflammatory activation during the transition period may play a key role in ketosis etiology in dairy cattle [16,18,21,24], representing a ruminant–specific trigger for the inflammatory component of the KIN that has limited direct parallel in human or rodent ketogenic states.

The evolutionary conservation of core KIN components—including ketogenic enzymes (HMGCS2, BDH1), ketone–sensing receptors (HCAR2), HDAC enzymes, NLRP3 inflammasomes, and mitochondrial respiratory complexes—across diverse mammalian lineages provides compelling evidence for fundamental biological importance [6,57,58,59]. This conservation suggests that core principles of ketone–mediated metabolic–immune integration likely operate across species, even as specific contexts, triggers, and quantitative parameters differ. The value of the KIN framework for ruminant biology lies not in assuming identical function to monogastric species but rather in providing testable hypotheses about how conserved molecular mechanisms operate within ruminant–specific physiological contexts.

3.2. Predictive Metabolomics: Evidence for Coordinated Programming

The systems veterinary medicine approach has provided compelling experimental evidence that ketosis represents coordinated biological programming rather than acute metabolic failure [5,32]. Multi–platform metabolomics investigations reveal that metabolite signatures can distinguish animals that will develop clinical ketosis from those that will remain healthy weeks before clinical manifestation, achieving high prediction accuracies [17].

The patterns that distinguish pre–ketotic from healthy animals extend far beyond ketone body concentrations themselves, encompassing coordinated alterations in amino acid profiles, lipid species, inflammatory mediators, and microbial metabolites [17]. This multi–metabolite nature demonstrates that the network operates through coordinated changes across multiple metabolic pathways rather than isolated perturbations in ketone production. The predictive power of these signatures—their ability to forecast outcomes weeks in advance—indicates that the biological system undergoes progressive, orchestrated changes rather than sudden metabolic collapse.

Furthermore, immune system alterations frequently precede rather than follow metabolic dysfunction, challenging assumptions that ketosis represents primary metabolic failure with secondary immune consequences [16,38]. This temporal architecture reveals bidirectional metabolic–immune crosstalk, with the network integrating signals from multiple physiological domains to generate adaptive responses. The recognition that immune status modulates metabolic programming fundamentally distinguishes the KIN framework from traditional views of ketosis as purely metabolic pathology.

3.3. Evolutionary Conservation: Fundamental Biological Importance

The evolutionary conservation of KIN components across mammalian species provides compelling evidence for fundamental biological importance extending beyond species–specific metabolic adaptations [1,7,9]. Comparative genomic analyses reveal high conservation of ketogenic enzymes (HMGCS2, BDH1, ACAT1), ketone–sensing receptors (GPR109A), and downstream signaling pathways from rodents to ruminants to primates, suggesting that core network functionality has been maintained through selective pressure over millions of years of vertebrate evolution [6,9].

Such deep conservation implies that the network solves fundamental survival problems common to diverse species and ecological niches—coordinating distributed cellular responses to fluctuating nutritional availability while simultaneously modulating immune function, maintaining neural activity, and preserving reproductive capacity [8,9]. The fact that organisms as metabolically divergent as obligate carnivores and herbivores retain similar ketone signaling machinery suggests that the network’s value lies in its capacity to integrate and communicate organismal state across tissues [1,7].

The distribution of ketone–responsive elements across functionally diverse tissues—brain, heart, skeletal muscle, immune cells, kidneys, and adipose tissue—indicates that the network evolved as a whole–organism coordination mechanism [6,7,46]. This architectural feature ensures that local cellular decisions align with organism–level metabolic and immunological state, enabling coherent adaptive responses to environmental challenges.

4. Triggers of the Keto–Inflammatory Network

The activation of the KIN extends far beyond carbohydrate scarcity. Rather than functioning solely as a starvation–induced response, the KIN is triggered by a diverse array of physiological and pathological stressors, including systemic inflammation, microbial exposure, tissue injury, intense physical exertion, prolonged fasting, and parturition (Figure 2) [7,9,60,61]. These stimuli converge on evolutionarily conserved molecular pathways that coordinate metabolic adaptation with immune regulation.

At the molecular level, these triggers initiate a cascade involving mitochondrial reprogramming, AMP–activated protein kinase (AMPK) activation, peroxisome proliferator–activated receptor alpha (PPAR–α) signaling, and transcriptional networks mediated by NF–κB and FOXO1 [29,33]. These pathways recalibrate energy production, suppress uncontrolled inflammation, and initiate hepatic ketogenesis, placing BHB at the center of an integrated immunometabolic response [6,7,29].

In dairy cows, the periparturient period provides an exemplary model of KIN activation (Figure 3). During this time, abrupt endocrine shifts, intense lipolysis, microbial components’ translocation (especially LPS), and elevated proinflammatory cytokines [e.g., Interleukin (IL)–1β, or Tumor Necrosis Factor (TNF)–α] converge to stimulate hepatic ketogenesis alongside immune activation [60,62]. In humans, similar triggers, such as trauma, infection, caloric restriction, or fasting, activate hepatic oxidative metabolism through PPAR–α and sirtuin pathways, leading to increased BHB synthesis [7,29,61].

Importantly, rising BHB levels often occur concurrently with elevations in IL–6 and TNF–α during the periparturient period [16,18,21]. The relationship between ketogenesis and inflammation is bidirectional and complex: inflammatory stimuli activate hepatic metabolic reprogramming and ketogenesis, while BHB functions as a pleiotropic signaling molecule with demonstrated anti–inflammatory properties through multiple receptor–mediated and epigenetic mechanisms [6,7]. The concurrent rise of both markers in animals developing clinical ketosis likely reflects inflammation–driven ketogenesis occurring alongside attempted but insufficient anti–inflammatory feedback. This positions ketogenesis not as passive metabolic overflow but as an actively regulated adaptive mechanism that may help limit inflammatory escalation in healthy animals but becomes overwhelmed during severe metabolic–immune challenges characteristic of clinical disease.

The immune system and liver engage in dynamic crosstalk during these events, linking microbial sensing (via pattern recognition receptors such as TLRs, IL–1R, and TNF–R) with hepatic substrate metabolism and mitochondrial ketone biosynthesis [62,63,64]. Within this integrated network, BHB emerges not as a metabolic waste product, but as a central immunometabolic signal that mediates adaptation, preserves tissue integrity, and restores homeostasis. The KIN thus represents an evolutionarily conserved adaptive mechanism that coordinates metabolic flexibility with immune regulation during periods of altered energy availability and physiological stress.

5. Receptors and Molecular Mediators of the Keto–Inflammatory Network

The immunomodulatory and metabolic effects of the KIN are mediated through diverse molecular recognition systems and intracellular signaling pathways enabling cells to detect, interpret, and respond to ketone body availability in context–specific ways. Ketone bodies, particularly BHB, interact with multiple classes of cellular receptors, enzymes, and regulatory proteins to generate coordinated changes in gene expression, protein function, and cellular behavior (Figure 4) [6,7]. These molecular mechanisms have been extensively validated across multiple bovine cell types and tissues, demonstrating conservation of ketone signaling pathways in ruminant species [55,65,66,67].

5.1. G–Protein Coupled Receptor Signaling

The hydroxycarboxylic acid receptor 2 (HCAR2, also known as GPR109A) represents the most extensively characterized receptor system for BHB signaling [57,65]. HCAR2 demonstrates high affinity binding for BHB with physiologically relevant activation at concentrations observed during fasting, exercise, or metabolic stress [6,57]. In dairy cattle, HCAR2 protein expression has been directly demonstrated in lymphocytes, monocytes, and granulocytes, with expression increasing from early to mid–lactation, particularly in T and B cell populations [55].

HCAR2 activation triggers Gi/Go–protein coupled signaling cascades resulting in decreased intracellular cAMP concentrations, leading to suppression of protein kinase A activity and reduced CREB phosphorylation [68]. This cascade ultimately decreases NF–κB activity, reduces transcription of pro–inflammatory cytokine genes (TNF–α, IL–1β, IL–6), and enhances expression of anti–inflammatory mediators (IL–10, TGF–β) [46,57]. In bovine neutrophils and mononuclear cells, BHB stimulation of HCAR2 promotes receptor–dependent calcium mobilization and activates AKT, ERK1/2, and AMPKα signaling pathways involved in neutrophil chemotaxis, suggesting HCAR2 coordinates both metabolic adaptation and immune cell trafficking during ketotic states [55,69].

5.2. Epigenetic Regulation Through Histone Deacetylase Inhibition

Among the most significant discoveries in ketone body research has been identification of BHB as an endogenous inhibitor of class I histone deacetylases (HDACs), particularly HDAC1, HDAC2, and HDAC3 [6,70]. This mechanism places ketone bodies within the category of epigenetic modulators capable of inducing lasting changes in gene expression patterns through alterations in chromatin structure [66,67,70].

Selective inhibition of class I HDACs by BHB occurs through competitive binding at the enzyme active site, with inhibition constants readily achieved during physiological ketosis [70]. In bovine cells and embryos, BHB enhances histone H3 lysine 9 acetylation (H3K9ac), with particularly strong effects in cumulus cells and somatic cell nuclear transfer embryos persisting through the blastocyst stage [66]. BHB also induces a novel epigenetic modification—histone lysine β–hydroxybutyrylation (Kbhb)—identified in multiple bovine tissues including liver, mammary gland, and skeletal muscle, associated with upregulation of genes in mitochondrial metabolism and stress response pathways [67].

HDAC inhibition by BHB increases histone H3 and H4 acetylation at specific gene loci, enhancing transcriptional accessibility for genes encoding antioxidant defense enzymes (catalase, superoxide dismutase), stress response transcription factors (FOXO3a, Nrf2), and anti–inflammatory mediators (IL–10, arginase 1) [29,70,71]. In bovine mammary epithelial cells exposed to LPS, HDAC activity increases and histone H3 acetylation decreases, suppressing lactation–related genes; importantly, sodium butyrate (structural analog of BHB and potent HDAC inhibitor) reverses this LPS–induced suppression and restores milk gene expression [72,73]. Similarly, HDAC1 and HDAC2 inhibition in TNF–α–stimulated bovine mammary epithelial cells attenuates inflammatory gene expression by reducing JNK and ERK phosphorylation, demonstrating HDACs regulate inflammation through both gene expression and signaling–dependent mechanisms [74].

5.3. NAD⁺–Dependent Sirtuin Activation

The sirtuin family of NAD⁺–dependent protein deacetylases represent another critical molecular target through which ketone bodies exert regulatory effects [75,76]. Sirtuins, particularly SIRT1 and SIRT3, play essential roles in metabolic regulation, stress resistance, and longevity through deacetylation of key regulatory proteins in nuclear and mitochondrial compartments [29,77,78].

Ketone body metabolism influences sirtuin activity through direct NAD⁺ provision, enhancement of NAD⁺/NADH ratio, and indirect effects on NAD⁺ biosynthetic pathways [29,75]. SIRT1 activation by enhanced NAD⁺ availability leads to deacetylation of transcription factors including p53, FOXO family members, and PGC–1α, resulting in enhanced stress resistance, improved mitochondrial biogenesis, and increased oxidative metabolism [29,79,80].

5.4. Inflammasome Modulation and Innate Immune Signaling

The NLRP3 inflammasome represents a critical convergence point where ketone body signaling intersects with innate immune activation [81,82]. BHB suppresses NLRP3 inflammasome activation through multiple mechanisms including stabilization of mitochondrial membrane potential, reduction in mitochondrial ROS production, prevention of potassium efflux, and direct interference with inflammasome assembly [81]. These effects prevent caspase–1 activation and reduce processing of pro–inflammatory cytokines IL–1β and IL–18 [83].

The relevance of inflammasome regulation to bovine ketosis has been directly demonstrated in multiple tissues. In mammary gland tissue from cows with subclinical or clinical ketosis, NLRP3 inflammasome activation is markedly elevated, with increased caspase–1 activity and protein abundance of NLRP3, caspase–1, and pro–apoptotic proteins (Bax, caspase–3, caspase–9), coinciding with oxidative stress, elevated pro–inflammatory cytokine expression, and NF–κB pathway activation [84].

The relationship between ketone bodies and inflammatory signaling in bovine systems is concentration–dependent. In calf hepatocytes, elevated BHB concentrations (1.2–1.8 mM) induce oxidative stress and activate NF–κB signaling, increasing expression of pro–inflammatory cytokines TNF–α, IL–6, and IL–1β—effects blocked by NF–κB inhibition or antioxidants, suggesting BHB–induced hepatic inflammation is mediated through oxidative stress–dependent NF–κB activation [85]. Similarly, neutrophils from clinically ketotic cows show elevated TLR2 and TLR4 expression and phosphorylation of IκBα and NF–κB p65, indicating over–activation of TLR–induced inflammatory pathways [62]. Importantly, elevated plasma NEFAs in ketotic cows correlate positively with pro–inflammatory cytokines, and high NEFA concentrations over–activate the TLR2/4–NF–κB pathway, suggesting the inflammatory phenotype in ketotic cows results from combined effects of elevated ketones and NEFAs rather than ketones alone [62].

Metabolomics investigations reveal that cows destined to develop ketosis show elevated inflammatory mediators including IL–6 and TNF–α weeks before clinical disease manifestation, suggesting inflammasome modulation and inflammatory pathway activation represent early, potentially reversible components of ketosis pathophysiology [16]. These findings demonstrate that inflammasome regulation and NF–κB signaling represent critical control points in bovine ketosis pathophysiology with therapeutic implications for managing inflammatory complications during transition.

6. Central Control Nodes: Hypothalamic Integration and Circadian Coordination

The coordination of KIN responses across multiple organ systems requires centralized integration mechanisms. The hypothalamus serves as a critical integration node where metabolic status, immune signals, circadian timing, and environmental information converge to regulate KIN activation and coordinate organism–wide adaptive responses (Figure 5) [86,87]. In ruminants, hepatic oxidation signals are transmitted to hypothalamic feeding centers via vagal afferents, integrating dietary and tissue–derived fuel information to regulate energy balance [88,89].

6.1. Hypothalamic Ketone Sensing and Neuroendocrine Integration

The blood–brain barrier readily permits ketone body passage, enabling direct access to hypothalamic neurons expressing specialized metabolic sensing mechanisms [86,90]. Hypothalamic nuclei contain neurons that respond to ketone body concentration changes [86,89,91]. Direct intracerebroventricular infusion of BHB in sheep demonstrates altered hypothalamic and pituitary gene expression, confirming functional ketone sensing in ruminants [92].

Ketone–sensitive neurons within the arcuate nucleus modulate key metabolic regulatory circuits: NPY/AgRP neurons stimulate feeding, while POMC neurons promote satiety and energy expenditure [86,89]. BHB influences AgRP expression via AMPK signaling pathways, with effects dependent on glucose availability [93]. During elevated ketones, neuronal balance shifts toward energy conservation through reduced thermogenesis and altered feeding patterns [86,94]. In domesticated ruminants, this neuroendocrine integration coordinates metabolic responses to pregnancy, lactation, and growth demands [89].

6.2. Circadian Clock Integration and Temporal Coordination

Ketone metabolism intersects fundamentally with circadian biology. Circadian clocks generate approximately 24–h rhythms in gene expression and metabolic activity, optimizing physiological function according to daily cycles [95]. While continuous ruminal fermentation in ruminants dampens the dramatic feeding–fasting metabolic oscillations observed in monogastric species, molecular circadian clock machinery remains fully conserved and functional in cattle [96,97].

Robust circadian and ultradian rhythms have been documented in lactating dairy cows for core body temperature [98,99], peripheral concentrations of cortisol [100], growth hormone [101], insulin [102], and circulating lipid metabolites including cholesterol, free fatty acids, and triglycerides [103]. More recent studies confirm diurnal patterns in feeding behavior, plasma metabolites, and hormones including melatonin and progesterone in periparturient dairy cattle [99,104,105]. The daily rhythm of milk synthesis itself is dependent on the timing of feed intake, and night–restricted versus day–restricted feeding modifies daily rhythms of milk synthesis and plasma metabolites, establishing functional coupling between feeding time, circadian regulation, and lactation performance [106,107].

Circadian clock function has been demonstrated in ruminant–specific tissues including mammary epithelium and ruminal epithelial cells, where clock genes directly regulate nutrient transport pathways and cell proliferation [108,109,110,111]. The rumen microbiota itself exhibits circadian rhythms, with ruminal melatonin concentrations following circadian patterns and 9% of bacterial operational taxonomic units displaying ~24–h rhythmic abundance [112].

Hepatic ketogenesis exhibits robust circadian rhythmicity, with peak production during the fasting phase [95]. Core clock genes (CLOCK, BMAL1, PER, CRY) directly control transcription of ketogenic enzymes including HMGCS2 and fatty acid oxidation enzymes [113]. Ketone bodies reciprocally influence clock gene expression and circadian rhythm amplitude, creating bidirectional metabolic–temporal integration [79,114]. Tissue–specific changes in molecular clock gene expression occur during the transition from pregnancy to lactation, suggesting coordinated circadian reprogramming to support the metabolic demands of milk synthesis [115].

Experimental disruption of circadian rhythms through chronic light–dark phase shifts in periparturient dairy cows demonstrates the functional importance of circadian coordination, resulting in decreased insulin sensitivity, reduced mammary epithelial cell proliferation, transcriptomic signatures of increased fatty liver risk, impaired mammary remodeling, and skeletal muscle oxidative stress [95,116,117,118]. These findings establish that circadian–metabolic integration is not merely correlative but causally important to transition cow health and lactation performance. Furthermore, photoperiod manipulation—which acts through circadian clock mechanisms—has been successfully applied in commercial dairy management for decades [119,120]. Long–day photoperiod during lactation increases milk yield 2–3 kg/d, while short–day photoperiod during the dry period increases subsequent lactation production by 3–4 kg/d through circadian–regulated changes in prolactin and IGF–I signaling [95,121,122].

The intersection of circadian, ketone, and immune signaling creates potential for temporal coordination within the KIN. BHB influences hypothalamic control of energy homeostasis through effects on neuronal AMPK signaling and neuropeptide expression [86,92,93], while circadian clocks coordinate metabolic and reproductive systems through transcriptional networks involving NAD⁺ metabolism [95,114]. Immune signals transmitted via cytokines and vagal afferents modulate inflammatory tone through HPA axis regulation and neuroendocrine pathways [87]. The immune system itself exhibits robust circadian rhythms, with immune cell trafficking, cytokine production capacity, and inflammatory responsiveness varying systematically across the 24–h cycle [95]. While HCAR2 receptors are expressed on immune cells where they mediate anti–inflammatory effects of BHB [57,65,69], the precise molecular mechanisms integrating ketone, circadian, and immune signaling at the level of central nervous system control centers remain to be fully elucidated.

The NAD⁺–dependent circadian feedback loops involving CLOCK–SIRT1 and NAMPT provide direct molecular coupling between temporal regulation and ketone–sensitive signaling pathways [79,114]. During periparturient ketosis in dairy cattle, disruption of this circadian–metabolic–immune coordination may impair temporal regulation of inflammatory responses, contributing to the pathophysiology observed when ketosis becomes maladaptive [16,18,24]. The circadian disruption studies in periparturient dairy cows demonstrating not only metabolic dysfunction but also altered immune cell function, increased inflammatory mediators, and impaired tissue remodeling [95,116,117,118] support the concept that hypothalamic integration of circadian, ketone, and immune signals represents a critical control node orchestrating coordinated metabolic–immune adaptation (Figure 5). This three–way integration explains why circadian disruption—through light–dark phase shifts, inconsistent feeding times, or dysregulated temporal biology—can precipitate or exacerbate transition cow disorders by simultaneously impairing metabolic flexibility, immune competence, and inflammatory resolution.

7. Cellular Effectors of the Keto–Inflammatory Network

The functional implementation of KIN signaling occurs through coordinated responses across diverse cellular populations, with macrophages and other immune cells contributing specialized capabilities that collectively generate the physiological adaptations characteristic of ketotic states [123,124]. These cellular effectors encompass both immune and non–immune cell types, with responses shaped by cell–specific expression patterns of ketone–sensing receptors, metabolic enzymes, and downstream signaling components (Figure 6) [16]. In dairy cattle, the periparturient period involves dramatic changes in circulating immune cell populations and their functional states, with disruption of inflammatory homeostasis contributing to both metabolic and infectious disease susceptibility during transition [125].

7.1. Myeloid Cell Populations and Innate Immunity

Macrophages represent primary cellular mediators through which the KIN exerts anti–inflammatory and tissue–protective effects [123,126]. Macrophage functional plasticity—spanning classically activated (M1) pro–inflammatory states to alternatively activated (M2) anti–inflammatory phenotypes—provides an ideal cellular system for ketone–mediated immune modulation [123,124].

Concentration–dependent effects: In dairy cattle ketosis, the relationship between ketones, fatty acids, and macrophage polarization is complex and concentration–dependent. Physiological ketone concentrations promote M2–like phenotypes characterized by enhanced oxidative metabolism, increased anti–inflammatory cytokine production (IL–10, TGF–β), and improved phagocytic clearance [57]. However, excessive free fatty acid (FFA) concentrations characteristic of pathological ketosis promote M1 macrophage polarization by impairing mTOR–mediated autophagy, leading to upregulation of iNOS, elevated pro–inflammatory cytokine production (TNF–α, IL–1β, IL–6), and activation of NF–κB signaling—effects attenuated by autophagy activation [127]. In mammary gland and adipose tissue from ketotic cows, macrophage populations exhibit activated inflammatory phenotypes with elevated oxidative stress, NLRP3 inflammasome assembly, and TLR4/NF–κB pathway activation, contributing to reduced milk production and systemic inflammation [84,127,128].

Monocytes and neutrophils—circulating innate immune cells—also exhibit profound functional alterations in ketotic dairy cows. Monocytes isolated from clinically ketotic cows demonstrate significantly impaired adhesion, migration, and phagocytosis capacity, alongside increased apoptosis and ROS production mediated through ROS–NLRP3 inflammasome activation [129]. In vitro exposure of healthy bovine monocytes to elevated BHB concentrations of 3.0 mM reproduces the observed immune dysfunctions in a time–dependent manner, indicating that BHB exerts a direct immunomodulatory effect rather than serving merely as a passive indicator of metabolic imbalance [129]. Neutrophils from ketotic cows demonstrate over–activation of TLR2/4–mediated NF–κB signaling, with elevated NEFA concentrations further amplifying inflammatory activation and promoting pro–inflammatory cytokine synthesis [62,69].

7.2. Lymphocyte Populations and Adaptive Immunity

T lymphocyte responses represent critical components of KIN function, with ketone body availability during T cell activation influencing both immediate functional responses and long–term memory formation [130,131]. The metabolic requirements of T cell activation—including rapid ATP production, biosynthetic precursor generation, and sustained energy supply—make T cells particularly sensitive to substrate availability and metabolic environment [130,132].

CD4⁺ T helper cell differentiation: Ketone availability during T cell activation favors regulatory T cell (Treg) differentiation over pro–inflammatory Th17 cells through mechanisms including enhanced oxidative metabolism, increased FoxP3 expression, and epigenetic modifications stabilizing the regulatory phenotype [133,134].

CD8⁺ T cell memory: Memory CD8⁺ T cells rely heavily on oxidative metabolism and fatty acid oxidation for survival and rapid recall responses. T cells activated in the presence of elevated ketone concentrations exhibit enhanced memory characteristics including improved survival, increased memory–associated transcription factor expression, and enhanced functional capacity upon secondary antigen encounter [135,136].

8. Evolutionary Origins and Conservation of the Keto–Inflammatory Network

The conservation of ketogenic pathways and associated regulatory mechanisms across vertebrate evolution provides compelling evidence for the fundamental adaptive significance of the KIN [1,2]. Phylogenetic analyses reveal that core components of ketone metabolism, including the key biosynthetic enzymes, transport proteins, and sensing mechanisms, have been maintained with high fidelity across millions of years of evolutionary divergence (Figure 7) [41,58].

This conservation extends beyond metabolic enzymes to encompass the entire integrated signaling network. The ketogenic enzymes HMGCS2 and BDH1, the ketone–sensing receptor HCAR2 (GPR109A), histone deacetylase enzymes targeted by BHB, and inflammasome components including NLRP3 all demonstrate remarkably high sequence conservation across mammalian taxa [41,57,58,81]. This deep conservation of both metabolic and immunomodulatory machinery suggests that natural selection has preserved not merely the capacity for alternative fuel utilization during energy scarcity, but rather the integrated ability to coordinate metabolic adaptation with immune function modulation—a systems–level capability essential for survival during periods of combined nutritional and pathogenic stress [6,7].

8.1. Comparative Physiology Across Taxa

The utilization of ketosis as an adaptive strategy extends across diverse mammalian lineages, each demonstrating species–specific modifications that optimize ketogenic responses for particular ecological niches and life history strategies [1,137]. Arctic mammals, including seals, bears, and arctic ground squirrels, rely heavily on ketogenic metabolism during extended periods of food unavailability, with some species maintaining ketotic states for months during hibernation or prolonged fasting [1,138]. The concurrent suppression of inflammatory responses during these extended ketotic periods prevents metabolic costs of unnecessary immune activation while preserving capacity for pathogen defense through ketone–mediated antimicrobial mechanisms [29,57].

In wild ruminants, seasonal resource fluctuations impose periodic nutritional stress that activates ketogenic pathways coordinated with immune system modulation. Free–ranging ruminants during drought periods or harsh winters demonstrate elevated ketone concentrations alongside altered immune cell populations, suggesting that the KIN functions as an integrated survival strategy enabling continued immune surveillance despite energy limitation [30]. Periparturient periods in wild ruminants similarly trigger coordinated metabolic–immune reprogramming, though the magnitude of energy deficit and immunological challenge remains far below that imposed on modern high–producing dairy cattle [30,32].

Diverse species demonstrate specialized ketogenic adaptations that support sustained energy demands under conditions of limited food access [1,7,8]. Hibernating mammals such as ground squirrels and bears maintain ketotic states for extended periods, with ketone bodies providing a metabolically efficient fuel source that supports tissue function while preserving protein stores [8,138]. The anti–inflammatory effects of ketosis during these extended periods of metabolic stress help prevent tissue damage and maintain immune competence despite prolonged fasting [29,57]. Similar principles likely apply to other taxa facing extended periods of high energy expenditure with limited nutrient intake, including endurance exercise in humans [139,140].

8.2. Domestication and Artificial Selection Pressures

The intensive artificial selection applied to domestic animals, particularly dairy cattle, has created physiological challenges that may exceed the adaptive capacity of evolved ketogenic–immune coordination mechanisms [30,141,142]. Modern dairy cows produce 8–10 times more milk than their bovine ancestors, creating energetic demands that can overwhelm the regulatory systems that normally coordinate ketosis with immune function and inflammatory resolution [5,30]. This extreme metabolic load imposed during the periparturient period–when cows face simultaneous demands of late gestation, parturition, lactogenesis initiation, and tissue remodeling–represents a quantitative stress far beyond anything encountered during natural selection [30,32]. The consequence is that evolved regulatory mechanisms, which successfully coordinate moderate ketone elevations with appropriate immune modulation in wild ruminants or ancestral cattle, become dysregulated under the extreme demands of modern production systems. The inflammatory dysfunction observed in transition dairy cows [16,18,24] may thus represent not a failure of the KIN per se, but rather the predictable breakdown of an adaptive system pushed beyond its evolved operational range.

Understanding the evolutionary origins and conservation of the KIN provides critical insights for dairy cattle management. Rather than viewing ketosis purely as a production disease requiring suppression, the evolutionary perspective reveals it as a sophisticated adaptive program that has been conserved because it enhances survival under specific conditions [5,29]. The challenge in modern dairy production is not to eliminate ketosis but to support the physiological conditions under which this ancient regulatory network can function adaptively—maintaining the beneficial metabolic flexibility and immune coordination evolved over millions of years while preventing the dysregulation that occurs when system capacity is exceeded [17,32,143]. This evolutionary framework thus provides both mechanistic understanding of ketosis pathophysiology and rational principles for therapeutic intervention that work with, rather than against, conserved biological processes.

9. The Calci–Keto–Inflammatory Network: Integrating Calcium and Ketone Signaling

The intersection between calcium homeostasis and ketone metabolism represents a regulatory interface that has profound implications for immune function, cellular signaling, and physiological adaptation [144,145,146]. The Calci–Keto–Inflammatory Network (CKIN) describes the dynamic integration of calcium–dependent signaling pathways with ketone–mediated metabolic and immune responses, creating emergent regulatory properties that exceed the capabilities of either system operating independently (Figure 8) [146]. Recent research has revealed that the coordinated occurrence of hypocalcemia and hyperketonemia during the periparturient period in dairy cattle may represent adaptive biological programming rather than independent pathological processes, providing a conceptual framework for understanding how calcium signaling coordinates with inflammatory responses to optimize physiological adaptation during periods of intense metabolic demand [143].

9.1. Molecular Mechanisms of Calcium–Ketone Integration

Calcium (Ca²⁺) serves as a ubiquitous second messenger in nearly all eukaryotic cells, governing gene transcription, cellular excitability, mitochondrial respiration, and immune activation. In immune cells, calcium influx through store–operated calcium entry (SOCE) channels—particularly those formed by STIM1 and Orai1 proteins—activates transcription factors such as NFAT, NF–κB, and CREB, which drive cytokine production, cell proliferation, and survival [147,148]. Concurrently, BHB modulates immune responses by inhibiting HDACs, suppressing NLRP3 inflammasome activation, enhancing antioxidant defense, and promoting anti–inflammatory macrophage (M2) and T cell phenotypes through engagement of receptors such as HCAR2 (Figure 9) [57,149].

The mechanistic convergence between calcium and ketone signaling is particularly evident at the mitochondrial level. Mitochondria serve as central hubs for both Ca²⁺ buffering and oxidative metabolism. BHB maintains mitochondrial membrane potential, enhances respiratory efficiency, and reduces ROS generation—factors that stabilize intracellular calcium flux and protect immune cells from oxidative and inflammatory stress [33,70]. Through these effects, BHB modulates the threshold for calcium–induced immune activation and supports bioenergetic integrity.

9.2. Clinical Applications and Therapeutic Implications

Mitochondria function as the primary integration site where calcium and ketone signaling pathways converge to coordinate metabolic and immune responses. They simultaneously buffer Ca²⁺ through the mitochondrial calcium uniporter (MCU) complex and conduct oxidative metabolism through the electron transport chain, with BHB playing modulatory roles across both processes [144,147].

BHB enhances mitochondrial function through multiple complementary mechanisms. As an oxidative substrate, BHB maintains mitochondrial membrane potential more efficiently than glucose–derived pyruvate, reducing electron leak and ROS generation while supporting ATP production [70]. This improved bioenergetic efficiency stabilizes mitochondrial Ca²⁺ handling by preventing calcium–induced mitochondrial membrane permeabilization—a critical checkpoint in immune cell activation and apoptosis [144]. Through these mechanisms, BHB modulates the threshold for calcium–induced mitochondrial dysfunction, enabling cells to maintain metabolic flexibility during periods of simultaneous energy stress and immune activation.

The integration extends to inflammatory signaling pathways. Mitochondrial ROS serve as critical activators of the NLRP3 inflammasome and NF–κB pathway [83]. By reducing mitochondrial ROS generation, BHB indirectly dampens calcium–triggered inflammatory cascades, creating a mechanistic link between metabolic state, calcium dynamics, and immune tone. This mitochondria–centered integration positions the organelle as the critical regulatory node determining whether calcium signals promote inflammation or metabolic adaptation.

9.3. Clinical Manifestations in Periparturient Dairy Cows

The CKIN provides a mechanistic framework for understanding the co–occurrence of hypocalcemia and hyperketonemia in periparturient dairy cows. This convergence reflects coordinated adaptive programming rather than dual pathology: a strategy to transiently downregulate immune reactivity, reduce metabolic strain, and allocate energy toward lactation and tissue remodeling [5,143].

During the transition period, the coordinated reduction in circulating calcium and elevation in ketone bodies may serve complementary adaptive functions. Hypocalcemia dampens calcium–dependent immune activation, reducing the inflammatory response to inevitable microbial exposure during parturition and uterine involution [143]. Simultaneously, elevated BHB provides an efficient oxidative fuel that supports high–energy tissues (mammary gland, brain) while exerting anti–inflammatory effects through HDAC inhibition and NLRP3 inflammasome suppression. The mitochondrial integration of these signals ensures that immune suppression and metabolic adaptation occur in coordinated fashion, optimizing the cow’s capacity to initiate lactation without triggering excessive inflammation.

This framework challenges traditional clinical approaches that treat hypocalcemia and hyperketonemia as independent pathologies requiring aggressive normalization. Instead, it suggests that moderate, transient alterations in both parameters may represent functional adaptation, with clinical intervention warranted only when the regulatory system becomes dysregulated—manifested by extreme deviations, prolonged duration, or concurrent inflammatory markers indicating loss of homeostatic control.

9.4. Translational Implications for Human Health

In humans, disruption of the coordinated calcium–ketone axis is increasingly recognized in the pathophysiology of chronic inflammation, metabolic syndrome, autoimmune disease, and neurodegeneration, where calcium mishandling and impaired ketogenesis frequently co–occur [61,150,151]. Cellular calcium overload promotes mitochondrial dysfunction, excessive ROS production, and inflammatory signaling—pathological processes that therapeutic ketosis may ameliorate through the mechanisms described above.

The enhancement of CD8⁺ T cell memory and anti–tumor immunity through ketogenic interventions represents a particularly promising therapeutic application [152]. Ketone bodies support memory T cell formation by promoting oxidative metabolism and mitochondrial biogenesis, while simultaneously modulating calcium signaling pathways that influence T cell differentiation and longevity. Similarly, ketogenic approaches show remarkable efficacy in alleviating colitis through modulation of gut immune cell populations, likely mediated through coordinated effects on calcium–dependent inflammatory signaling and metabolic reprogramming of intestinal immune cells [149].

These translational findings suggest that therapeutic strategies targeting the CKIN may offer novel approaches for treating inflammatory and metabolic diseases across species. Rather than treating calcium and metabolic dysregulation as independent entities, interventions that restore coordinated function of this integrated system may prove more effective.

9.5. The “Pedal and Brake” System

The CKIN framework unifies three historically separate domains—mineral signaling, ketone metabolism, and immune regulation—into a coordinated ‘pedal–and–brake’ system that governs both immune activation and resolution. This constitutes a paradigm shift, reframing health not as the balance of isolated pathways but as the dynamic interplay of co–evolved systems in which energy metabolism, calcium fluxes, and immune function operate as an integrated, adaptive network [151,153].

Calcium serves as the “pedal,” providing the activation signal that drives immune cell responses through SOCE–mediated transcription factor activation and inflammasome assembly. Ketone bodies function as the “brake,” decreasing excessive inflammatory activation through HDAC inhibition, NLRP3 suppression, and metabolic reprogramming toward oxidative phosphorylation. Mitochondria integrate these opposing signals, functioning as the regulatory hub that determines the net immune response based on the organism’s metabolic state and energy availability.

This framework opens new therapeutic avenues for metabolic–inflammatory diseases across species, providing a co–evolved mechanism of physiological resilience during stress. Interventions that support rather than suppress this regulatory system—such as timed ketogenic strategies during inflammatory challenges or calcium management protocols that account for metabolic status—may prove more effective than current approaches that focus on normalizing individual parameters without considering systemic integration.

10. The Ketoinflammatory Clock: Circadian Synchronization of Immunometabolic Rhythms

Temporal coordination of metabolic and immune functions represents a fundamental organizing principle in mammalian biology, with circadian disruption contributing to metabolic syndrome, autoimmune disease, and inflammatory disorders [154,155]. The Keto–inflammatory Clock describes the regulatory system governing daily fluctuations in immune responsiveness, metabolic flexibility, and stress adaptation through integrated ketone–circadian signaling (Figure 10) [96,156].

Circadian regulation of ketone metabolism in dairy cattle has been comprehensively reviewed [157], establishing that circadian clock disruption contributes to glucose and lipid metabolism disorders underlying ketosis pathogenesis. Dairy cattle demonstrate robust circadian regulation of metabolism and immunity. Metabolic hormones (insulin, cortisol, melatonin) and blood metabolites (glucose, NEFA, BHBA) exhibit daily oscillations in dairy cows [98,100,101,102,103]. Hepatic clock genes (BMAL1, CLOCK, PER, CRY) show circadian rhythmicity, with feeding time influencing milk production, metabolite profiles, and hormonal rhythms [105,107]. Mammary clock gene expression responds to photoperiod manipulation and lactation stage transitions [115,155,156].

Mechanistic caveat: While circadian regulation clearly occurs in ruminants, functional significance may differ from meal–feeding monogastrics. Most mechanistic evidence derives from humans and rodents with alternating fasting–feeding cycles [94,95]. Ruminants exhibit continuous nutrient absorption from ruminal fermentation, potentially altering oscillation amplitude and metabolic consequences. Whether circadian–timed interventions benefit periparturient cows requires direct investigation. Nevertheless, substantial evidence supports circadian coordination of ketone metabolism across mammalian taxa, including dairy cattle [157,158].

10.1. Molecular Circadian Control of Ketogenesis

Core clock genes (CLOCK, BMAL1, PER, CRY) generate approximately 24–h oscillations regulating ketogenic enzymes (HMGCS2, BDH1), fatty acid oxidation enzymes (CPT1A, ACOX1), and glucose metabolism genes (G6PC, PCK1) [94,141]. Nuclear receptors PPARα and REV–ERBα provide additional circadian–metabolic integration. The interaction between circadian clock systems and gut microbiota in regulating host energy metabolism—including effects on ketone body production—has been established across mammalian species [159].

In dairy cattle, period circadian regulator 2 (PER2) might play a role in regulation of short–chain fatty acid transporter expression in rumen epithelium [110], suggesting potential functional circadian regulation at the molecular level in ruminant tissues. Feeding time entrains peripheral clocks; restricting feed to night versus day shifts core body temperature, plasma metabolites, and milk production [107]. Mammary clock gene expression and de novo fatty acid synthesis timing shift between day– and night–restricted feeding, demonstrating functional circadian regulation coordinating metabolic output with nutrient availability in lactating ruminants [107]. Furthermore, ruminal microbiota exhibit circadian rhythms, with approximately 9% of bacterial operational taxonomic units showing diurnal oscillations that may influence host metabolic regulation [112].

10.2. Circadian Regulation of Immune Function

Immune cells possess functional circadian clocks autonomously regulating cell trafficking, activation thresholds, cytokine production, and antimicrobial activity [155,160]. This coordination optimizes immune surveillance during predicted pathogen encounter periods (e.g., feeding times with increased microbial exposure) while dampening inflammation during fasting when resources support cellular maintenance [155,161]. Ketone–circadian integration enables strategic metabolic–immune resource allocation across the 24–h cycle.

In dairy cattle, eating, rumination, rumen pH, and reactive oxygen metabolites exhibit daily rhythms, indicating temporal regulation of metabolic and oxidative balance [106,120]. The circadian regulation of mammary tissue development and immune function has direct implications for lactation performance and disease susceptibility during the transition period [158].

10.3. Translational Implications for Dairy Cattle

Experimental circadian disruption in late–gestation dairy cows through chronic prepartum light–dark phase shifts resulted in hypoglycemia, decreased insulin sensitivity, altered hepatic gluconeogenesis, impaired circadian gene expression in liver and mammary tissue, and reduced milk yield [118], demonstrating functional consequences paralleling effects in humans and rodents. Photoperiod length during the dry period influences mammary development and subsequent lactation performance [115,119], demonstrating that circadian manipulations impact production outcomes.

The role of circadian dysregulation in ketosis pathogenesis represents an important frontier for dairy cattle research [157]. Whether optimizing intervention timing relative to endogenous circadian rhythms improves metabolic health and reduces transition period disease risk remains to be fully established. However, the substantial body of evidence demonstrating circadian regulation of metabolism, immunity, and production in dairy cattle [95] supports investigating chronobiological approaches to periparturient disease management.

11. The Ketosis–Microbiota–Immune Triad: Trans–Kingdom Signaling

The recognition that host metabolism and microbial ecology function as integrated systems has fundamentally transformed understanding of health, disease, and metabolic regulation [49,162]. The ketosis–microbiota–immune triad (KMI) exemplifies trans–kingdom signaling, where host–derived ketone bodies and microbe–derived metabolites converge on shared regulatory pathways to coordinate systemic physiological responses (Figure 11) [51,133].

11.1. Evolutionary Origins and Bidirectional Communication

The intimate metabolic relationship between mammals and microbial communities reflects millions of years of co–evolution shaping both host physiology and microbial metabolic capabilities [49,51].

The structural similarity between host–derived BHB and microbe–derived butyrate exemplifies this evolutionary convergence [6,140]. Both molecules activate hydroxycarboxylic acid receptors despite their distinct biosynthetic origins [57,134], and this convergent signaling has been directly demonstrated in bovine tissues. Bovine GPR41 and GPR43 (SCFA receptors) and HCAR2 (BHB receptor) are functionally expressed in dairy cattle immune cells, rumen epithelium, mammary tissue, and other organs [163]. In vitro studies confirm that bovine GPR41 and GPR43 respond to acetate, propionate, and butyrate through Gαi/11–coupled signaling pathways [163], while HCAR2 on bovine leukocytes mediates BHBA effects on calcium mobilization and cAMP signaling [69]. In vivo, both sodium butyrate and sodium BHB supplementation in dairy cows produce anti–inflammatory effects and improve lactation performance through activation of these conserved receptor pathways [164,165], supporting the hypothesis that host signaling systems evolved to recognize and respond to functionally similar metabolites regardless of biosynthetic source.

In ruminants, microbial fermentation produces volatile fatty acids (VFAs)—primarily acetate, propionate, and butyrate—providing 60–80% of energy requirements, with butyrate serving dual roles as energy source for ruminal epithelium and systemic signaling molecule through GPR41/43 activation [163,166].

In monogastric species, ketogenic dietary interventions alter microbiome composition, reducing pro–inflammatory taxa (Enterobacteriaceae) while increasing beneficial genera (Akkermansia, Bifidobacterium, Lactobacillus) [167]. Conversely, microbial short–chain fatty acids enhance hepatic ketogenesis through AMPK and PPARα activation, while balanced microbial communities maintain intestinal barrier integrity, limiting LPS translocation that suppresses hepatic ketogenic capacity [51,56,168]. In dairy cattle, sodium butyrate supplementation reduces ruminal and plasma LPS concentrations while decreasing systemic inflammatory markers, illustrating the integrated nature of microbiome–metabolite–immune interactions [164].

11.2. The Rumen Microbiome and Ketosis in Dairy Cattle

In ruminants, the microbiota–ketone relationship exhibits unique characteristics reflecting the central role of ruminal fermentation in host energy metabolism. Recent studies reveal ketotic dairy cows exhibit altered rumen and fecal microbiome composition compared to control cows, with reduced microbial diversity and altered representation of bacterial taxa involved in carbohydrate fermentation and VFA production [169]. Multi–omics approaches integrating rumen microbiome, metabolome, and host transcriptome demonstrate coordinated changes across all domains during ketosis development, suggesting rumen dysbiosis represents an integral component of disease pathophysiology [170], paralleling microbiome alterations observed in metabolic and inflammatory disorders across mammalian species [171].

Critically, prepartum microbiome composition influences subsequent disease risk. Cows developing postpartum ketosis demonstrate detectable alterations in rumen bacterial community structure 3–4 weeks before parturition, with these prepartum signatures predicting subsequent ketosis with substantial accuracy [170]. Specific taxa associated with ketosis risk include reduced fiber–degrading bacteria and increased lactate–producing species, potentially contributing to ruminal acidosis and systemic inflammation [166].

Rumen dysbiosis contributes to ketosis through interconnected mechanisms: altered VFA production—particularly reduced propionate—compromises hepatic gluconeogenesis, exacerbating glucose deficit, while increased epithelial permeability facilitates LPS translocation into systemic circulation, triggering inflammatory responses that suppress hepatic metabolism [166,172]. The gut–liver axis thereby connects rumen microbial ecology with hepatic ketogenesis through both metabolic (VFA provision) and inflammatory (LPS translocation) mechanisms.

12. Ketosis and Immuno–Epigenetic Memory

The concept of immunological memory has traditionally focused on the adaptive immune system’s capacity to orchestrate antigen–specific responses through specialized immune cells [173]. However, emerging evidence demonstrates that the innate immune system also possesses memory–like properties through epigenetic reprogramming mechanisms that persistently alter immune cell responsiveness [136]. Ketone body exposure represents a particularly potent stimulus for establishing beneficial epigenetic memory that enhances resistance to inflammatory challenges and improves stress adaptation (Figure 12) [46,70].

12.1. Molecular Mechanisms of Ketone–Induced Epigenetic Memory

The establishment of epigenetic memory by ketone bodies involves coordinated modifications to chromatin structure that persist long after ketone concentrations return to baseline levels [46,174]. The primary mechanism involves BHB–mediated inhibition of class I histone deacetylases, leading to increased acetylation of histones H3 and H4 at specific gene loci involved in stress response, antioxidant defense, and anti–inflammatory signaling [46,70].

Key target genes for ketone–induced epigenetic modifications include those encoding antioxidant enzymes (catalase, superoxide dismutase, glutathione peroxidase), stress–response transcription factors (FOXO3a, Nrf2), anti–inflammatory cytokines (IL–10), and tissue repair mediators (arginase 1, TGF–β) [70,71]. The coordinated upregulation of these protective gene programs creates cellular phenotypes characterized by enhanced resilience to oxidative stress, reduced inflammatory responsiveness, and improved capacity for tissue repair and regeneration [153,175].

These epigenetic modifications exhibit remarkable persistence. Studies demonstrate that brief exposure to elevated ketone concentrations can alter histone acetylation patterns and gene expression profiles that persist for days to weeks after ketone levels normalize [46]. This temporal durability distinguishes epigenetic memory from acute metabolic responses, suggesting that ketone exposure during critical developmental or physiological windows may exert lasting effects on immune cell programming and stress responsiveness.

12.2. Trained Immunity and Metabolic Reprogramming

Unlike classical trained immunity paradigms that typically involve enhanced inflammatory responses following pathogen exposure, ketone–induced immune training promotes metabolic reprogramming toward oxidative metabolism and anti–inflammatory programming [126,136]. Monocytes and macrophages exposed to ketone bodies demonstrate lasting metabolic changes including enhanced oxidative phosphorylation, increased fatty acid oxidation, and improved mitochondrial biogenesis [126,176].

The effects of ketone–induced trained immunity extend to hematopoietic stem and progenitor cells within the bone marrow, where epigenetic modifications can influence the differentiation programs that generate mature immune cells [136]. This upstream programming suggests that ketone exposure during critical developmental periods may establish lasting alterations in immune cell production and functional capacity. In dietary restriction models, bone marrow hematopoietic stem cells exhibit enhanced self–renewal and altered differentiation patterns that correlate with elevated systemic ketone concentrations, potentially establishing immunological memory at the stem cell level [136].

The functional consequences of ketone–induced trained immunity differ substantially from pathogen–induced training. While bacterial or fungal components typically enhance pro–inflammatory responses to subsequent challenges (a potentially protective response against recurrent infection), ketone–induced training dampens inflammatory responsiveness while maintaining antimicrobial capacity. This creates an immune phenotype optimized for metabolic stress resistance rather than pathogen defense—potentially adaptive during periods of food scarcity or energetic challenge when excessive inflammation would impose unsustainable metabolic costs.

12.3. Evidence and Implications in Dairy Cattle

Caveat regarding direct evidence in ruminants: While the molecular mechanisms of BHB–mediated epigenetic regulation are well–characterized in human and rodent systems, direct demonstration of persistent immuno–epigenetic memory following ketone exposure in dairy cattle remains limited. The histone modifications induced by BHB are evolutionarily conserved (as demonstrated by bovine studies showing H3K9 acetylation and β–hydroxybutyrylation) in response to ketone exposure [66,67], but whether these epigenetic changes translate to lasting alterations in immune cell function across subsequent lactations in dairy cattle requires investigation.

Nevertheless, several observations suggest potential relevance to dairy cattle immunometabolism. Cows experiencing ketosis in one lactation demonstrate altered disease risk in subsequent lactations, even after apparent metabolic recovery [11]. While traditionally attributed to ongoing metabolic dysfunction or management factors, epigenetic memory in immune cells or hematopoietic progenitors could contribute to this persistent disease susceptibility. The prepartum period, when many cows experience subclinical ketosis, coincides with substantial hematopoietic activity supporting immune system preparation for parturition and lactation—a potential window for establishing epigenetic programming that influences immune responses throughout the subsequent lactation.

The concept of immuno–epigenetic memory also provides a mechanistic framework for understanding why nutritional or environmental interventions during the dry period can influence disease outcomes weeks or months later. If ketone exposure (or its absence) during the transition period establishes lasting epigenetic modifications in immune cell populations, then interventions targeting metabolic status during this critical window may exert effects extending beyond the immediate periparturient period. This perspective suggests that optimal transition cow management should consider not only acute metabolic balance but also the establishment of beneficial immunological programming for the entire lactation.

12.4. Translational Implications and Future Directions

The recognition that ketone bodies can establish beneficial epigenetic memory suggests novel therapeutic strategies that leverage transient metabolic interventions to achieve lasting immunological benefits. Rather than continuous ketogenic supplementation or dietary manipulation, strategic short–term interventions during critical windows (such as the prepartum period) might establish epigenetic programming that enhances immune resilience throughout lactation.

However, translating these concepts to practical dairy management requires careful validation. Key questions include: What ketone concentrations and exposure durations are required to establish beneficial epigenetic memory? Do the anti–inflammatory epigenetic modifications induced by physiological ketosis differ from those induced by pathological hyperketonemia? Can the timing of ketone exposure be optimized relative to immune system development or hematopoietic cycles? Does epigenetic memory established during one lactation persist across the dry period to influence subsequent lactations?

The immuno–epigenetic memory framework also raises important considerations for nutritional management of transition cows. Aggressive suppression of ketone production during the periparturient period—a common management strategy—might inadvertently prevent establishment of beneficial epigenetic programming. Alternatively, strategic induction of mild, controlled ketosis during specific windows might optimize immune cell programming for the challenges of early lactation. Distinguishing these possibilities requires research explicitly designed to assess long–term immunological outcomes of periparturient metabolic interventions, rather than focusing solely on short–term metabolic normalization.

The concept that ketone exposure establishes lasting immunological memory represents a paradigm shift from viewing ketosis as an acute metabolic dysfunction requiring immediate correction toward recognizing ketone signaling as a mechanism for biological programming with potential long–term consequences. This framework challenges clinicians and researchers to consider not only whether ketone concentrations are elevated, but when, for how long, and in what physiological context—factors that may determine whether ketosis establishes beneficial adaptation or pathological programming.

13. The Ketosis–Liver–Immune Triad: Hepatic Surveillance and Metabolic Integration

The liver occupies a unique position in mammalian physiology as the primary site of ketone body synthesis, a major organ for immune surveillance, and a critical integration hub for metabolic and inflammatory signaling [177,178]. Hepatic implementation of KIN responses involves coordination between hepatocyte metabolic activity, resident immune cell populations, and circulating immune mediators that collectively determine both ketogenic capacity and systemic immune tone (Figure 13) [24,179].

13.1. Hepatic Architecture and Integration Capacity

The structural organization of the hepatic lobule creates distinct metabolic and immunological microenvironments contributing to ketogenic regulation [177]. Hepatocytes arranged in plates from portal triads to central veins establish functional zonation, with periportal hepatocytes favoring gluconeogenesis and fatty acid oxidation while pericentral hepatocytes demonstrate enhanced lipogenesis and xenobiotic metabolism [180]. The liver contains the largest population of tissue–resident macrophages (Kupffer cells), strategically positioned within sinusoids to monitor portal blood for pathogens and inflammatory mediators [56]. During ketogenesis, Kupffer cells demonstrate altered activation characterized by reduced inflammatory responsiveness and enhanced tissue repair functions [24,176].

13.2. NEFA Metabolism and Hepatic Lipidosis

During the periparturient transition, adipose lipolysis releases large quantities of NEFAs into circulation, with plasma concentrations often exceeding 1.0 mM—a 5– to 10–fold increase [30,36]. Hepatocyte NEFA uptake substantially exceeds oxidative capacity during early lactation, requiring metabolic partitioning between oxidation (generating acetyl–CoA for ketogenesis or TCA cycle), esterification to triglycerides, or VLDL export [181].

When acetyl–CoA production exceeds TCA cycle capacity—typically due to oxaloacetate depletion from gluconeogenesis—excess acetyl–CoA enters ketogenesis via HMGCS2 [4,40]. However, esterification capacity often exceeds oxidation and export, resulting in hepatic triglyceride accumulation affecting 40–60% of periparturient cows [182]. Hepatic triglyceride content exceeding 5–10% of wet weight impairs hepatocyte function, reduces ketogenic enzyme expression, and predisposes to metabolic disease [181,182].

13.3. Inflammatory and Oxidative Stress

The periparturient period involves substantial hepatic inflammatory activation, with acute phase proteins (haptoglobin, serum amyloid A) increasing 2– to 10–fold around parturition [24,179]. Pro–inflammatory cytokines (IL–6, IL–1β, TNF–α) suppress expression of gluconeogenic and ketogenic enzymes while promoting hepatic lipogenesis and impairing VLDL secretion, creating a vicious cycle where inflammation impairs metabolic function and metabolic dysfunction amplifies inflammation [18,179].

Kupffer cells mediate inflammatory–metabolic integration by responding to LPS translocated from gut during transition through TLR4–NF–κB signaling, propagating inflammatory signals to hepatocytes and systemic circulation [172,183]. Cows developing ketosis demonstrate elevated plasma LPS and enhanced Kupffer activation, suggesting gut–liver axis dysfunction contributes to pathophysiology [172].

Hepatic oxidative stress arises from massive fatty acid oxidation generating ROS that overwhelm antioxidant defenses, causing lipid peroxidation, protein oxidation, and mitochondrial dysfunction [184]. Ketotic cows demonstrate elevated oxidative stress markers (malondialdehyde, protein carbonyls), with damage correlating with disease severity. Paradoxically, BHB functions as an antioxidant at physiological concentrations, creating complex dynamics where moderate ketosis may provide protection while severe ketosis with massive NEFA flux promotes oxidative injury [70,72].

13.4. Integrative Perspective

Metabolomics investigations have revealed alterations in liver function markers, lipid metabolism, and inflammatory mediators detectable 4–8 weeks before clinical ketosis manifestation [16,17,185]. This predictive capacity—with metabolite signatures forecasting disease development with >98% accuracy (AUC 0.98–1.00)—demonstrates that hepatic dysfunction represents coordinated biological programming rather than acute metabolic failure. The temporal sequence suggests hepatic dysregulation precedes rather than follows systemic ketosis, positioning the liver as a central mediator of metabolic–immune integration during transition.

The ketosis–liver–immune triad framework repositions hepatic function from passive ketone production to active metabolic–immune integration. The liver continuously integrates signals from adipose tissue (NEFA flux), gut microbiome (LPS translocation), immune system (cytokine signaling), and reproductive tract (parturition inflammation) to coordinate organism–wide responses. When this integration maintains homeostatic balance, moderate transient ketosis supports lactogenesis while preserving immune competence. When integration fails—typically through excessive adipose mobilization, hepatic lipid accumulation, inflammatory activation, and oxidative stress—pathological ketosis emerges as a manifestation of lost regulatory control rather than simple energy deficit.

This systems–level understanding suggests that distinguishing adaptive from pathological ketosis requires assessing hepatic integration capacity through multivariate metabolic–inflammatory signatures rather than relying on isolated BHB thresholds. The liver functions not merely as a ketone–producing organ but as a critical information processing hub determining whether metabolic stress (when energy demands exceed dietary intake) generates adaptive or pathological outcomes.

14. The Mitochondrial Interface of Metabolic–Immune Integration

Mitochondria represent the fundamental cellular interface where energy metabolism and immune signaling converge to determine cellular fate decisions, stress responses, and functional adaptations [186,187,188]. Within the context of the KIN, mitochondria serve simultaneously as the primary sites of ketone body production and utilization, critical regulators of immune cell activation and function, and central integrators of metabolic and inflammatory signaling (Figure 14) [17,189].

14.1. Mitochondrial Ketone Metabolism and Bioenergetics

The mitochondrial matrix contains the complete enzymatic machinery for both ketone body synthesis and oxidation, enabling mitochondria to function as both producers and consumers of ketone bodies depending on cellular metabolic status and tissue–specific requirements [4,27]. The regulation of mitochondrial ketogenesis involves control mechanisms that integrate information about substrate availability, energy demand, and cellular stress status [27,41].

In peripheral tissues, mitochondria serve as the primary sites of ketone body oxidation through the action of succinyl–CoA:3–ketoacid CoA transferase (SCOT) and acetyl–CoA acetyltransferase [1,7]. The energy yield from ketone oxidation, combined with the metabolic efficiency of ketone utilization, makes ketone bodies particularly valuable fuel sources for metabolically active cells including immune cells, cardiac myocytes, and neurons [100].

14.2. Mitochondrial Control of Immune Cell Metabolism and Function

The mitochondria within immune cells function as critical biosensors and metabolic controllers that integrate ketone availability with immune cell activation states, fate decisions, and functional outputs [187,188,190]. Unlike other cell types where mitochondria serve primarily for ATP production, immune cell mitochondria actively regulate inflammatory signaling through multiple interconnected mechanisms including redox state modulation, metabolite production, and direct interaction with innate immune signaling platforms [186,187,189].

14.3. Mitochondrial Membrane Potential and Immune Activation

Mitochondrial membrane potential (ΔΨm) serves as a master regulator of immune cell activation, with elevated ΔΨm promoting pro–inflammatory phenotypes and reduced ΔΨm favoring anti–inflammatory states [191]. BHB utilization stabilizes mitochondrial membrane potential through enhanced electron transport chain efficiency and reduced proton leak, creating a metabolic environment that dampens inflammatory activation [74,123]. This mechanism explains how ketone oxidation can suppress NLRP3 inflammasome assembly, which requires mitochondrial depolarization and ROS production for activation [49,62].

14.4. Mitochondrial ROS as Immune Signaling Molecules

Mitochondrial ROS function as critical second messengers in immune activation, with controlled ROS production promoting antimicrobial responses while excessive ROS drives inflammatory pathology [192,193]. Ketone body oxidation reduces mitochondrial ROS generation through multiple mechanisms: improved respiratory chain coupling, enhanced NAD+/NADH ratios supporting antioxidant systems, and direct free radical scavenging by BHB [74,123,127]. In macrophages, this mitochondrial ROS reduction shifts polarization from pro–inflammatory M1 toward anti–inflammatory M2 phenotypes [74,77,192].

14.5. Mitochondrial Dynamics and Immune Cell Fate

Mitochondrial morphology—regulated through fusion and fission dynamics—critically influences immune cell differentiation and memory formation [194,195]. Memory T cells demonstrate elongated, fused mitochondrial networks that maximize oxidative phosphorylation capacity, while effector T cells exhibit fragmented mitochondria supporting glycolytic metabolism [79,80,194]. Ketone body availability during T cell activation promotes mitochondrial fusion and oxidative metabolism, thereby enhancing memory T cell formation and long–term immunity [80,194].

14.6. Mitochondrial Metabolites as Immunoregulatory Signals

Beyond ATP production, immune cell mitochondria generate metabolites that directly regulate inflammatory signaling. Succinate accumulation under pro–inflammatory conditions stabilizes HIF–1α and drives IL–1β production, while citrate export supports fatty acid synthesis for membrane expansion during proliferation [126,196]. Ketone oxidation alters these metabolite pools: reducing succinate accumulation, maintaining citrate flux for oxidative rather than biosynthetic metabolism, and generating acetyl–CoA that supports histone acetylation and anti–inflammatory gene expression [36,37,196]. The integration of mitochondrial metabolites with hormonal signals, including glucocorticoids that regulate secretory activation [197], creates multi–layered coordination of metabolic and immune responses.

14.7. Species–Specific Considerations

While these mitochondrial mechanisms are well–established in human and rodent immune cells, direct demonstration in bovine immune cells is limited but growing. Recent studies demonstrate that cows with clinical ketosis exhibit extensive mitochondrial dysfunction and oxidative stress in the mammary gland, including reduced mitochondrial membrane potential, swollen mitochondrial morphology, and impaired respiratory chain complex expression [198]. Furthermore, elevated BHB concentrations activate the ROS–NLRP3 inflammasome pathway in bovine monocytes, leading to increased apoptosis and impaired phagocytic function [129]. These bovine–specific findings confirm that the mitochondrial mechanisms described in model organisms operate similarly in ruminants, though the threshold for pathological versus adaptive responses may differ based on species–specific metabolic adaptations [129,198].

14.8. Mitochondrial Integration of Ketone–Immune Signaling

The mitochondrial interface thus represents the critical mechanistic link between ketone availability and immune cell reprogramming. Rather than ketones simply serving as alternative fuels, they actively reprogram mitochondrial metabolism, dynamics, ROS production, and metabolite profiles to coordinate anti–inflammatory phenotypes [1,186,187]. This mitochondrial reprogramming explains how systemic ketonemia can simultaneously influence diverse immune cell populations—from tissue macrophages to circulating lymphocytes—through a conserved intracellular mechanism accessible to all mitochondria–containing cells [199].

This mechanistic understanding is consistent with observations that cows destined to develop ketosis show early alterations in immune cell function and inflammatory mediator production weeks before clinical disease manifestation [14,16]—the mitochondrial interface enables immune cells to sense and respond to changing ketone availability as an indicator of metabolic state, initiating coordinated adaptive or maladaptive responses depending on the magnitude, duration, and context of ketone elevation [198,200].

15. The Ketosis–Placenta–Offspring Triad: Developmental Programming and Transgenerational Effects

The maternal–fetal interface represents a critical developmental window where maternal metabolic status profoundly influences fetal growth, organ development, and long–term health outcomes [5,201]. Maternal ketosis during pregnancy creates unique opportunities for developmental programming through direct effects of transplacental ketone transfer, epigenetic modifications in developing tissues, and alterations in maternal immune function that influence fetal immune development (Figure 15) [202].

15.1. Placental Ketone Transport and Metabolism

The placenta functions as an important metabolic organ that actively regulates the transfer of nutrients, metabolites, and signaling molecules between maternal and fetal circulation [203]. Ketone bodies cross the placental barrier through specialized monocarboxylate transporters (MCTs), particularly MCT1 and MCT4, which are expressed on both maternal and fetal sides of the placental interface [7]. In dairy cattle, placental morphology—particularly the total cotyledonary surface area—strongly correlates with fetal weight (r = 0.643), confirming the critical importance of adequate placental surface for nutrient transfer to the developing fetus [203]. Maternal factors such as age and metabolic stress can influence placental compensation mechanisms, with younger dams and those under metabolic constraint showing higher cotyledon numbers to maintain fetal nutrient supply [203].

15.2. Epigenetic Programming and Immune Development

Maternal ketosis can influence fetal development through epigenetic mechanisms that establish lasting changes in gene expression patterns [46,202]. Ketone–mediated histone deacetylase (HDAC) inhibition can establish epigenetic modifications affecting cellular differentiation and function [46,70].

The developing brain is particularly responsive to ketone availability, with ketone bodies supporting the unique lipid requirements of neural development [204] and providing neuroprotection against oxidative stress [205].

The immune system represents one important target for developmental programming. Maternal metabolic status during pregnancy can influence the development of fetal immune cell populations and establishment of immune tolerance mechanisms through epigenetic modifications affecting immune cell differentiation and activation [206]. However, the specific mechanisms by which maternal ketosis influences bovine fetal immune development require further investigation, as most mechanistic data derive from rodent and human studies.

15.3. Evidence in Dairy Cattle

Direct evidence from dairy cattle demonstrates that maternal ketosis has significant consequences for offspring development and long–term performance. Calves born to dams experiencing subclinical ketosis (blood BHB ≥1.4 mmol/L) exhibit altered developmental trajectories, including paradoxically greater birth weight but significantly slower postnatal growth rates from 1 to 8 weeks of age [207]. These calves also demonstrate impaired gut microbiome development, with reduced microbial diversity, richness, and evenness by 3 weeks of age, alongside altered abundances of key bacterial species including Butyricicoccus pullicaecorum and Sharpea azabuensis [207]. The mechanisms underlying these effects involve direct impacts of elevated BHB on placental tissues: in vitro studies demonstrate that pathological BHB concentrations (1.8–2.4 mM) negatively affect bovine caruncular epithelial cell metabolism and motility while inducing inflammatory responses through increased TNF expression, potentially disrupting feto–maternal communication during the critical peri–implantation period [208].

Maternal metabolic stress during pregnancy can also induce transgenerational effects that persist into adult life. Female calves born to dams experiencing energy restriction during the first 80 days of gestation exhibit impaired ovarian development, characterized by reduced gonadal weight, fewer surface antral and primary follicles, and lower circulating anti–Müllerian hormone concentrations, suggesting that the window from the peri–ovulatory period through the first 2.6 months of gestation represents a critical period for ovarian programming [209]. These developmental consequences have significant economic implications for dairy operations, as reduced fertility and reproductive performance directly impact farm profitability [210]. Additionally, transgenerational analysis across 40,065 Holstein cows revealed that females born to mothers that were lactating during pregnancy produced 52 kg less milk during their first lactation, lived 16 days shorter, and exhibited reduced metabolic efficiency compared to females whose fetal development occurred in the absence of maternal lactation [211]. The magnitude of these negative effects increased with maternal milk yield during embryogenesis, indicating that the metabolic demands of lactation compete with fetal development for nutrients, potentially preventing offspring from fully expressing their genetic potential [211].

16. Reframing Ketosis: From Pathology to Adaptive Programming

16.1. Historical Context and Conceptual Evolution

The medical and veterinary establishment’s historical perspective on ketosis has been profoundly shaped by observations made primarily within extreme pathological contexts—diabetic ketoacidosis in humans and severe postpartum ketosis in dairy cattle—where massive ketone elevations were associated with life–threatening conditions [9,10]. This pathocentric perspective created a conceptual framework in which any elevation of ketone bodies was viewed as inherently pathological, requiring immediate correction.

This bias was reinforced by three factors: first, limited analytical capabilities during early clinical biochemistry development that could only detect grossly elevated ketones [10,12]; second, the agricultural economics of dairy production where any reduction in milk yield was classified as “loss” rather than potential physiological trade–off [11]; and third, reductionist diagnostic paradigms that evaluated single biomarkers in isolation rather than integrated physiological patterns [25].

The evolution toward a more balanced understanding has been driven by technological advances enabling precise ketone measurement across physiological and pathological ranges, molecular characterization of ketone body signaling functions beyond fuel provision, and longitudinal immunometabolomics investigations revealing coordinated physiological programming underlying ketosis development [5,6,16,17].

16.2. Evolutionary Conservation: Ketosis as Survival Strategy

The reconceptualization of ketosis from pathology to adaptation gains powerful support from cross–species evolutionary evidence. Ketogenic capacity is not a metabolic defect but rather a highly conserved survival mechanism present across vertebrate taxa facing predictable metabolic challenges [1,2,153].

Hibernating mammals demonstrate the most dramatic physiological ketosis, with bears maintaining BHB concentrations of 1.5–4.0 mmol/L throughout months of winter dormancy while preserving muscle mass, immune competence, and reproductive capacity [152,153]. These concentrations would trigger aggressive therapeutic intervention in a dairy cow or hospitalized human, yet represent optimal physiological adaptation in the hibernation context.

Migratory birds upregulate ketogenesis during long–distance flight, using ketone bodies to fuel both sustained muscular work and maintain cognitive function for navigation [154]. Fasting–adapted species across taxa demonstrate that ketosis enables survival during predictable periods of nutrient scarcity, suggesting this metabolic program evolved as a beneficial adaptation rather than a disease state [1,29].

Neonatal mammals, including human infants, exhibit physiological ketosis during the transition from placental to enteral nutrition, with ketone bodies serving as preferential fuel for the developing brain during this critical period [36,201,202]. The conservation of this neonatal ketogenic program across mammalian species underscores its fundamental biological importance.

Lactating mammals face metabolic challenges paralleling other ketosis–inducing states: mobilization of body reserves to meet demands exceeding intake capacity. From an evolutionary perspective, the periparturient ketosis observed in high–producing dairy cows may represent an exaggerated but fundamentally conserved adaptive response—one that has been pushed beyond its evolved operating range by artificial selection for milk production [5,6,205].

16.3. The Paradigm Shift: From Suppression to Support

Recognition that ketosis can serve adaptive functions has profound implications for how we conceptualize metabolic health and disease [29]. The traditional therapeutic goal of “normalizing” ketone concentrations assumes that pre–ketotic metabolism represents the optimal state. However, the evolutionary and cross–species evidence suggests an alternative framing: ketosis represents a distinct metabolic program with its own logic, regulatory mechanisms, and health implications.

This paradigm shift aligns with growing recognition across medicine that many metabolic “diseases” represent adaptive responses to environmental challenges that become pathological only when adaptive capacity is exceeded, regulatory mechanisms fail, or responses persist beyond optimal duration [20,153]. The challenge is not eliminating ketosis but rather supporting successful ketotic adaptation while preventing dysregulated progression.

The distinction between adaptive and pathological ketosis cannot be made based on ketone concentrations alone. Rather, it requires integrated assessment of the metabolic–inflammatory–immune context in which ketosis occurs. This recognition motivates the systems–level framework presented in the following section: the CKIC, which provides a structured approach for distinguishing beneficial adaptation from harmful dysregulation through pattern–based assessment of coordinated physiological signatures.

17. The Calci–Keto–Inflammatory Code: A Systems Integration Framework

The synthesis of evidence presented throughout this review converges on a fundamental reconceptualization: ketosis represents not merely a metabolic disorder caused by energy deficiency, but rather an integral component of a complex biological information processing system. The CKIC is described here as a unifying framework that integrates calcium homeostasis, ketone metabolism, and immune regulation into a coordinated system capable of encoding, transmitting, and decoding physiological information.

The transition from viewing these processes as independent “pathways” to recognizing them as an integrated “code” is conceptually significant. A code requires vocabulary (molecular signals), grammar (temporal dynamics), semantics (biological meaning), and pragmatics (context–dependent interpretation) [48,49]. Mammalian metabolic–immune systems exhibit precisely these properties, with calcium functioning as a master regulator gating cross–talk between subsystems [143,212].

What does it mean to describe the CKIC as a biological “language” or “code”? Unlike simple cause–and–effect pathways where A always leads to B, a true code exhibits linguistic properties that enable context–dependent information processing. We propose that the CKIC operates as a biological language with four essential components: First, vocabulary—the molecular signals themselves (BHB concentrations, calcium flux patterns, cytokine profiles including TNF–α, acute phase proteins such as haptoglobin and SAA, NEFA levels) may function as the “words” of this biological language. Second, grammar—the temporal dynamics and patterns of change (rising vs. falling BHB, rate of calcium flux, trajectory curvature, TNF–α kinetics) appear to constitute the syntactic rules governing how molecular signals combine to create meaning. Third, semantics—identical molecular concentrations convey different biological meanings depending on pattern and context; BHB 1.2 mmol/L with low calcium, controlled haptoglobin/SAA, and stable TNF–α is associated with adaptive ketosis, whereas the same BHB level with elevated calcium, rising acute phase proteins, and increasing TNF–α is associated with impending metabolic decompensation [14,16,24,32]. Fourth, pragmatics—the system interprets signals differently based on physiological context, circadian phase, immune status, and developmental stage, just as human language interprets “bank” differently depending on whether discussing finance or rivers [95,97].

While this linguistic framework is grounded in established molecular mechanisms and extensive observational data, it remains a conceptual model requiring rigorous experimental validation. Prospective validation through information–theoretic analyses (transfer entropy, mutual information quantifying actual information flow between system components) and randomized clinical trials testing code–based diagnostic and therapeutic approaches will be essential to confirm these conceptual insights translate to practical utility. If validated, this code–based framework has the potential to transform our understanding from asking “Is BHB elevated?” to “What pattern do BHB, calcium, and inflammatory markers (haptoglobin, SAA, TNF–α) form, what does this pattern signify about physiological state, and what trajectory is predicted?” This linguistic perspective could enable earlier disease prediction, more precise therapeutic targeting, and management strategies that work with rather than against the biological information processing systems refined through millions of years of mammalian evolution.

17.1. Calcium as a Master Regulatory Node: The Adaptive Hypocalcemia Hypothesis

Central to the CKIC framework is recognition of calcium’s dual function as both metabolic signal and immune regulator. Beyond well–established roles in muscle contraction and bone metabolism, calcium serves as a dynamic immune rheostat [143,145]. Calcium–dependent signaling through store–operated calcium entry (SOCE) channels activates transcription factors including NFAT, NF–κB, and CREB, driving pro–inflammatory cytokine production, with elevated extracellular calcium enhancing immune activation thresholds [147,213,214].

17.1.1. The Adaptive Hypocalcemia Hypothesis

The CKIC proposes—as a testable hypothesis—that certain patterns of periparturient hypocalcemia may represent an adaptive strategy to prevent excessive immune activation during tissue remodeling, microbial exposure, and metabolic challenge [143]. Strategic calcium reduction could dampen immune sensitivity, preventing inflammatory overactivation while maintaining pathogen defense. This hypothesis applies specifically to mild–moderate hypocalcemia without clinical milk fever, not severe hypocalcemia causing paresis or life–threatening complications. Prospective validation is required.

Within the CKIC framework, calcium–inflammation patterns must be interpreted alongside ketone body status: the same calcium trajectory acquires different biological meaning depending on concurrent BHB concentrations and temporal dynamics. For example, BHB concentrations of 1.2–1.4 mmol/L combined with rapid, self–resolving hypocalcemia and controlled inflammatory markers (haptoglobin < 0.5 g/L, SAA < 15 mg/L, TNF–α < 0.3 ng/mL) may represent adaptive metabolic–immune programming, whereas similar calcium patterns with BHB ≥3.0 mmol/L or rapidly rising BHB coupled with elevated inflammatory markers (haptoglobin > 0.5 g/L, SAA > 15 mg/L, TNF–α > 0.3 ng/mL) signal metabolic decompensation requiring intervention [11,13,14,22,23]. To systematically classify these integrated patterns, a formal nomenclature system is proposed, described below.

17.1.2. The Calci–Keto–Inflammatory Pattern Classification

To capture the temporal dynamics and biological integration essential to the CKIC framework, an integrated pattern classification system is proposed. This system recognizes that biological meaning emerges from the coordinated pattern of five components: calcium trajectory, haptoglobin, SAA, TNF–α, BHB dynamics, and temporal trajectories (

Table 1. The Calci–Keto–Inflammatory Code: Integrated Pattern Assessment Framework ¹.

Component	Coordinated (Adaptive)	Discordant (Maladaptive)	Clinical Interpretation
Calcium Trajectory	Phasic (72–96 h recovery)	Sustained (>5 d), Biphasic, Progressive	Primary classification axis
Haptoglobin	<0.5 g/L	>0.5 g/L	Acute phase protein marker [14,24,179]
SAA	<15 mg/L	>15 mg/L	Inflammatory status indicator [179,215]
TNF–α	<0.3 ng/mL	>0.3 ng/mL	Pro–inflammatory cytokine [14]
BHB Status	1.2–1.4 mmol/L	1.4–2.9 mmol/L (moderate); ≥3.0 mmol/L (severe)	Metabolic context [11,13,22,23]
Temporal Pattern	Stable or declining	Rising or accelerating	Trajectory prediction [17,185]

¹ This framework integrates calcium dynamics, inflammatory markers (haptoglobin, SAA, TNF–α), β–hydroxybutyrate (BHB), and temporal trajectories to enable pattern–based prediction of metabolic outcomes. The TNF–α threshold of >0.3 ng/mL was established based on longitudinal data demonstrating cows developing ketosis exhibit TNF–α concentrations of 0.64 ± 0.06 ng/mL at –4 weeks prepartum versus 0.27 ± 0.05 ng/mL in healthy controls (p = 0.03), with elevation persisting at diagnosis (0.49 ± 0.09 ng/mL vs. 0.06 ± 0.03 ng/mL, p < 0.01) [14]. Thresholds represent preliminary guidelines derived from metabolomics investigations [14,17,185] and independent validation datasets [11,13,215,216,217,218,219,220], requiring prospective validation across diverse cohorts and management systems. Pattern integration achieves >90% predictive accuracy for clinical ketosis weeks before disease manifestation [17,185], substantially exceeding threshold–based approaches.

).

Table 1 represents a transitional interpretive framework rather than a definitive diagnostic classification system. While conventional practice relies on single–variable thresholds (BHB > 1.2 mmol/L or BHB > 3.0 mmol/L), Table 1 demonstrates how integrating calcium trajectory, inflammatory markers (haptoglobin, SAA, TNF–α), and temporal dynamics improves biological interpretation. However, this multi–component assessment remains a substantial simplification of the full CKIC complexity and is subject to diagnostic limitations including assay method variability and temporal sampling challenges.

The manuscript’s ultimate vision—multi–platform metabolomics achieving >98% accuracy (AUC 0.98–1.00) in anticipatory prediction 4–8 weeks prepartum [185]—transcends categorical classification entirely, moving toward continuous risk probability assessment that captures the full dimensionality of metabolic–immune network status. Table 1 provides clinically accessible guidance for researchers and practitioners transitioning from threshold–based to pattern–based thinking, acknowledging that comprehensive metabolomic platforms remain research tools requiring validation before widespread implementation. It represents an intermediate step in conceptual evolution:

Reductionist thresholds (current practice) → Integrated pattern recognition (Table 1) → Comprehensive metabolomic prediction (future goal) [185].

Table 1 is presented not as the endpoint but as a bridge—demonstrating to researchers and practitioners why single–variable thresholds fail while pointing toward the superior predictive power of multi–dimensional assessment that our metabolomics work exemplifies [185].

Calcium trajectories are classified as: Phasic (rapid postpartum decline with spontaneous recovery within 72–96 h), Sustained (prolonged hypocalcemia >5 days), Biphasic (initial decline, partial recovery, then secondary decline), or Progressive (continuous worsening).

Inflammatory status determines whether calcium trajectories represent adaptive or maladaptive responses: Coordinated patterns (hypocalcemia with controlled inflammatory markers: haptoglobin < 0.5 g/L, SAA < 15 mg/L, TNF–α < 0.3 ng/mL) typically represent adaptive responses, whereas Discordant patterns (hypocalcemia with elevated haptoglobin > 0.5 g/L, SAA > 15 mg/L, or TNF–α > 0.3 ng/mL) signal maladaptive dysregulation requiring intervention [13,14,215].

The 15 mg/L SAA cutoff was established based on our investigation showing progression from 8.4 mg/L at –8 weeks to 35.8 mg/L at disease diagnosis [14], aligned with independent validation demonstrating diseased cattle exhibit SAA concentrations of 31.2–54.8 mg/L versus 1.5–2.8 mg/L in healthy cows [215]. The TNF–α threshold of >0.3 ng/mL was established based on our longitudinal investigation [14], where cows that developed ketosis exhibited TNF–α concentrations of 0.64 ± 0.06 ng/mL at –4 wks prepartum versus 0.27 ± 0.05 ng/mL in healthy controls (p = 0.03), with this elevation persisting at diagnosis (0.49 ± 0.09 ng/mL vs. 0.06 ± 0.03 ng/mL, p < 0.01). Notably, TNF–α elevation at –4 wks prepartum represents an early predictive indicator, preceding clinical diagnosis by several weeks.

Ketone body status provides the metabolic context that modulates interpretation of calcium–inflammation patterns. The same calcium–inflammation pattern acquires different biological meaning depending on BHB concentrations: BHB concentrations of 1.2–1.4 mmol/L suggest coordinated metabolic adaptation potentially supporting lactogenesis, BHB concentrations of 1.4–2.9 mmol/L indicate concerning metabolic stress, BHB ≥ 3.0 mmol/L signals severe metabolic decompensation, and rising BHB trajectories (regardless of absolute concentration) indicate progression toward dysregulation [11,13,22,23,216].

Temporal dynamics represent the fifth critical dimension: stable or declining biomarker trajectories suggest effective adaptive responses, whereas rising trajectories—particularly concurrent elevation of multiple markers—indicate failing homeostatic mechanisms and impending crisis.

Integrated interpretation: The CKIC employs a five–component assessment (calcium trajectory + haptoglobin + SAA + TNF–α + ketone status + temporal dynamics) that enables predictive pattern recognition. For example, Phasic–Coordinated hypocalcemia with haptoglobin < 0.5 g/L, SAA < 15 mg/L, TNF–α < 0.3 ng/mL, and BHB concentrations of 1.2–1.4 mmol/L represents optimal physiological postpartum adaptation supporting lactogenesis. In contrast, Sustained–Discordant hypocalcemia with haptoglobin > 0.5 g/L, SAA > 15 mg/L, TNF–α > 0.3 ng/mL, and BHB ≥ 3.0 mmol/L with rising trajectories signals severe maladaptation requiring immediate intervention.

This integrated classification system achieves >90% accuracy in predicting clinical ketosis weeks before disease manifestation [17,185], demonstrating the predictive power of pattern–based versus threshold–based assessment. The CKIC framework thus repositions periparturient monitoring from reactive disease treatment to proactive pattern recognition enabling precision interventions that support adaptive physiology while preventing maladaptive progression.

17.1.3. Evidence Supporting Adaptive Hypocalcemia

Recent experimental and observational evidence supports this adaptive hypocalcemia hypothesis through multiple independent lines of investigation within the CKIC framework.

First, temporal studies demonstrate that inflammation precedes and predicts hypocalcemia rather than resulting from it: dyscalcemic cows show elevated acute phase proteins (SAA, haptoglobin) at 2 DIM, before dyscalcemia diagnosis at 4 DIM [217].

Second, different calcium–inflammation patterns display distinct outcomes: Sustained–Discordant hypocalcemia (prolonged low calcium with highest SAA, haptoglobin, and TNF–α) predicts poorest outcomes, while Biphasic–Discordant patterns show elevated TNF–α [218].

Third, Phasic–Coordinated hypocalcemia associates with superior performance: cows with this pattern produce more milk (49.1 vs. 44.6 kg/d in multiparous cows) and experience fewer disease events than cows with normocalcemia, Sustained–Discordant, or Biphasic patterns [218,219].

Fourth, experimental evidence demonstrates that preventing adaptive hypocalcemia during immune activation intensifies inflammation: maintaining eucalcemia during LPS challenge increases LPS–binding protein by 80%, reduces neutrophil counts by 40%, and adversely affects lactation performance [220].

These converging lines of evidence support the hypothesis that Phasic–Coordinated hypocalcemia functions as an endogenous anti–inflammatory adaptation within the KIN, positioning calcium as a regulatory gate modulating immune defense versus inflammatory tissue damage. The integration of calcium patterns with ketone trajectories and inflammatory signatures (haptoglobin, SAA, TNF–α)—rather than any single variable—determines adaptive versus maladaptive outcomes within the CKIC framework.

17.1.4. Therapeutic Implications

If validated through prospective trials, this framework suggests that therapeutic decisions during the transition period may benefit from integrated assessment of calcium dynamics, ketone concentrations, and inflammatory status (haptoglobin, SAA, TNF–α) rather than treating each variable independently. The CKIC framework proposes that biological meaning emerges from the pattern of these five components.

For example, BHB concentrations of 1.2–1.4 mmol/L combined with Phasic–Coordinated hypocalcemia and controlled inflammatory markers (haptoglobin < 0.5 g/L, SAA < 15 mg/L, TNF–α < 0.3 ng/mL) may represent adaptive metabolic programming supporting lactogenesis, whereas similar BHB concentrations with Sustained–Discordant hypocalcemia (elevated haptoglobin > 0.5 g/L, SAA > 15 mg/L, and TNF–α > 0.3 ng/mL with rising acute phase proteins) signal maladaptation requiring intervention.

Current practice reflexively administers calcium to hypocalcemic animals and treats elevated BHB with glucogenic precursors, without considering the integrated metabolic–inflammatory context; the experimental and observational evidence suggests that interventions may require careful consideration of the complete calcium–ketone–inflammation pattern rather than isolated threshold–based responses. This applies specifically to management of subclinical conditions, not treatment of clinical milk fever or severe ketosis requiring immediate intervention.

17.2. Code–Based Interpretation: Pattern over Threshold

The CKIC operates through hierarchical information processing where biological meaning emerges from temporal patterns rather than static concentrations [25]. Rates of change, acceleration, and trajectory curvature distinguish transient perturbations from sustained threats. The same biochemical pattern generates different outcomes depending on context: BHB 1.2 mmol/L with low calcium, controlled haptoglobin/SAA, and stable TNF–α may represent adaptive ketosis, whereas identical BHB with elevated calcium, rising acute phase proteins, and increasing TNF–α signals pathological progression.

Multi–platform metabolomics integrating amino acids, acylcarnitines, phosphatidylcholines, and inflammatory markers achieves >98% accuracy (AUC 0.98–1.00) in predicting clinical ketosis at 4–8 wks prepartum, substantially exceeding threshold–based screening [17,185]. This predictive power derives from recognizing that current biochemical patterns encode information about future trajectories—a fundamental property of biological codes where molecular signatures function as predictive signals rather than merely diagnostic markers.

17.3. Implications and Future Directions

The CKIC framework’s primary clinical value lies in prevention through early pattern recognition rather than treatment of established disease. Multi–platform immunometabolomics integrating calcium, ketone, and inflammatory signatures (haptoglobin, SAA, TNF–α) achieves >98% accuracy (AUC 0.98–1.00) in predicting clinical ketosis at 4–8 wks prepartum [17,185]—substantially earlier than threshold–based screening and during the critical window when adaptive mechanisms remain modifiable. This predictive power enables preventive intervention before pathological cascades become irreversible, fundamentally shifting from reactive disease management to proactive health maintenance.

Pattern–based risk stratification enables precision prevention by distinguishing animals requiring different interventions: those with elevated BHB and controlled inflammation (low haptoglobin, SAA, TNF–α) may benefit from metabolic support; those with elevated BHB and rising inflammatory markers (SAA, haptoglobin, TNF–α) require anti–inflammatory intervention; those with moderate BHB, short temporary hypocalcemia, and controlled inflammation may be experiencing adaptive responses requiring monitoring rather than treatment. This stratification prevents both under–treatment of animals progressing toward disease and over–treatment of animals undergoing physiological adaptation.

Mechanistic understanding that inflammation often precedes and drives metabolic dysfunction [14,16,219,220] redirects preventive strategies from exclusive focus on energy supplementation toward integrated approaches addressing immune dysregulation, barrier function, and oxidative stress during the dry period. Preventing inflammatory activation prevents the cascade of metabolic consequences rather than managing established pathology.

Beyond transition dairy cattle, the CKIC framework applies to any mammalian system navigating metabolic–immune stress—human metabolic syndrome, perioperative critical illness, fasting adaptation, and lactation across species. The conservation of calcium signaling, ketone metabolism, and innate immunity across taxa suggests this represents a fundamental biological organizing principle refined through vertebrate evolution.

This framework exemplifies a paradigm shift from reductionist approaches treating individual abnormal values toward systems–level prevention through integrated pattern recognition. Health represents maintenance of code integrity—the capacity to accurately encode, transmit, and decode physiological signals. Disease represents information processing failure—errors preventing appropriate adaptive responses. The transition from threshold–based reaction to pattern–based prevention represents the practical implementation of this conceptual paradigm shift.

The present review establishes the conceptual foundation and synthesizes evidence supporting this integrative framework. Rigorous prospective validation through randomized trials comparing pattern–based preventive interventions to conventional threshold–driven management will determine whether this paradigm delivers its theoretical promise: preventing metabolic disease by working with, rather than against, evolved adaptive mechanisms.