Nutritional Characteristics, Health Impact, and Applications of Kefir

A global epidemiological shift has been observed in recent decades, characterized by an increase in age-related disorders, notably non-communicable chronic diseases, such as type 2 diabetes mellitus, cardiovascular and neurodegenerative diseases, and cancer. An appreciable causal link between changes in the gut microbiota and the onset of these maladies has been recognized, offering an avenue for effective management. Kefir, a probiotic-enriched fermented food, has gained significance in this setting due to its promising resource for the development of functional or value-added food formulations and its ability to reshape gut microbial composition. This has led to increasing commercial interest worldwide as it presents a natural beverage replete with health-promoting microbes and several bioactive compounds. Given the substantial role of the gut microbiota in human health and the etiology of several diseases, we conducted a comprehensive synthesis covering a total of 33 investigations involving experimental animal models, aimed to elucidate the regulatory influence of bioactive compounds present in kefir on gut microbiota and their potential in promoting optimal health. This review underscores the outstanding nutritional properties of kefir as a central repository of bioactive compounds encompassing micronutrients and amino acids and delineates their regulatory effects at deficient, adequate, and supra-nutritional intakes on the gut microbiota and their broader physiological consequences. Furthermore, an investigation of putative mechanisms that govern the regulatory effects of kefir on the gut microbiota and its connections with various human diseases was discussed, along with potential applications in the food industry.


Introduction
The global population of individuals aged 65 and older is anticipated to increase significantly, from 10% in 2022 to 16% by 2050, reflecting an ongoing and rapid global aging phenomenon [1].This accelerated trend in life expectancy presents substantial challenges to both public health and socioeconomic systems.As individuals age, they become more susceptible to non-communicable diseases such as type 2 diabetes (T2D), cardiovascular illnesses, cancer, and neurodegenerative conditions [2], necessitating continuous monitoring and medical intervention.
Several interventions including dietary changes, physical activity, metformin, nicotinamide adenine dinucleotide (NAD + ) precursors, sirtuin agonists, senescence-associated secretory phenotype (SASP) inhibitors, and senolytics are currently being developed to target underlying aging mechanisms with the potential to impact multiple fundamental aging pathways [3].These all have a significant impact on the makeup of the gut microbiome, as do the changes in lifestyle choices that follow, such as a reduction in the quality of food and physical activity and an increase in drug use, which have been linked to the onset of age-related disorders [4][5][6].Thus, one of the most important treatment approaches might be to alter the microbiome through food to slow down the physiological deterioration that comes with age [7].The consumption of dietary components may either feed or restrict the growth of specific members of the gut microbial population, making nutrition a significant regulator of the microbiome [8].
Kefir is a fermented milk beverage produced by the synergistic action of bacteria and yeasts contained in kefir grains that have an inert matrix of polysaccharides and proteins and are home to a diverse range of microorganisms, including lactic acid bacteria (LAB), acetic acid bacteria, and yeasts [9,10].The intricate interactions of these multiple microbes, as well as several bioactive substances produced by their metabolic processes, contribute to kefir's reputation as a natural probiotic [9,[11][12][13][14].The microbial composition of kefir can vary based on factors like geographical origin, fermentation duration, substrate, and processing methods, yet kefir grains consistently maintain a relatively stable and distinctive microbiota, often characterized by the prevalence of specific Lactobacillus species [15].
The interplay between diet and gut microbiota has a major impact on species abundance, diversity, and overall influence on human health [16].As a result, an in-depth investigation of the impacts of kefir's bioactive compounds on the gut microbiota is required, which will contribute to a better understanding of the underlying processes driving kefir's role in human health.In this perspective, we discussed 33 studies from 2012 to 2024 that investigate the impact of kefir's bioactive compounds on the gut microbiota and metabolic physiology under deficient and supplemented conditions and, in addition, extended this discussion to age-related diseases.

Gut Microbiota Changes with Age
Microorganisms that make up the intestinal microbiota may exist in two states: balanced or unbalanced.The microbiota in the first situation, known as eubiosis, is flexible enough to maintain its equilibrium by tolerating minor changes in the environment, the diet, or the water drunk.However, instances of significant changes, such as the translocation or expansion of a particular bacterial group, colonization by pathogenic bacteria, the use of antibiotics, and alterations in lifestyle, result in imbalance or dysbiosis [17,18].Since the microbiota is primarily governed by physiology, the relative abundance of some microbes is affected by age-related changes in intestinal physiology with respect to the host's diet, lifestyle, and medications [19,20].
Mucin, for example, functions as a protective barrier in the gastrointestinal system, preventing direct contact between microbes and epithelial cells.Nonetheless, in mice, mucin production declines with age, resulting in a thinner and less uniform mucus coating.Different microbial strains, such as those in the Clostridiaceae, Akkermansiaceae, Bifidobacteriaceae, and Bacteroidaceae families, all display age-related alterations and use mucin as a nutrient [21].Although the role of mucin-metabolizing microbes like Akkermansia muciniphila, Bacteriodes fragilis, Bacteroides vulgatus, Bifidobacterium spp., and Prevotella spp. in mucin layer degradation with age remains unclear, Bacteroides spp.and A. muciniphila increase in centenarians, hence implying potential benefits [22].As a result, probiotic-enriched foods or supplementation with certain strains may improve age-related mucin loss, as well as good benefits on health, immunity, and lifespan [23].Increased amounts of short-chain fatty acids (SCFAs) produced as byproducts by the gut microbiota during the fermentation of partly and non-digestible polysaccharides resulted in a decrease in colon pH, which contributed to the enhancement of the intestinal barrier [24,25].
However, as the protective mucin layer in the digestive system deteriorates, microbes that typically would not contact the epithelial layer can now potentially trigger inflammatory responses [6,26].Consequently, aging, as well as the chronic health and metabolic disorders that come with it, is marked by a rise in low-grade, chronic inflammation [6].Therefore, enhancing the preservation of intestinal physiology in model organisms can mitigate age-related alterations, ultimately minimizing microbial dysbiosis and extending lifespan [27].
The intestinal microbiota has an impact on the operation of several organs, including the heart, brain, liver, pancreas, and gut, and its regulation may be a crucial step in the treatment of illnesses and the preservation of health since the intestinal microbiota also plays a role in the development and maturation of organs and physiological systems [28,29].Increased intestinal permeability relates to an imbalance between Firmicutes and Bacteroidetes, which permits bacterial byproducts to penetrate and induce inflammatory reactions associated with diabetes and other metabolic disease [30].LPS from Gram-negative bacteria induces insulin resistance and impairs insulin signaling in a variety of organs, including muscle, adipose tissue, liver, and the hypothalamus, via activation of NF-kB and JNK pathways.Additionally, LPS activates toll-like receptors, triggering the production of inflammatory cytokines, thereby activating the immune system [31,32].

Occurrence
Kefir is a fermented beverage made from kefir grains and milk or water that is acidic, frothy, and low in alcohol [33,34].Its origins may be found in the Caucasus, Eastern Europe, and the Balkans, and due to its beneficial health effects, its consumption has spread worldwide throughout time [35].People in nations like the United States of America, Japan, France, and Brazil have become accustomed to drinking this sour, viscous beverage [36].The distinctive quality of its starter, the kefir grains, is how kefir differs from other fermented foods.
Kefir grains are irregularly shaped and lobed, ranging from white to light yellow [37].They are 1 to 4 cm long and resemble miniature cauliflower florets.Lactic acid bacteria (LAB), yeasts, and acetic acid bacteria (AAB) coexist in symbiotic association inside the natural matrix of exopolysaccharides (EPS), kefiran, and proteins that make up this gelatinous and slimy structure [34].The use of kefir beverages has been linked to significant health advantages, including improved lactose digestion, anti-carcinogenic, anti-hypertensive, and anti-diabetic effects [38][39][40].The kefir bacteria, their interactions, and their metabolic products throughout the fermentation process are responsible for all these health-promoting qualities [34].

Microbial Diversity
Lactic acid bacteria, acetic bacteria, yeasts, and fungi are the most prevalent microbial species in kefir grains, among other complicated microbial species [41], and have been divided into four groups: homo-and heterofermentative as well as lactose-and nonlactose-assimilating yeasts [42].Lactobacillus kefiranofaciens, Lacticaseibacillus paracasei (also known as Lactobacillus paracasei), Lactiplantibacillus plantarum (also known as Lactobacillus plantarum), Lactobacillus acidophilus, and Lactobacillus delbrueckii subsp.bulgaricus are the most prevalent bacterial species found in kefir grains, while the main yeast species found in kefir are Saccharomyces cerevisiae, S. unisporus, Candida kefyr, and Kluyveromyces marxianus subsp.marxianus [35].
According to the geographic origin of the kefir grains, which is inextricably linked to the climatic conditions, the microbiota of the kefir grains may vary [43].In actuality, the ratio of kefir grains to substrate, fermentation period, temperature, and degree of agitation are all factors that might affect the microbiota composition and nutritional content of kefir [44,45].Although some significant Lactobacillus species always occur due to their probiotic strainspecific capabilities, it is acknowledged that this microbial diversity is responsible for the physicochemical characteristics and biological activities of each kefir [44,46].
Many of these compounds have different absorption rates in the brain and may enter as rapidly as glucose to affect metabolic processes.For example, since essential amino acids cannot be synthesized by the brain, they must be supplied from protein breakdown and diet.Hence, these components are essential for controlling energy balance, serving as precursors to neurotransmitters synthesized in the brain, supporting metabolic activities, enhancing immunomodulation, and fostering homeostasis [53].Kefiran, a microbial polysaccharide from kefir grains, aids in the mental recovery of individuals with severe traumatic brain injuries and lengthens the healthy lifetime of the elderly [54,55].Using database mining approaches, we revealed that kefiran binds to different protein targets that may partake in several molecular events (Figure 1).Hence, kefir is suggested to exert control of organism homeostasis through a direct impact on the gut-brain axis [17].roles of Kefiran on different disease phenotypes using network analysis.Kefiran-protein interaction generated from BindingDB [56] and gene-disease association curated from DisGeNet [57].Integrated biomolecular interaction was created using Cytoscape [58] and STRING.db[59].Target genes are listed as follows: Abl1, non-receptor tyrosine kinase; ca2, carbonic anhydrase II; cdk2, cyclin-dependent kinase 2; cdk1, cyclin-dependent kinase; fgf1, fibroblast growth factor 1; fgf2, fibroblast growth factor 2; hdac, histone deacetylase; atp1b1, ATPase Na + /K + transporting subunit beta 1; vegfc, vascular endothelial growth factor C.
Numerous studies have been conducted to investigate the modulatory effects of these bioactive substances on the gut microbiota in experimental animal models, as well as the resulting metabolic implications.Table 1 summarizes pertinent information about the experimental models, designs used, and results obtained from the research.T2D is a complicated chronic disorder that puts patients at risk for long-term macroand microvascular consequences due to deficiencies in insulin secretion, glucose metabolism, or both [43].Chronic low-grade inflammation is correlated with the onset of T2D.An unbalanced intestinal microbiota, which is promoted by changes in intestinal permeability caused by LPS-induced endotoxemia, favors the development of this inflammation, resulting in systemic insulin resistance and the subsequent onset of metabolic syndrome and T2D [17].
A study using high-fat diet-fed mice found that Lactobacillus mali APS1 produced from kefir grains lowered blood glucose and HOMA index while increasing glucagonlike peptide (GLP-1) and butyrate levels [90,91].Lowering the HOMA index indicates better glycemic control while increasing GLP-1 implies appetite modulation and possibly protection for insulin-producing pancreatic beta cells, which are essential for controlling blood sugar levels [92].Another study on Wistar rats with monosodium glutamate-induced metabolic syndrome revealed that whole milk kefir consumption for 10 weeks reduced insulin resistance.This improvement was linked to the calcium content in kefir and the bioactive compounds generated during fermentation.Additionally, kefir enhanced glucose uptake by muscle cells, further reducing insulin resistance [93].
Patients with type 2 diabetes have a microbiota that is characterized by a reduction in butyrate-producing bacteria, a moderate dysbiosis, an environment that is proinflammatory, a decrease in the expression of genes involved in vitamin synthesis, an increase in serum LPS levels, and an increased intestinal permeability [94].However, the gut microbiota contributes to energy generation via anaerobic digestion of food components, which results in SCFA such as acetate, propionate, and butyrate.Notably, butyrate, a major source of energy for colonocytes and the main byproduct of SCFA fermentation, is known to regulate body weight and improve insulin sensitivity by boosting GLP-1 secretion via GPR-mediated signaling and decreasing adipocyte inflammation [90].

Cardiovascular Health
Dyslipidemia, a known risk factor for cardiovascular disease, causes decreased diversity in the gut microbiota, rendering people more vulnerable to dysbiosis.This increased susceptibility causes inflammation and alterations in intestinal permeability, resulting in negative health effects for the host.SCFAs contribute to energy generation, lipogenesis, gluconeogenesis, and cholesterol synthesis, in addition to primary bile acids capable of binding to the farnesoid X receptor, which is an important protein in the etiology of obesity [101,102].However, kefir blocked intestinal lipid uptake in obese mice through the reduction in hepatic and serum triglycerides, total cholesterol, and LDL-c as well as reduced expression of genes linked to adipogenesis, lipogenesis, and proinflammatory cytokines in epididymal fat [103].
Despite the little exploration of its biological profile, several biologically active peptides are produced by the symbiotic metabolic interactions between different bacterial and yeast species in kefir, including ACE-inhibitory peptides that block the angiotensinconverting enzyme (ACE), preventing the conversion of angiotensin I to the vasoconstrictor angiotensin II [104].This inhibits the production of aldosterone, which typically increases serum sodium levels and elevates blood pressure, while also impacting bradykinin, a vasodilatory hormone, resulting in decreased blood pressure [50,93].
In addition, the discovery of 16 peptides released from caseins, including two (sequences PYVRYL and LVYPFTGPIPN) exhibiting strong ACE-inhibitory capabilities, shows that commercial kefir made from caprine milk demonstrates considerable angiotensinconverting enzyme (ACE)-inhibitory activity [105].These findings imply that kefir may harbor a wide variety of bioactive chemicals with the ability to work independently or synergistically, making it a suitable therapeutic option or adjunct in the treatment of hypertension.
Furthermore, a soluble non-bacterial fraction of kefir reduced cardiac hypertrophy in spontaneously hypertensive rats (SHRs), possibly through ACE inhibition and lowering the TNF-α-to-IL10 ratio and improved baroreflex sensitivity in hypertensive rats.It also decreased mean arterial pressure (MAP) and heart rate (HR) [39].Kefir was also shown to increase baroreflex sensitivity (BRS) in SHR and to reduce cardiac autonomic control impairment when taken regularly in modest doses [106].
In another separate study, a 60-day kefir treatment improved endothelial function in SHR, which was attributed to the partial restoration of the ROS/NO balance and the recruitment of endothelial progenitor cells, both of which contributed to the improvement of the endothelial architecture [107].Also, administration of pitched (traditional) kefir (350 g of kefir/day) for 4 weeks in adult males reduced high LDL-c, ICAM-1, VCAM-1, IL-8, TNF-α, and CRP, indicating the metabolic impact of kefir intake [108].
As shown in Figure 2 [109], LDL oxidation is regarded as the primary cause of atherosclerotic plaque development and a substantial contributor to proinflammatory responses in the subendothelial area [110].The involvement of HDL in lipid distribution, enabling the absorption and movement of cholesterol deposited in atherosclerotic plaque foam cells back to the liver and bile (cholesterol reverse transport), highlights the antiatherogenic and anti-inflammatory effects of HDL [111,112], NO-promoting effects, and inhibition of TNF-α-induced endothelial cell apoptosis [113,114].HDL inhibits the cytotoxic impact of oxidized LDL on the vascular endothelium that triggers the atherogenesis process, reducing inflammation by inhibiting the expression of adhesion molecules on the endothelium surface, such as P-selectin, E-selectin, ICAM-1, and VCAM-1 leading to reduced T-cell and monocyte adherence to the vascular endothelium, hence restricting their movement to atheromatous focus.Pro-inflammatory processes are initiated when soluble forms of endothelial adhesion molecules, sP-selectin, sEselectin, sICAM-1, and sVCAM-1 are released into the bloodstream from the surface of active endotheliocytes [115].The significant reduction in lipid profile parameters including LDL-c, cell adhesion molecules ICAM-1 and VCAM-1, as well as proinflammatory cytokines such as IL-8, TNF-α, and IL-17a after kefir intake reveals its atheroprotective effect, protecting against cardiovascular disease.

Cognitive Function and Alzheimer's Disease
A wealth of data shows that diverse stimuli affect the intestinal mucosa, influencing gut integrity via the hypothalamic-pituitary-adrenal axis [116,117].This link extends to neurodegenerative conditions and gut dysbiosis, with gut microbiota-derived molecules impacting the blood-brain barrier and brain function [118].The development of Alzheimer's disease is linked to an imbalance in the microbiota-gut-brain axis, which may be aggravated by a lack of probiotic bacteria owing to an imbalanced diet [119].Microglial maturation and function, blood-brain barrier construction and stability, myelination, and neurogenesis, among other neurodevelopmental processes, have all been discovered to be influenced by gut microbiota and contribute to neurological health [17,120].HDL inhibits the cytotoxic impact of oxidized LDL on the vascular endothelium that triggers the atherogenesis process, reducing inflammation by inhibiting the expression of adhesion molecules on the endothelium surface, such as P-selectin, E-selectin, ICAM-1, and VCAM-1 leading to reduced T-cell and monocyte adherence to the vascular endothelium, hence restricting their movement to atheromatous focus.Pro-inflammatory processes are initiated when soluble forms of endothelial adhesion molecules, sP-selectin, sE-selectin, sICAM-1, and sVCAM-1 are released into the bloodstream from the surface of active endotheliocytes [115].The significant reduction in lipid profile parameters including LDLc, cell adhesion molecules ICAM-1 and VCAM-1, as well as proinflammatory cytokines such as IL-8, TNF-α, and IL-17a after kefir intake reveals its atheroprotective effect, protecting against cardiovascular disease.

Cognitive Function and Alzheimer's Disease
A wealth of data shows that diverse stimuli affect the intestinal mucosa, influencing gut integrity via the hypothalamic-pituitary-adrenal axis [116,117].This link extends to neurodegenerative conditions and gut dysbiosis, with gut microbiota-derived molecules impacting the blood-brain barrier and brain function [118].The development of Alzheimer's disease is linked to an imbalance in the microbiota-gut-brain axis, which may be aggravated by a lack of probiotic bacteria owing to an imbalanced diet [119].Microglial maturation and function, blood-brain barrier construction and stability, myelination, and neurogenesis, among other neurodevelopmental processes, have all been discovered to be influenced by gut microbiota and contribute to neurological health [17,120].
In an experimental study involving an animal model exposed to nicotine-induced stress, Noori et al. [121] investigated the potential therapeutic effects of fermented kefir made from both soy and cow's milk on depression, anxiety, and cognitive impairment.A battery of tests, including the elevated plus maze (EPM) for anxiety assessment, the open field test (OFT) to evaluate locomotor activity and anxiety, and the forced swim test (FST) for measuring depression severity, was employed.Throughout the treatment period, a significant improvement in anxiety reduction, depression severity, and cognitive performance was observed and is thought to be related to kefir's high tryptophan content, which is a precursor to serotonin, a neuromodulator that plays an important role in fostering neuroplasticity and neuronal growth associated with depression [122].
Similarly, in an animal model mirroring human depression induced by six weeks of exposure to seven stressors, mice supplemented with Lactobacillus kefiranofaciens ZW3, sourced from kefir, displayed increased activity, a greater preference for sucrose, and reduced risk of constipation, a condition associated with depression.This supplementation also led to improved tryptophan metabolism, elevated anti-inflammatory cytokines, decreased proinflammatory cytokines, and notable changes in the gut microbiota composition, including increases in Actinobacteria, Bacteroides, Lachnospiraceae, Coriobacteriaceae, Bifidobacteriaceae, and Akkermansia and decrease in Proteobacteria [123].
Oxidative stress is important to the pathophysiology of Alzheimer's disease (AD), impacting the brain more deeply due to mitochondrial malfunction, increased metal levels, inflammation, and β-amyloid peptides [131].An in vivo study utilizing aged mice reveals that probiotics fermentation technology (PFT) containing specific microbes namely Lactobacillus kefiri P-IF, L. kefiri P-B1, Kazachstania turicensis, Kazachstania unispora, and Kluyveromyces marxianus reduces age-related oxidative stress.A six-week oral daily dosage of 2 mg/kg body weight PFT suppresses oxygen radical generation, enhances GSH and total antioxidant capacity, and reduces NO and MDA levels, restoring age-related oxidative alterations to levels comparable to those seen in young, untreated mice [132].
Furthermore, in a study using a fly model of AD, supplementing their diet with kefir made from grains containing Lactobacillus species (L.kefiranofaciens, L. kefiri, Acetobacter fabarum, L. lactis, and Rickettsiales) resulted in significant improvements.This included a 1.6-fold increase in survival rates, a 2-fold enhancement in climbing ability, and reduced severity of brain vacuolar lesions, all indicating a positive impact on the neurodegenerative phenotype of the flies [133].Kefir-fed animals had a unique gut microbiota composition, which is thought to have a favorable effect on the gut-brain axis.Microbiome analysis suggests that kefir may boost the synthesis of gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter.This action is thought to be mediated by the conversion of 2-oxoglutarate to glutamate, which may be aided by Lactobacillus reuteri.The manipulation of the intestinal microbiota by kefir results in an increased presence of Lactobacillus reuteri, a bacterial species renowned for its positive effects on the host's immunological and metabolic systems [134].
In humans, kefir was shown to improve the symptoms of AD in human studies.Kefir consumption by patients with probable AD for 90 days showed substantial cognitive improvements, including higher memory test scores, enhanced visual-spatial and abstraction abilities, better executive, and language functions, and improved Mini-Mental State Examination (MMSE) scores, which were accompanied by reduced serum TNF-α, IL-12p70, IL-8, enhanced mitochondrial membrane potential, and reduced intracellular ROS in erythrocytes [135].

Cancer
Since cancer cells proliferate rapidly and are resistant to apoptosis, with a strong relationship to poor dietary habits, it is critical to investigate dietary components, particularly probiotics such as kefir as a possible therapeutic option for mitigating cancer cell growth [136].During neoplastic growth, tumor cells evade the immune response, and probiotic bacteria have been found to promote immune system effector activities in co-cultures with peripheral blood mononuclear cells (PBMCs), as evidenced by cytokine profiles [137,138].
In a study to examine the effects of kefir on colorectal cancer (CRC) via gut microbiota composition regulation using internally transcribed spacer 2 (ITS2) and 16S rRNA highthroughput sequencing on azoxymethane/dextran sulfate sodium (AOM/DSS)-induced CRC mouse model, kefir supplementation decreased the ratios of Firmicutes/Bacteriodetes and Ascomycota/Basidiomycota ratio, as well as the relative abundance of pathogenic bacteria Clostridium sensu stricto, Aspergillus and Talaromyces.This was accompanied by a decrease in oncocyte proliferation indicator (Ki67, NF-κB, and β-catenin) and immunity regulators (TNF-α, IL-6, and IL-17a) while the relative abundance of probiotics increased [24].
Further study showed that administration of Lactobacillus kefiranofaciens JKSP109 (LK) and Saccharomyces cerevisiae JKSP39 (SC) from Tibetan kefir grain and their combination on an AOM/DSS-induced mouse model of CRC led to an increased expression of TUNELpositive tumor epithelial cells and the content of short-chain fatty acids in fecal samples, as well as increase in body weights while disease activity index, tumor multiplicity, and proinflammatory cytokines were reduced [150].Likewise, several similar studies have reported the anticancer properties of kefir including its anti-tumor effect via the promotion of tumor immunotherapy by modulating the gut microbiota composition [151], regulation of intestinal inflammation, and subsequent reduction of DMH-induced CRC in Wistar rat's offspring programmed for adulthood through neonatal overfeeding [152].

Applications in Food Product Development
Kefir, known for its diverse beneficial microorganisms and bioactive compounds, offers a range of health benefits in dairy products, although milk-related hypersensitivities pose challenges.In response, fermented non-dairy beverages are gaining popularity, prompting scientific exploration of kefir adaptation onto non-dairy substrates like fruits, vegetables, and molasses for diverse fermentation bioprocesses [157].
A recent study examined how the amount of kefir grains and fermentation time affected the composition, sensory aspects, and color of a probiotic beverage.The study found substantial impacts, such as a drop in sugar content, and an increase in acidity, total phenols, carbon dioxide, and organic acids.The study also discovered that fermentation altered sensory qualities such as color, brightness, chroma values, density, antioxidant activity, citric acid, and hue values, with differences related to the quantity of kefir grains inoculum and fermentation period [158].
Natural materials are increasingly being used in the food and packaging industries to provide ecologically friendly and biodegradable packaging solutions [159].A recent study investigated the effects of kefir on the quality of gelatine-based edible films.The findings revealed that, while the thickness of the films remained constant, density rose and hydrophilic characteristics improved.Although kefir improved surface morphology, excessive application resulted in hazy formations and a minor loss in mechanical characteristics.The inclusion of kefir enhanced the greenness, yellowness, and opacity of the films.This suggests that kefir could be a more environmentally sustainable alternative to petrochemical packaging, as it showed no growth of harmful microorganisms over a 10-day period, highlighting potential benefits for both environmental sustainability and human health [160].
Besides food product development, a recent study examined the potential of kefiran, a biopolymer, for tissue engineering and regenerative medicine applications [161].Extracted kefiran was used to produce scaffolds by cryogelation and freeze-drying, with molecular structures validated by proton NMR and FTIR spectra.The results indicated a high molecular weight, acceptable rheological properties, and scaffold traits like porosity and wall thickness.The kefiran extracts and scaffolds had no cytotoxic impact on L929 cells and showed no significant differences in cytocompatibility, which altogether positions them as potential options in tissue engineering and regenerative medicine.
Additionally, while traditional kefir made from natural grain-based kefir is reported to elicit health-benefiting properties, commercial kefirs made of defined mixtures of microorganisms are emerging with different functional effects.Due to the complexities of using grains and the resulting limitations on product shelf-life, modern commercial kefirs employ artificial microbial blends.Commercial kefir production now involves various starter culture producers and dairy product manufacturers.Research indicates that commercial kefirs differ significantly from traditional grain-based kefir in microbial composition and metabolite characteristics, which therefore necessitates further investigation on their potential in the food industry and functional effects on consumers [157,162].

Perspectives
The manipulation of the intestinal microbiota has emerged as a promising strategy for both disease prevention and therapeutic intervention, and the utilization of fermented foods possessing probiotic properties offers a nutritional approach as an alternative to synthesized drugs.Kefir, distinguished by its established safety profile in both animal and human, cost-effectiveness, ease of preparation, and microbiological composition enriched with bioactive compounds, metabolites, and peptides, stands out as a potential functional food with substantial health benefits.
Kefir contains bioactive compounds and unique peptides; it harbors specific bacterial strains known to orchestrate shifts in the composition of gut microbiota, alleviate low-grade inflammation, and promote optimal health.These result in a wide range of health advantages that may lessen the growing prevalence of these age-related disorders.Additionally, the modern food market is seeing the rise of novel functional foods, fueled by the remarkable therapeutic capabilities of kefir grains.This has resulted in a greater demand for healthier and more sustainable food items infused with kefir and its value-added derivatives.Furthermore, specialists in nutrition and food science are expressing a strong interest in extending research disciplines to explore the therapeutic qualities of kefir.
However, further research is necessary to exploit the advantages that highlight kefir's potential in healthy aging, specifically to (1) comprehend the precise microorganisms orchestrating its beneficial effects, their complex molecular interactions with other bioactive compounds within the gut (whether as synbiotics or postbiotics), and how they impact the gut-brain axis; and (2) perform controlled human intervention trials that might enable a more robust approach to elucidating the specific functional mechanisms by which kefir exerts its biological benefits, particularly in the context of promoting healthy aging.

Table 1 .
Roles of kefir bioactive constituents on gut microbiota and physiological parameters in both health and disease conditions.

Table 2 .
Select randomized controlled trials on the physiological roles of kefir.