Crime and Nourishment: A Narrative Review Examining Ultra-Processed Foods, Brain, and Behavior

Abstract

Recently, there has been increased scientific and clinical interest in the potential harms associated with ultra-processed foods, including poor mental health, aggression, and antisocial behavior. Research spanning epidemiology, mechanistic pre-clinical work, addiction science, microbiome and exposome science, and human intervention trials has underscored that nutrition is of relevance along the criminal justice continuum. As such, the emerging dietetics research is salient to the thousands of international psychologists and allied mental health professionals that are engaged in justice work, including forensics, prevention, and intervention. In addition, relationships between nutrition and behavior relate to “food crime”, an emergent area unifying criminal justice researchers with psychology, public health, and other interdisciplinary sectors. Food crime scrutinizes the vast harms, including non-communicable diseases and adverse behavioral outcomes, as influenced by the distribution of addictive ultra-processed food products. Here, we examine the emergent research, including biophysiological mechanisms, and evidence indicating that dietary patterns/components intersect with psychosocial vulnerabilities linked with risks of antisocial behavior and justice involvement. Viewed through a prevention lens, the study of nutrition and aggressive behavior should be prioritized, especially if the outcomes emerge as externalities of the global consumption of ultra-processed food. In the context of criminal justice and behavior, there is a need for forensic examination of how industry influence and power structures can undermine matters of food justice.

1. Introduction

“The undernourishment and malnourishment of convicts is, in fact, one of the worst crimes…criminals might be generated by prison food alone…yet, knowing as we do that it [undernourishment] is perpetrated upon the human being in our prisons, we sit supine and acquiescent, and thereby make the crime our own”.

Julian Hawthorne, Banquet of the Damned, 1914

The early twentieth century perspectives of investigative journalist and author Julian Hawthorne, extracted above from the ‘Banquet of the Damned’ chapter of his book The Subterranean Brotherhood [1], were part of a growing social concern that nutrition and crime were somehow related. Mostly, this concern was directed at the overt malnourishment associated with poverty and socioeconomic disadvantage, especially in children [2]. Some scholars argued that, due to the importance of quality nutrition on the developing brain, inadequate nutrition could be a causative factor in the development of antisocial behavior and subsequent delinquency. In his syndicated newspaper column, science journalist Henry Addington Bruce noted in 1925 that “the malnourished child is likely to be afflicted with nervous disorders both in childhood and in later life…there is reason, too, for affirming a frequent connection between malnutrition and conduct disorders, ranging from minor delinquencies to actual criminality” [3]. By the 1950s, the first controlled studies using multivitamin–mineral formulas indicated that nutrients are important to mental wellbeing in adults, even in the absence of overt malnutrition [4,5]. Nevertheless, until recently, the role of dietary patterns and nutrition as they relate to behavior in general, and the criminal justice system in particular, remained at the periphery of mainstream psychiatry and psychology [6,7].

Here, in our narrative review, we examine the historical and emerging research connecting ultra-processed foods with facets of neuropsychiatry and behavior, with a special emphasis on antisocial behavior—the basis of nutritional criminology [8] (Figure 1). Our method included the use of traditional research databases such as PubMed, PsycINFO, CINAHL, and Google Scholar, supplemented with Google Books and newspapers.com; the latter two databases aided in the identification and analysis of research (and more specifically, the historical deliberation of research) that may otherwise escape notice. Included in this analysis is a synthesis of nutritional epidemiology, mechanistic bench discoveries, ultra-processed/hyperpalatable food addiction science, microbiome and exposome science, and human intervention trials. The emergent research is contextualized through the lens of the criminal justice continuum, from early-life prevention through potential intervention and treatment in carceral systems. Importantly, the racial implications of the industry-designed proliferation of ultra-processed food products in marginalized communities, including Black and Indigenous communities [9], provides a contextual frame to the discussions of food and behavior.

At the outset, we underscore that the antecedents of aggression, violence, and crime in general, are complex and multifactorial. It is not our position that dietary patterns and/or nutritional factors are, in general, primary explanations for antisocial behavior. However, as we argue below, contemporary research in the areas of nutrition and neuropsychiatry, as well as public health epidemiology and microbiome sciences, suggest that there are meaningful intersections between nutrition and known socioeconomic and psychological vulnerabilities related to justice involvement. It is our contention that progress in this area will require a greater understanding of the emerging research area of “food crime”, which broadly encompasses the extent to which the commercial/corporate determinants of health, including power structures, industry influence in policymaking and science, as well as marketing issues, increase non-communicable disease risk and underpin widespread food and other social injustices [10,11].

2. Primer on Ultra-Processed Foods

To translate the ways in which bottom–up (e.g., microbiome) and top–down (e.g., epidemiology) research supports the mechanistic theories linking nutrition to antisocial behavior, a brief examination of “ultra-processed” foods can provide context. Historically, a multitude of terms, such as “junk food”, “convenience food”, “highly processed food”, and “fast-food”, have been used interchangeably in reference to dietary items that are calorie-dense and nutritionally poor. These foods are produced with processes that exacerbate ecological destruction. Moreover, these terms have generally been directed at foods and beverages that contain high amounts of refined sugar and/or fats and/or sodium, low amounts of dietary fiber, and inclusive of synthetic ingredients such as emulsifiers, flavor enhancers (e.g., monosodium glutamate (MSG), colors, and other additives [12]). Although researchers have been successfully examining unhealthy dietary patterns through identification of macro-nutrients (e.g., high fat or high sugar) and the absence of micronutrients (e.g., folate), essential fats (e.g., omega-3 fatty acids), and fiber, the field lacked an effective tool for evaluating the potential synergistic, reinforcing effects of the rewarding ingredients that comprise highly palatable foods and beverages. That is, traditional nutrient profiling investigating the impact of single macro- or micro-nutrients was not, according to a growing number of researchers, capturing the potentially harmful effects of these foods and beverages at they holistically exist in the food system [13]. In such evaluation approaches, the externalities of these food products on biodiversity, water, and land resources are often not offered any place.

The development of the NOVA food classification system, in 2009, transformed nutritional epidemiology by easing the identification of foods that meet specific “ultra-processed food” criteria [14,15]. Monteiro and colleagues showed that it is possible to separate foods into four major groups based on the degree of processing, with a distinct group referred to as ultra-processed foods [16,17]; the four-category NOVA food classification system is separated into (i) unprocessed/minimally processed foods, (ii) processed culinary ingredients, (iii) processed food products, and (iv) ultra-processed products [18]. Although the NOVA system has been the subject of criticism, including a lack of specificity in relation to individual foods that are nutrient-dense and generally regarded as healthy, and at the same time, missing items that are high in sugar, sodium, and saturated fat [19], the system has been useful in illuminating links between highly processed foods and disorders and diseases [20].

It is important to recognize that most foods consumed by humans have been processed to some degree through the removal of inedible fractions, grating, squeezing, draining, flaking, drying, parboiling, fermentation, pasteurization, pressing, crushing, milling, etc. When naturally occurring foods undergo these forms of processing, they are still classified as Category 1 (minimally processed) by NOVA, reflecting the adaptive necessity for aspects of food processing that have provided value to human health and wellbeing. NOVA 1 foods are relatively easy to identify—examples include eggs, unprocessed meat and seafood, unflavored and unsweetened dairy products (such as yogurt), rice, pasta, whole potato, and culinary spices. NOVA 2 foods involve relatively little processing and include fat sources such as olive oil, butter, and sugar sources such as honey and maple syrup. Category 3 food items resemble their natural form but contain added salt, sugar, and/or oil to enhance palatability (e.g., fruits in sugar syrup) and/or shelf life (e.g., canned vegetables preserved in salt). Thus, nearly all forms of naturally occurring foods are encompassed by NOVA Categories 1 through 3 [16].

In contrast, Category 4’s ultra-processed foods are industrially formulated and subjected to processing techniques that permit the creation of highly palatable, durable, accessible, and convenient foods/beverages that are mostly ready-to-eat or to-heat. Ultra-processed foods contain little, if any, naturally occurring foods/ingredients. As such, they are perhaps better defined as “products” [21], with the additional processing carrying higher levels of sugar, fat, emulsifiers, flavor enhancers, synthetic ingredients, plant isolates, and/or extruded meat remnants [22,23]. Examples of NOVA 4 products include ready-to-heat meals, instant noodles, pastries, pre-prepared pies (including those containing processed meats), powdered and packaged soups, cakes, and cake mixes, multi-ingredient breakfast cereals, soft and so-called “energy” drinks, dairy-based beverages and yogurts with fruit flavor and other additives, hot dogs, sausages, and burgers with flavor enhancers, fillers, and extenders [16].

Although ultra-processed products can have isolated food fibers, vitamins, minerals, and essential fats retrofitted into the finished item, consumption is generally associated with diminished micronutrient intake and dietary diversity of plant-based antioxidants (e.g., flavonoids) [24]. Indeed, ultra-processed products are most readily identified by the presence of emulsifiers, flavor enhancers, colors, and isolated fibers, rather than sugar, fat, and sodium, per se [25]. There is one major exception to the generalized loss of dietary flavonoids coincident with ultra-processed food consumption—soy isoflavones with estrogen-like properties [26]. This is not surprising given the massive increases in textured soy protein (which contains eight times more isoflavones than traditional soy milk [27]) and other soy-derived additives that have been insinuated into the processed food supply as meat “extenders” and for other purposes [28,29].

Over the last decade and a half, rigorous prospective cohort trials and cross-sectional studies alike have robustly linked ultra-processed foods to non-communicable diseases, including cardiovascular disease, cancer, obesity, type 2 diabetes, and overall, increased hazard of all-cause mortality [30,31,32,33,34,35,36,37,38]. More recently, and of high relevance to our context here, ultra-processed food consumption has been linked to diagnosable mental disorders and various neuropsychiatric outcomes, including depressive symptoms, anxiety [39,40,41,42,43,44,45,46,47], and antisocial and/or aggressive behavior [48,49,50,51,52,53,54]. Links between non-communicable diseases and criminal behavior cannot be viewed in isolation; for example, involvement with the criminal justice system in early life is associated with much higher risks of non-communicable disease in subsequent decades [55]. Given the research to be discussed below, we suggest that diet may be at least one mediating factor.

In a tightly controlled study conducted in an institutional setting, researchers showed that ultra-processed food consumption was associated with greater caloric intake and weight gain vs. ad libitum minimally processed food access and consumption; this observation was despite the ultra-processed diet being matched to the unprocessed diet for presented calories, sugar, fat, sodium, fiber, and macronutrients [56]. Moreover, the subjects consuming the ultra-processed dietary pattern had differing blood and urine metabolites (metabolome), endogenous and exogenous compounds that represent activity through a wide range of biochemical pathways [57]. Ultra-processed food consumption is associated with low-grade systemic inflammation [58,59] and appears to influence systemic metabolites and energy uptake via changes to the gut microbiome [60,61,62]. Mechanistic topics are discussed in more detail below.

In recent years, a growing body of research has demonstrated that ultra-processed foods produce similar neurobiological and behavioral responses as addictive substances [63,64]. The Yale Food Addiction Scale (and its modified Version 2.0, along with translation into multiple languages) is a self-report measure that assesses the diagnostic criteria for substance-use disorders with respect to UPF intake and has yielded a cross-cultural body of evidence demonstrating the validity and utility of “ultra-processed food addiction” as a novel presentation [65]. Based on a meta-analysis of international research, the overall pooled prevalence of ultra-processed food addiction using the Yale Food Addiction Scale has been estimated at 14% in adults and 12% in children. The types of food that provoke food addiction are almost exclusively NOVA-categorized ultra-processed foods, inclusive of engineered combinations of sugar, fat, sodium, emulsifiers, and flavor enhancers [66]. These are the very foods that have witnessed an increased shelf presence in US retail outlets in recent decades [67,68], aided in part by formulation and marketing efforts taken from tobacco company blueprints [69].

The precise biopsychosocial underpinnings of ultra-processed food addiction remain understudied, though existing evidence has demonstrated support for similarities with substance-use disorders. For instance, addictive-like ultra-processed food intake has been associated with adverse childhood experiences (e.g., early-life violence victimization) and/or familial aggregation of addictive disorders [70,71]. Furthermore, overlaps between food addiction with impulse control disorders and gambling addiction have also been observed [72,73]. In addition, diets high in processed fats and added sugar, ingredients typically found in UPFs, have been shown to drive impulsive choice behavior in preclinical models [74], and human studies have similarly linked greater UPF intake with risk-taking [75], suggesting the plausibility of UPFs directly contributing to reward-driven, impulsive behavior. Of high relevance to criminology and carceral systems, research involving healthy adults indicates that perceived social isolation is associated with altered neural reactivity to food cues within specific brain regions responsible for processing internal appetite-related states, compromised executive control and attentional bias, and motivation toward external food cues. Put simply, social isolation may be a driver of cravings and addiction to ultra-processed foods [76].

As discussed below, ultra-processed food addiction rates have been estimated as being higher in vulnerable populations, such as persons living with food insecurity [77]. Thus, there is an urgent need for criminal justice professionals to examine the tactics of the harmful product industries, including the purveyors of ultra-processed products [78]. In many ways, empirical study of the potentially addictive properties of ultra-processed foods and their implication in a presentation that resembles a clinically significant substance-use disorder overlaps with the scope of the industry-driven public health concerns conceptualized as food crime [10,11].

3. Dietary Patterns vs. Individual Nutrients

The expansion of ultra-processed foods has been facilitated, at least in part, by industry’s emphasis of isolated nutrients that may, or may not be, in specific products. Our primary focus here is on holistic dietary patterns and the behavioral implications of the ultra-processed products within. However, in order to frame later mechanistic discussions related to brain and behavior, a brief discussion of select nutrients is worthwhile. Links between low blood and brain tissue levels of omega-3 fatty acids and human aggression, homicide, and suicide by violent means, have been discussed for over three decades [79,80,81]. Omega-3 fatty acids have been found to reduce aggression, antisocial behaviors, and self-harm, in the healthy general population and populations with known neuropsychiatric disorders [82,83,84,85,86]. Researchers have found that omega-3 blood levels are lower in marginalized and socioeconomically disadvantaged persons [87,88]. Inmates in correctional facilities may have lower blood levels of omega-3 than those found in the wider population, with research further indicating that inmates with lower blood levels of omega-3 fatty acids are more aggressive than inmates with high Omega 3 Index scores [89]. In a recent meta-analysis, researchers concluded that omega-3 fatty acids reduce both reactive and proactive forms of aggression [90]. The relevance of these findings to mechanistic theories will be discussed shortly. For now, though, it is important to note that low omega-3 levels are certainly not the only nutrient-related elements tied to aggressive and antisocial behavior. For example, elevated blood copper/zinc ratios have been linked to aggression, lowered mood, cognitive difficulties, and criminality [91,92,93]. Whether or not these blood micronutrient ratios and other observations linking single-nutrient deficiencies (e.g., magnesium or zinc) to antisocial behavior are the result of poor diets lacking nutritional diversity and/or increased demands for nutrients while under psychological/physiological stress is an active area of investigation [94,95,96].

Multiple isolated nutrients have been implicated in aggression, antisocial behavior, and mental disorders. These range from vitamin A to zinc and have been reviewed in detail elsewhere [97,98]. While studies show that isolated supplements may improve mood, reduce anger, irritability, and/or aggression [99,100,101] the available evidence, at least in young populations, supports a multi-nutrient approach rather than single-nutrient supplementation [102]. In any case, the increasingly robust research under the umbrella term “nutritional psychiatry” suggests that attention to dietary patterns, rather than a particular dietary supplement, is an urgent priority in the field of mental health and behavior. Indeed, the research surrounding food deserts indicates that a focus on dietary patterns that address all six dimensions of food security—availability, access, utilization, stability, agency, and sustainability [103]—is central to the discourse on “food crime”.

4. Nutrition and Mental Health—Interventions

Decades worth of pre-clinical experimentation have shown that lab chow high in fat, and/or sugar, and/or sodium, and/or synthetic additives, can influence brain function and behavior [104]. Moreover, animal studies demonstrate that maternal consumption of ultra-processed-food-like patterns can influence offspring aggression and antisocial behavior [105,106,107,108]. These pre-clinical studies are supported by findings from nutritional epidemiology [109]. However, it was not until recently that researchers transitioned back toward the whole-of-diet intervention studies as initiated by Schoenthaler (discussed below) in the 1980s. Recent years have witnessed controlled human intervention studies examining dietary patterns with various neuropsychiatric outcomes. These studies are of relevance to individual and community mental health, and by extension, criminology. The intervention studies described below have generally shared an emphasis on fruits, vegetables, whole grains, nuts, dairy without added sugars, paired with the exclusion or minimization of sweets/desserts, refined cereals, fried food, fast-food, processed meats, and sugar-sweetened beverages [110]. Even though the methodologies have improved compared to the original 1980s efforts, with more careful analysis of food intake by participants and efforts to include some version of a control group (e.g., a social support group or non-dietary treatment), it should be acknowledged that contemporary dietary interventions are difficult to fully blind and suffer from expectancy effects [111,112].

Notwithstanding the absence of fully blinded randomized controlled intervention trials comparing minimally processed dietary patterns vs. ultra-processed patterns, head-to-head, the available evidence lends support to the idea that dietary patterns influence brain and behavior in meaningful ways. One study that has received considerable attention is the SMILES trial (n = 67), a 3-month dietary intervention focusing on healthy choices. Here, the researchers reported improved ratings of depression on a clinical rating scale compared to a social support control group [113]. In a recent reanalysis of the SMILES trial, the group reported that it was the elimination of ultra-processed foods (rather than the inclusion of particular healthy foods) that seemed to produce the positive results [114]. Research suggests that improved mental outlook after dietary interventions can be observed in a relatively short time. For example, a 3-week randomized dietary intervention study showed that young adults (n = 101) switching to a healthy dietary pattern (vs. habitual diet) report significantly lower depression symptoms; these differences were also noted at a follow-up 3 months later [115]. In a multi-center, randomized controlled trial (n = 292) involving a low-fat, low-glycemic index, plant-based diet intervention, adults in the diet group reported improved mental outlook and productivity [116]. Other randomized controlled intervention studies with similar designs have demonstrated that improvements in mental health are noted with greater inclusion of fruits, vegetables, lean meats, fish, whole grains, and exclusion of highly processed snacks and fast-food [113,117,118,119,120].

5. Experimentation in Correctional Facilities

The emerging research on ultra-processed foods at the epidemiological level neatly fits with studies conducted in the 1980s, namely those examining dietary patterns and behavioral outcomes in correctional settings. In a series of quasi-experimental studies across twelve juvenile correctional institutions in the United States, Schoenthaler and colleagues examined culinary swaps wherein foods with high amounts of added sugar and fat were traded for similar, less processed options [121,122,123,124]. One of the studies was more narrowly focused in that there was a simple introduction of orange juice at mealtimes [125]. The outcomes were based on examination of officially documented incidents of antisocial behavior among subjects who had experienced approximately three months on the newly modified diet. These results were compared to reports on those who had entered the facilities prior to the dietary transition. Taken as a whole, the studies involved several thousand juveniles with the results showing an average 47% reduction in documented offenses, infractions, and other indicators of antisocial behavior. These included reductions in overt violence, acts of theft, verbal aggression, and insubordination to corrections personnel [126,127]. The interventions were nutritionally adequate and the study designs involved cooperation and supervision by government nutritionists [6,128]. Since these studies were not randomized, interpretations of causation are limited [6]. Nevertheless, as discussed below, contemporary reanalysis of successful dietary intervention studies used to improve mental health indicates that it is the removal of ultra-processed foods, rather than the presence of specific “healthy foods”, that seems to be the central factor in improvements to mental health [114].

In addition to the whole-of-diet interventions, researchers have also examined the usefulness of nutritional supplementation in correctional settings. Beginning with a small (n = 62) randomized controlled trial involving a juvenile correctional facility, researchers showed that subjects receiving a multivitamin–mineral formula had 28% less violent infractions vs. inmates consuming a placebo. This study was recently replicated with a larger sample size (n = 449), with the results showing that those consuming a basic low-dose multivitamin–mineral formula (similar to that found in most supermarkets) had 38% fewer serious rule infractions compared to the placebo group [129,130]. These US studies enjoy support from international intervention trials in diverse populations in confinement, including adult correctional settings. In a randomized, double-blind, placebo-controlled study in a United Kingdom prison (n = 231), researchers reported that adult inmates consuming a basic multivitamin–mineral formula with low amounts of added omega-3 fatty acids (80 mg eicosapentaenoic acid (EPA) and 44 mg docosahexaenoic acid (DHA)) committed an average of 26% fewer disciplinary offences [131], rule-breaking incidents [132], and aggression [133]. An ongoing study in Australia is evaluating the potential of omega-3 fatty acid supplementation to lower aggression among inmates within correctional facilities [134]. Previous placebo-controlled research in Singapore (n = 145) has indicated that a 200 mL omega-3 fatty acid-containing beverage (300 mg EPA, 300 mg DHA) can reduce antisocial and aggressive behavior over and above regular treatment programs in young offender institutions [135], and may reduce post-release recidivism rates [136]. For further information on controlled diet and nutritional intervention studies in relation to the mental health and behavior of prisoners, the implications of the existing research, and the need for further high-quality evidence, the reader is directed to a recent systematic review [137]. In the review, the authors note that, even though the first attempts to study diet and behavior outcomes in prison systems date back more than forty years, the area is still in its “infancy”.

Taken as a whole, and keeping in mind the aforementioned limitations of dietary intervention studies, an emphasis on less processed foods at the expense of high-sugar, high-fat, ultra-processed foods, appears worthy of greater scrutiny in criminology. Prison food in the United States and elsewhere is often dominated by low-cost ultra-processed and prepackaged products that are formulated to check boxes related to mandated nutrient and caloric intake [138]. This leads to robust business at prison commissaries (and an underground economy) wherein highly palatable foods with poor nutritional scores are used as coping resources [139,140]. As researchers examine methods to “nudge” prisoners toward healthier dietary choices in carceral settings [141], there is opportunity to measure behavioral and mental health outcomes. In one of the earliest examinations of the behavioral consequences of a dietary intervention to reduce high-sugar foods in a correctional setting, inmates reported irritability, nervousness, and other physiological changes “reminiscent” of drug withdrawal [142].

At present, the state of Maine, under the direction of Corrections Commissioner Randall Liberty, is conducting a large-scale transformation of its prison food. Specifically, the Commissioner has partnered with Brigaid, a team of culinary experts known to transform institutional food service by, among other things, swapping out ultra-processed foods for less processed versions. Liberty is hopeful that the Maine project will be an example for other states to follow [143]. From our perspective, we see large-scale projects like that in Maine to be a ripe opportunity for experts in criminology to join with interdisciplinary teams and apply research tools in the analysis of potentially important endpoints, including biophysiological and behavioral outcomes. Although our focus here is on mental health and behavior, it is important to underscore that prisoners are at very high risk of NCDs [144], and diet represents an important pathway in reducing non-communicable disease risk.

6. Mechanistic Pathways

Setting aside the methodological limitations of the quasi-experimental dietary studies conducted in correctional facilities in the 1980s, and accepting for a moment that the positive findings are in line with contemporary intervention studies in the realm of nutritional psychiatry, the question of explanatory mechanisms looms large. How might dietary choices influence the risk of antisocial and/or aggressive behavior? As mentioned, early theoretical efforts focused on sugar—most notably the idea that reactive hypoglycemia (after high sugar consumption) places a vulnerable person at risk for aggression and antisocial behavior as blood sugar rapidly drops. The relationship between high dietary sugar intake and neurocognition/behavior is an enduring research question [145,146,147,148], and pre-clinical studies continue to indicate that chronic early-life sugar consumption promotes aggression [149]. These observations are supported by epidemiological studies linking early-life confectionary and soft-drink consumption with aggression, violence [150], and later-life involvement with the justice system [151]. From a mechanistic standpoint, sugar consumption is part of a dietary composite linked to chronic, low-grade inflammation [152], which in turn is linked to immune-mediated neuroinflammation and alterations in neurotransmission [153].

In any case, aspartame, and additives such as MSG (and similar chemicals often labeled covertly as “yeast extract” on packaging), are known as dietary excitotoxins due to their propensity to cause neuronal hyperexcitability [154]. In addition to the aforementioned studies connecting aspartame to changes in neurocognition, emerging human studies are demonstrating that the elimination (or minimization of) dietary excitotoxin additives such as monosodium glutamate and aspartame has the potential to improve symptoms of depression, anxiety, posttraumatic stress disorder (PTSD), pain, and Gulf War Illness [155,156,157,158,159,160]. Susceptibility to the effects of dietary excitotoxins may be mediated by differences in blood–brain barrier permeability, allowing for increased access to the brain [161]; disturbances to normal blood–brain barrier structure and function can be influenced by psychological trauma, acute and chronic stress, and may have a bidirectional relationship with mental illness [162]. Aspartame is known to lower the uptake of tryptophan into the mammalian brain, leading to reduced serotonin production [163], which may explain the observation of aspartame-induced aggression in rodents [164]. It is interesting to note that, among the ultra-processed foods connected to depression, those containing artificial sweeteners appear to have the strongest relationship [47]. The extent to which MSG and other dietary excitotoxins can intersect with psychiatric vulnerabilities to promote aggression is an open question [165].

Since our focus here is mainly on ultra-processed vs. unprocessed/minimally processed foods, suffice it to say that, if a person is deficient in any essential nutrient, observations of neuropsychological disturbances are not unexpected. Vitamins, minerals, and essential fatty acids contribute to the structure and function of the human brain, both directly and indirectly through many metabolic pathways. Moreover, there is a vast physiological interdependence of nutrients; for example, folate, deficiencies of which have been linked to aggression and violence [166,167], is responsible for the blood transport of omega-3 fatty acids, which are, in turn, essential to neuronal structure and function [168]. Vitamin D is another example of a nutrient which has been linked to protection against aggression and antisocial behavior [169,170]. Vitamin D, too, operates through multiple pathways, including those that can directly influence the central nervous system, and others that operate through the immune system and the microbiome [171,172]. Examining nutrients in isolation can be helpful, of course, but the complexity of enzymatic reactions, which are highly dependent on various nutrients, suggests that the study of isolated single-nutrient vitamin/mineral supplementation may provide limited value [102]. One the other hand, careful study of the omega-3 fatty acids EPA and DHA, which play important roles in neuronal structure and function, membrane fluidity, intracellular signaling, and gene expression [173], has taught us much about the interactions between immune system mediators and mental health. That is, the potential of omega-3 fatty acids, EPA in particular, to act as signaling molecules throughout the immune system, limiting systemic low-grade inflammation, appears to be a leading mechanistic explanation for positive outcomes [174,175].

Human research indicates that elevated markers of low-grade inflammation (e.g., C-reactive protein) are a characteristic finding in persons with mental illness and aggressive tendencies [176,177,178,179]. In a recent study involving 686 adults with bipolar disorder (vs. 343 healthy controls), researchers found that the systemic immune–inflammatory index (a combined measurement of neutrophils, lymphocytes, and platelet counts) was higher among persons with bipolar disorder who had committed criminal offenses [180]. Similar findings of increased inflammatory immune markers have been reported for adults with schizophrenia who had committed crimes vs. non-criminal patients [181]. More generally, diet-induced low-grade inflammation is associated with behavioral disinhibition in adults [182]. EPA is known to limit the production of proinflammatory cytokines (e.g., interleukin-6, interleukin-1 beta, and tumor necrosis factor-alpha) [183], immune chemicals that have been implicated in human aggression [184,185,186]. The observation that omega-3 fatty acids (vs. other dietary fats) have differential effects on gut microbiota helped to strengthen the early argument that the intestinal ecosystem plays an underestimated role in the diet–mental health linkage [187,188]. Given its centrality to mechanistic theories, we will discuss the microbiome in a dedicated section, below.

7. Microbiome Pathways

Dysbiosis (from the Greek “life in distress”) is a popular term used to describe alterations to the “normal” complexity of commensal microbial communities. In the gastrointestinal tract, dysbiosis may refer to losses of beneficial microorganisms, and/or the expansion of potentially harmful microbes, and/or the loss of overall microbial diversity [189]. In our current context, it is important to point out that many factors of importance to criminology—ranging from alcohol and drug use to poverty and food insecurity—have been connected to dysbiosis [190,191,192,193]. Several recent studies have shown that individual and community-level socioeconomic disadvantage is associated with losses in gut microbial diversity and other signs of dysbiosis [194,195,196,197,198]. Volumes of preclinical studies demonstrate that various physical and psychological stressors can provoke gut dysbiosis [199,200], and these are supported by a growing number of human studies [201,202,203,204,205]. We are only at the beginning stages of understanding the long-term biophysiological and clinical implications of trauma-induced dysbiosis in humans, although the available evidence indicates that systemic metabolic changes are mediated by the microbiome [206].

Although it has been nearly four decades since researchers first demonstrated, via using germ-free vs. conventional animal models, that the gut microbiota can influence brain physiology [207], the relevancy of these findings to daily mental health and cognition escaped mainstream discussion. In the mid-2000s, following the demonstration that specific microbes in the gastrointestinal tract can cause distress and anxious behavior through activation of visceral sensory nuclei in the brainstem [208], facilitated by direct gut-to-brain communication via the vagus nerve [209,210,211], microbiome–neuropsychiatry research began in earnest. It was also shown through germ-free rodent models that gut microbes influence the early-life development of the hypothalamic–pituitary–adrenal stress response, and the production of neurochemicals such as the brain-derived neurotrophic factor (BDNF) [212]. The BDNF has been linked to aggressive behavior in multiple animal and human studies [213,214,215]. At the same time, other groups were conducting human research, demonstrating that the oral administration of probiotics can lower markers of systemic inflammation [216] and oxidative stress [217]; removed from their silos and viewed through a transdisciplinary lens, these and other studies provided sound biophysiological mechanisms to support a topic—gut microbes and neuropsychology—that was once considered outlandish [218,219]. At this point, it is clear that the vagus and spinal nerves are facilitating the delivery of gut microbe-associated information to the brain [220], and from the systemic route, gut microbiota are involved in the production of humoral signaling molecules (e.g., cytokines), neuropeptides, and hormonal messengers that influence cognition and behavior [221].

In the more specific realm of aggression and antisocial behavior, there has been surprisingly little human research examining the microbiome and criminology. In a small study involving inmates (n = 30) who had recently entered two different correctional facilities, the prisoners were reported to have significant differences in the phylogenetic structure and functional genes of the gut microbiota vs. healthy non-incarcerated controls [222]. In addition, a study in Michigan linked post-mortem microbiome samples (n = 188) from the larger Detroit area to a local area (i.e., census block group) crime level. Microbiome alpha diversity, and the richness and evenness of the gut bacteria, was significantly lower in high (versus medium and low) crime areas; alpha diversity could be used to predict block group category placement in low, medium, and high crime conditions. Moreover, specific taxa varied by crime level, including Lachnospiraceae [223], an anerobic family linked to aggression in separate preclinical work [224]. Unhealthy dietary patterns have been shown to increase Lachnospiraceae in animals, with accompanying inflammation [225]. Lachnospiraceae, including the genus Sellimonas, has been linked to major depression in human research [226,227].

These intriguing studies are supported by a growing body of preclinical research—evidence that continues to point toward the microbiome as at least one factor in a complex web of causation [228,229,230]. Emerging studies are indicating that certain gut microbial signatures are associated with mood [231,232], temperament [233], sociability [234], violent tendencies [235], and emotional regulation [236,237]. The potential relevance of these microbial signatures to criminology is obvious. It is also worth noting that a higher frequency of early-life antibiotic prescriptions has been linked to subsequent attention deficit hyperactivity disorder [238], and even in adulthood, antibiotic prescriptions have been linked to subsequent anxiety and depression [239]. Of course, questions of causality in topics as complex as human behavior and criminality will endure, but the available evidence supports the idea that the immune system and neuropsychology are deeply intertwined [240]. To the best of our knowledge, there has been no research examining the frequency of antibiotic prescriptions and subsequent indicators of justice system involvement. Animal models show that early-life antibiotic administration is associated with later-life aggressive behavior [241,242]. Remarkably, the transfer of fecal material from human infants with disruptions to normal microbiome development (via the administration of antibiotics) leads to aggressive-like behavior in recipient lab animals, observations not seen with transfer of microbiota from healthy infants [243]. This emerging research emphasizes the need for expanded research on the long-term neurobehavioral implications of early-life environmental exposures and vulnerabilities. It is worth noting that, in the United States, there is a strong, linear relationship between poverty and outpatient antibiotic prescribing rates, with approximately 40% of variability in prescribing explained by the prevalence of poverty in a state [244]. We will discuss other fecal transplant studies below.

Returning to our focus on nutrition, volumes of animal studies indicate that westernized ultra-processed food-like dietary patterns (with high sugar, refined fats, sodium, emulsifiers, and/or other additives) can lead to dysbiosis. Human research supports these findings [245,246,247,248,249]. Dietary patterns high in ultra-processed foods are not only characterized by their inclusion of these industrially refined ingredients, they are also generally identifiable by what they are missing—natural dietary fiber and polyphenols in the whole food context (vs. isolated fibers retrofitted into foods) [250]. These absences are important because fiber and polyphenols add to gut microbial diversity and aid in the production of metabolites with antioxidant and anti-inflammatory properties [251,252]. The formation of bioactive polyphenolic metabolites via gut microbe activity appears to play an important mechanistic role in mental and cognitive health [253]. The consumption of juices rich in naturally occurring flavonoids has been shown to improve mood in young adults with depressive symptoms [254] and lower scores on anger/hostility measures in otherwise healthy adults [255].

Further support for a microbial role can be found in so-called fecal transplant studies. Researchers have demonstrated that the transfer of fecal microbes, from animals reared on unhealthy dietary patterns to recipient animals reared on standard lab chow, leads to behavioral changes and cognitive deficits in the recipients [256,257,258]. The transfer of fecal material from animals that have experienced chronic unpredictable stress to healthy animals leads to anxiety and depressive-like signs among otherwise normal recipient animals [259]. Similar research has shown that the behavioral alterations associated with isolation stress-induced dysbiosis (e.g., diminished sociability, increased aggression) can be observed in otherwise healthy animals that are recipients of fecal material from dysbiotic donor animals; this observation is not found when the recipients receive fecal material that has been subjected to high heat (i.e., heat-inactivated microbes) [260]. In an avian model, fecal transfer from separate lines of chickens, one line known to be more aggressive, and the other less aggressive, into a neutral commercial line of recipient chicks, led to increased or decreased aggression based on the donor line tendencies; these behavioral observations were matched with differential changes in the activities of brain serotonergic and catecholaminergic systems [261]. These fecal transplant studies bring us closer to viewing microbes as a causative factor, yet the gut microbiome at the individual level cannot be uncoupled from larger social determinants of dysbiosis.

Whole food-derived dietary fiber and polyphenols also interact with gut microbes to help maintain the normal structure and function of the intestinal barrier. This is of relevance because one of the leading mechanistic theories linking gut microbes to abnormal psychology is the loss of integrity of the intestinal barrier, a process commonly known as “leaky gut” [262]. Much like the aforementioned blood–brain barrier, the intestinal lining also acts, normally, as a selective portal for nutrients through the exclusion or limitation of unwanted chemicals, microbes (or parts thereof, such as bacterial lipopolysaccharides), and other antigens. Psychological stress and/or ultra-processed-like dietary patterns can lead to increased intestinal permeability. This, in turn, leads to proinflammatory cytokine responses and the maintenance of low-grade inflammation [263,264]. The retro-fitting of isolated fiber into ultra-processed foods may contribute to dysbiosis, damage to the intestinal lining, and metabolic dysregulation [265,266,267,268].

The low-grade inflammatory cascade and associated metabolic dysregulation can influence mood and aggression [269,270]. Intestinal permeability allows gut microbial breakdown products, such as lipopolysaccharide endotoxin (LPS), to enter circulation. LPS works in tandem with dietary chemicals such as MSG to promote neuroinflammation and dysfunction of neurotransmission [271]. Since research is pointing toward a causal role for gut microbiota in systemic inflammation [272], scientists are actively pursuing gut microbial signatures that can tie together intestinal permeability, systemic low-grade inflammation, and risk of aggression [273,274].

In contrast to the protective nutrients derived from minimally processed foods, dietary emulsifiers, so common to ultra-processed foods, have been linked to disturbances to the microbiome and increased intestinal permeability [275,276,277,278,279,280]. In preclinical work, dietary emulsifiers, including carboxymethylcellulose and polysorbate-80, increase sensitivity to social stress and appear capable of altering gene expression in the amygdala [281,282]. It has also been shown that early-life exposure to dietary emulsifiers is related to later-life cognitive impairments and the expression of anxiety-like traits [283]. In continuation of our earlier discussion of dietary excitotoxins, it is important to note that emulsifiers are not the only dietary additives known to cause dysbiosis in animal models; monosodium glutamate (MSG) has also been shown to alter the gut microbiome [277,278,279,280] and preclinical work suggests that MSG may contribute to the addictive potentials of ultra-processed foods [284,285]. Multiple animal studies also show that diets rich in soy isoflavones are associated with increased anxiety-like behavior [286] and alcohol consumption in animal models of substance-use disorders [287,288,289]; such soy isoflavone-related increases in alcohol consumption may be due to changes in the microbiome [290]. It is also worth noting that early-life exposure to soy isoflavone metabolites is associated with adult anxiety and aggression in animals [291].

8. Where to Next?

The rapidly accumulating body of preclinical and intervention studies (those with objective findings) has provided crucial mechanistic clues for researchers to follow. Further research with dietary intervention approaches, nutritional supplementation, and independent replication of existing studies, will help the field. One of the most obvious areas for research attention is perinatal and early-life nutrition, and the study of potential links to criminal justice involvement during childhood development and over subsequent decades. In particular, researchers may examine the relationships between undernutrition and overnutrition (and the complication of ultra-processed foods in both low-income and high-income countries) as a path to antisocial behavior and differing crime rates [292,293].

Although a growing number of human studies indicate that targeting the gut microbiome with select “psychobiotic” agents (e.g., probiotics, prebiotics) can improve mood and lower stress [294,295], there is a specific need for follow-up on human studies suggesting that probiotics can lower aggressive thoughts [296], aggressive actions [297], and impulsivity [298]. Animal studies indicate that probiotics can positively influence oxytocin [299], the neuropeptide known to support prosocial behavior and limit aggression through empathy [300]. Translational research is required.

From our perspective, the isolated application of commercial probiotics to lower risks of real-world juvenile or adult antisocial behavior seems like a tall order; on the other hand, there may be an important place for probiotics and post-biotics in early-life neuroimmune programing with long-term neurobehavioral benefits [301,302]. In adult populations, research shows that healthy lifestyle behaviors predict positive mental health outcomes with supplementation, and unless other factors are attended to (such as reductions in ultra-processed foods), probiotics alone may be of limited value [303]. In other words, providing a probiotic capsule to juveniles that are actively consuming a diet heavy in ultra-processed foods, whether juvenile offenders in custody or food-insecure youth in the community, is unlikely to provide meaningful results. Instead, it may be time to return to whole-of-diet studies similar to those initiated by Schoenthaler, but with stricter designs and an eye toward microbiome and other objectively measured mechanistic biomarkers. Recent studies have shown that dietary interventions directed at the microbiome (e.g., whole grains, onions, leeks, cabbage, oats, fermented foods) can reduce psychological stress and improve mood [304,305], findings that should encourage research in prison systems and the broader realm of criminology.

Researchers in criminology are well-positioned to query the ways in which food policy and corporate practices influence antisocial behavior, aggression, and crime in general. Research shows that ultra-processed food consumption is higher in adults with lower income, less education, and persons living with food insecurity [306], and that those with food insecurity are as much as 3.8 times more likely to report clinically significant patterns of addictive-like ultra-processed food intake [77,307]. In a random selection of 362 US countries, research shows that, even after controlling for multiple factors, a 1 percent increase in food insecurity is associated with a 12 percent increase in violent crime in a given community [308]. Similar findings have connected food insecurity to gun violence [309], murder, rape, and robbery [310]. In households, food insecurity is associated with an increased parent-to-child aggression [311] and child maltreatment [312,313]. At the same time, fruit, vegetable, and omega-3 intake is lower in persons living with socioeconomic disadvantage [314] and fast-food consumption is particularly high in marginalized populations [315]. These findings are reflected in US studies using objective markers, including observations of low blood levels of vitamins and carotenoids in association with neighborhood deprivation and low socioeconomic position [316,317]. Moreover, human autopsy research shows that serum carotenoids correlate with brain carotenoids [318], and in living community samples, low carotenoid concentrations may be a surrogate marker of the biological manifestations (allostatic load) of chronic stress [319] (see Figure 2).

It is also important to recognize that aggressive behavior runs along a continuum, and although microaggressions have been a controversial topic, recent evidence has tied microaggressions to validated measurements of aggression. To what extent are minor forms of aggression linked to dietary patterns? To date, there has been no definitive explanation as to why judges provide harsher decisions and less favorable outcomes (e.g., dismissal and parole decisions) at the end of a morning session, compared to the beginning of a session [320,321,322,323].

In academic discourse concerning legal realism, discussions on the decision-making of judges based on psychological whims and wide discretion (vs. reliance upon facts, judicial rules, and the law) is referred to, as a somewhat sarcastic trope, as the “judicial breakfast” or “what the judge ate for breakfast” [324,325]. The “judicial breakfast” discourse is not literal in relation to food quality or dietary patterns and justice. Perhaps it should be. Research shows that a high carbohydrate meal increases subsequent social punishment and influences risk-related outcomes [326,327].

Finally, but certainly not least, the available research on the relationships between ultra-processed products and non-communicable diseases, including common mental disorders [328], demands increased scrutiny of the tactics used by the manufacturers, marketers, and distributors, of such products [329]. This is already ongoing in public health, under the banner of the commercial determinants of health (CDoH). For now, much of the CDoH discourse is directed at obesity, cardiovascular disease, and diabetes, with only minimal attention paid to antisocial behavior [330]. Since ultra-processed foods are increasingly associated with real-world harms, academics along the criminal justice spectrum are well-positioned to investigate the ways in which policies and practices intersect with harm prevention and matters of justice [11,331]. The ultra-processed product industry operates through a highly coordinated global network of interest groups spanning multiple levels, jurisdictions, and governance spaces. This hardened structure, which has successfully permeated itself into institutions of governance (from local to global [332]), represents a major challenge to efforts intended to attenuate the harms of ultra-processed products [333,334]. Taken as a whole, nutritional criminology and food crime can be viewed from an ecological perspective; that is, the aforementioned advances in microbiome science and “omics” technology underscore that each person maintains complex, biologically relevant microbial ecosystems, and those ecosystems are, in turn, a product of the lived experiences within larger social, political, and economic ecosystems [335].

9. Conclusions

Research spanning epidemiology, mechanistic bench science, addiction science, microbiome and exposome science, and human intervention trials, has underscored that nutrition is of relevance along the criminal justice continuum. In particular, there is increasing concern with the potential harms associated with ultra-processed foods, including non-communicable diseases, poor mental health, and antisocial behavior. Emergent research, including an enhanced understanding of biophysiological mechanisms, demonstrates that ultra-processed dietary patterns and additive components intersect with the many psychosocial vulnerabilities that otherwise lead to increased risk of being justice-involved. Collectively, the available research suggests that early-life interventions, under the rubric of prevention science, offers significant opportunity. Such scientific inquiry can also help advance the food industry’s accountability as it continues to propagate the consumption of unhealthy products, often circumventing regulations and leveraging industry-influenced science for its benefit [336].

The research advances have also allowed for a retrospective analysis of previous studies in carceral systems and provide support for a closer examination of nutrition and behavior in correctional facilities. The individual and community-level relationships between nutrition and behavior are also of high-level importance to the relatively new field of food crime—that which examines the vast harms, including grand-scale non-communicable diseases and behavioral outcomes, as caused by the manufacturers, distributors, and marketers, of ultra-processed food products. In the context of criminal justice and behavior, there is a need to consider how power structures, industry influence, and marketing issues underpin widespread food and social inequalities. This takes us back to the opening quote by Julian Hawthorne: To the extent that the criminal justice system ignores food-associated harms, we make the crime our own [1].