Sex differences in binge eating patterns in male and female adult rats

The duration of intermittent access to preferred sucrose-rich food affects binge-like intake, fat accumulation, and fasting glucose in male rats

Many people restrict their palatable food intake. In animal models, time-limiting access to palatable foods increases their intake while decreasing intake of less preferred alternatives; negative emotional withdrawal-like behavior is sometimes reported. In drug addiction models, intermittent extended access drives greater changes in use than brief access. When it comes to palatable food, the impact of briefer vs. longer access durations within intermittent access conditions remains unclear. Here, we provided male rats with chow or with weekday access to a preferred, sucrose-rich diet (PREF) (2, 4, or 8 h daily) with chow otherwise available. Despite normal energy intake, all restricted access conditions increased weight gain by 6 weeks and shifted diet acceptance within 1 week. They increased daily and 2-h intake of PREF with individual vulnerability and decreased chow intake. Rats with the briefest access had the greatest binge-like (2-h) intake, did not lose weight on weekends despite undereating chow, and were fattier by 12 weeks. Extended access rats (8 h) showed the greatest daily intake of preferred food and corresponding undereating of chow, slower weight gain when PREF was unavailable, and more variable daily energy intake from week to week. Increased fasting glucose was seen in 2-h and 8-h access rats. During acute withdrawal from PREF to chow diet, restricted access rats showed increased locomotor activity. Thus, intermittent access broadly promoted weight gain, fasting hyperglycemia and psychomotor arousal during early withdrawal. More restricted access promoted greater binge-like intake and fat accumulation, whereas longer access promoted evidence of greater food reward tolerance.

Intermittent, extended access to preferred food leads to escalated food reinforcement and cyclic whole-body metabolism in rats: Sex differences and individual vulnerability

Compulsive binge eating is a hallmark of binge eating disorder and bulimia nervosa and is implicated in some obesity cases. Eating disorders are sexually dimorphic, with females more often affected than males. Animal models of binge-like eating based on intermittent access to palatable food exist; but, little is known regarding sex differences or individual vulnerability in these models with respect to the reinforcing efficacy of food, the development of compulsive- and binge-like eating, or associated changes in whole-body metabolism or body composition. Adolescent male (n = 24) and female (n = 32) Wistar rats were maintained on chow or a preferred, high-sucrose, chocolate-flavored diet in continuous or intermittent, extended access conditions. Body weight and composition, intake, fixed- and progressive-ratio operant self-administration, and whole body energy expenditure and respiratory exchange ratios were measured across an 11-week study period. Subgroup analyses were conducted to differentiate compulsive-like "high responder" intermittent access rats that escalated to extreme progressive-ratio self-administration performance vs. more resistant "low responders." Female rats had greater reinforcing efficacy of food than males in all diet conditions and were more often classified as "high responders". In both sexes, rats with intermittent access showed cycling of fuel substrate utilization and whole-body energy expenditure. Further, "high-responding" intermittent access female rats had especially elevated respiratory exchange ratios, indicating a fat-sparing phenotype. Future studies are needed to better understand the molecular and neurobiological basis of the sex and individual differences we have observed in rats and their translational impact for humans with compulsive, binge eating disorders.

Exploring reward system responsivity in the nucleus accumbens across chronicity of binge eating in female rats

OBJECTIVE

Research has highlighted the importance of reward-based processes in binge eating (BE). However, both increased and decreased activation have been observed in reward related brain regions for BE. Differences may be similar to addiction research, where the reward system is initially hyper-responsive at early stages of use, but becomes hypo-responsive with prolonged drug/alcohol use. This study was the first to examine differences in reward system responsivity at early versus chronic BE stages.

METHOD

Using an animal model, Sprague-Dawley female rats were identified as BE prone (BEP) or BE resistant (BER) and randomly assigned to an early or chronic stage group. Neural activation (via Fos protein) was quantified in the nucleus accumbens core (NAC) and shell (NAS).

RESULTS

Early stage BEP rats had the highest levels of Fos expression of all of the study groups. By contrast, chronic stage BEP rats exhibited decreased activation in the NAS and NAC that was similar to the activation in chronic stage BER rats.

DISCUSSION

Findings are significant in suggesting hyper-neural activation to reward in the early stages of BE and decreased activation in later stages of BE. Additional studies are needed to elucidate how these differences may impact risk for and maintenance of BE.

A spontaneous binge-like eating model in mice using unpredictable once weekly access to palatable diets

Many pre-clinical models of binge-like eating involve predictable, scheduled, access to a palatable diet high in fat (HF), where access may be preceded by anticipatory behaviour. Here, to introduce spontaneity into the binge-type consumption of palatable diets, mice were allowed 2 h access on a random day once per week and at a random time within an 8 h window either side of the transition from dark phase to light phase. Despite normal intake of a stock diet prior to unpredictable access to HF diet, mice immediately initiated a substantial eating episode when presented with HF diet. Following this consumption, compensatory hypophagia was observed relative to stock diet-fed controls, and cumulative energy intakes converged. There were no effects of HF diet on body weight or body composition over a 12-week period. Binge-like consumption was also observed on unpredictable access to the complete liquid diet, chocolate Ensure, but not with a 10% sucrose solution. Binge-like responses to unpredictable access to HF diet or Ensure were similar in male and female mice, although there were effects of sex on caloric consumption from stock diet in the compensatory period following palatable diet intake, with higher intakes in females. The timing of the 2h access period relative to light phase transition affected intake of palatable diets, but less robustly than the equivalent effect on stock diet intake during the same timed periods - the diurnal patterning of energy intake was diet sensitive. The large spontaneous binge-like consumption on unpredictable access to either solid or liquid palatable diets in mice of either sex offers the potential to combine these attributes with other manipulations where a developing obesity is part of the binge-like eating phenotype.

Ovarian hormones mediate running-induced changes in high fat diet choice patterns in female rats

Physical inactivity and increased consumption of energy dense, high fat (HF) foods often leads to a state of positive energy balance. Regular exercise can facilitate the maintenance of a healthy body weight and mediate changes in dietary selection. Past studies using a two-diet choice (chow vs. HF) and voluntary wheel running paradigm found that when a novel HF diet and wheel running are simultaneously introduced, male rats show complete and persistent HF diet avoidance whereas the majority of females show HF diet avoidance for a few days, but then revert to HF diet preference. Ovariectomy (OVX) appears to decrease preference for the HF diet bringing it closer to that of males. Given that estradiol but not progesterone mediates changes in food intake and energy balance, we hypothesized that estradiol signaling is required for the reversal of HF diet avoidance in female rats. Accordingly, Experiment 1 compared the persistency of running-induced HF diet avoidance in males, sham-operated females, and OVX rats with replacement of oil vehicle, estradiol benzoate (E), progesterone (P), or both (E + P). The number of wheel running rats that either avoided or preferred the HF diet varied with hormone treatment. The reversal of HF diet avoidance in running females and OVX E + P rats occurred more rapidly and frequently than male running rats. E + P but not E or P replaced OVX wheel running rats significantly reversed HF diet avoidance. OVX oil rats avoided HF diet to the same extent as male rats for the first 11 days of diet choice and then rapidly increased HF diet intake and began preferring it. This incomplete elimination of sex differences suggests that developmental factors or androgens might play a role in sustaining running-induced HF diet avoidance. Subsequently, Experiment 2 aimed to determine the role of androgens in the persistency of running-associated HF diet avoidance with sham-operated and orchiectomized (GDX) male rats. Both intact and GDX male running rats persistently avoided the HF diet to the same extent. Taken together, these results suggest that activational effects of ovarian hormones play a role in female specific running-induced changes in diet choice patterns. Furthermore, the activational effects of androgens are not required for the expression of HF diet avoidance in males.

Ovarian hormones and obesity

BACKGROUND

Obesity is caused by an imbalance between energy intake, i.e. eating and energy expenditure (EE). Severe obesity is more prevalent in women than men worldwide, and obesity pathophysiology and the resultant obesity-related disease risks differ in women and men. The underlying mechanisms are largely unknown. Pre-clinical and clinical research indicate that ovarian hormones may play a major role.

OBJECTIVE AND RATIONALE

We systematically reviewed the clinical and pre-clinical literature on the effects of ovarian hormones on the physiology of adipose tissue (AT) and the regulation of AT mass by energy intake and EE.

SEARCH METHODS

Articles in English indexed in PubMed through January 2016 were searched using keywords related to: (i) reproductive hormones, (ii) weight regulation and (iii) central nervous system. We sought to identify emerging research foci with clinical translational potential rather than to provide a comprehensive review.

OUTCOMES

We find that estrogens play a leading role in the causes and consequences of female obesity. With respect to adiposity, estrogens synergize with AT genes to increase gluteofemoral subcutaneous AT mass and decrease central AT mass in reproductive-age women, which leads to protective cardiometabolic effects. Loss of estrogens after menopause, independent of aging, increases total AT mass and decreases lean body mass, so that there is little net effect on body weight. Menopause also partially reverses women's protective AT distribution. These effects can be counteracted by estrogen treatment. With respect to eating, increasing estrogen levels progressively decrease eating during the follicular and peri-ovulatory phases of the menstrual cycle. Progestin levels are associated with eating during the luteal phase, but there does not appear to be a causal relationship. Progestins may increase binge eating and eating stimulated by negative emotional states during the luteal phase. Pre-clinical research indicates that one mechanism for the pre-ovulatory decrease in eating is a central action of estrogens to increase the satiating potency of the gastrointestinal hormone cholecystokinin. Another mechanism involves a decrease in the preference for sweet foods during the follicular phase. Genetic defects in brain α-melanocycte-stimulating hormone-melanocortin receptor (melanocortin 4 receptor, MC4R) signaling lead to a syndrome of overeating and obesity that is particularly pronounced in women and in female animals. The syndrome appears around puberty in mice with genetic deletions of MC4R, suggesting a role of ovarian hormones. Emerging functional brain-imaging data indicates that fluctuations in ovarian hormones affect eating by influencing striatal dopaminergic processing of flavor hedonics and lateral prefrontal cortex processing of cognitive inhibitory controls of eating. There is a dearth of research on the neuroendocrine control of eating after menopause. There is also comparatively little research on the effects of ovarian hormones on EE, although changes in ovarian hormone levels during the menstrual cycle do affect resting EE.

WIDER IMPLICATIONS

The markedly greater obesity burden in women makes understanding the diverse effects of ovarian hormones on eating, EE and body adiposity urgent research challenges. A variety of research modalities can be used to investigate these effects in women, and most of the mechanisms reviewed are accessible in animal models. Therefore, human and translational research on the roles of ovarian hormones in women's obesity and its causes should be intensified to gain further mechanistic insights that may ultimately be translated into novel anti-obesity therapies and thereby improve women's health.

Sex Differences in Obesity and Mental Health

PURPOSE OF REVIEW

The purposes of this study were to examine the relationships between obesity and a wide range of mental health issues and to identify where sex differences exist and may vary across disorders.

RECENT FINDINGS

Research on sex differences in the relationship between obesity and psychiatric disorders is more abundant in some areas, such as depression and eating disorders, than others, such as anxiety, trauma, and substance use. However, for most of the disorders, their relationships with obesity and sex are complex and are usually moderated by additional variables. Thus, studies that find stronger relationships for women between depression and obesity cross-sectionally do not tell the whole story, as longitudinal studies suggest that this relationship may also be present among men, particularly when confounders are considered. For those with eating disorders, men and women with obesity are fairly equally affected, and weight and shape concerns may play a role in maintaining these behaviors for both sexes. Weight stigma, though, seems to have worse consequences for women than men with obesity. Sex differences exist in relation to the associations between mental health and obesity. However, these differences vary by disorder, with disorder-specific moderators playing a role, such as age for depressive disorders, comorbid depression for anxiety disorders, and weight and shape concerns for eating disorders. More work is needed to understand if sex differences play a role in the relationship between obesity and anxiety, trauma, and substance use disorders.

Estrogenic suppression of binge-like eating elicited by cyclic food restriction and frustrative-nonreward stress in female rats

Because binge eating and emotional eating vary through the menstrual cycle in human females, we investigated cyclic changes in binge-like eating in female rats and their control by estrogens. Binge-like eating was elicited by three cycles of 4 days of food restriction and 4 days of free feeding followed by a single frustrative nonreward-stress episode (15 min visual and olfactory exposure to a familiar palatable food) immediately before presentation of the palatable food. Intact rats showed binge-like eating during the diestrous and proestrous phases of the ovarian cycle, but not during the estrous (periovulatory) phase. Ovariectomized (OVX) rats not treated with estradiol (E2) displayed binge-like eating, whereas E2-treated OVX rats did not. The procedure did not increase signs of anxiety in an open-field test. OVX rats not treated with E2 that were subjected to food restriction and sacrificed immediately after frustrative nonreward had increased numbers of cells expressing phosphorylated extracellular signal-regulated kinases (ERK) in the central nucleus of the amygdala (CeA), paraventricular nucleus of hypothalamus (PVN), and dorsal and ventral bed nuclei of the stria terminalis (BNST) compared with nonrestricted or E2-treated rats. These data suggest that this female rat model is appropriate for mechanistic studies of some aspects of menstrual-cycle effects on emotional and binge eating in human females, that anxiety is not a sufficient cause of binge-like eating, and that the PVN, CeA, and BNST may contribute to information processing underlying binge-like eating.

Emotional Eating, Binge Eating and Animal Models of Binge-Type Eating Disorders

PURPOSE OF REVIEW

The objective of this paper is to review the role that hedonic factors, emotions and self-regulation systems have over eating behaviours from animal models to humans.

RECENT FINDINGS

Evidence has been found to suggest that for some high-risk individuals, obesity/binge eating may develop as an impulsive reaction to negative emotions that over time becomes a compulsive habit. Animal models highlight the neural mechanisms that might underlie this process and suggest similarities with substance use disorders. Emotional difficulties and neurobiological factors have a role in the aetiology of eating and weight disorders. Precise treatments targeted at these mechanisms may be of help for people who have difficulties with compulsive overeating.

Cytoplasmic FMR1-Interacting Protein 2 Is a Major Genetic Factor Underlying Binge Eating

BACKGROUND

Eating disorders are lethal and heritable; however, the underlying genetic factors are unknown. Binge eating is a highly heritable trait associated with eating disorders that is comorbid with mood and substance use disorders. Therefore, understanding its genetic basis will inform therapeutic development that could improve several comorbid neuropsychiatric conditions.

METHODS

We assessed binge eating in closely related C57BL/6 mouse substrains and in an F₂ cross to identify quantitative trait loci associated with binge eating. We used gene targeting to validate candidate genetic factors. Finally, we used transcriptome analysis of the striatum via messenger RNA sequencing to identify the premorbid transcriptome and the binge-induced transcriptome to inform molecular mechanisms mediating binge eating susceptibility and establishment.

RESULTS

C57BL/6NJ but not C57BL/6J mice showed rapid and robust escalation in palatable food consumption. We mapped a single genome-wide significant quantitative trait locus on chromosome 11 (logarithm of the odds = 7.4) to a missense mutation in cytoplasmic FMR1-interacting protein 2 (Cyfip2). We validated Cyfip2 as a major genetic factor underlying binge eating in heterozygous knockout mice on a C57BL/6N background that showed reduced binge eating toward a wild-type C57BL/6J-like level. Transcriptome analysis of premorbid genetic risk identified the enrichment terms morphine addiction and retrograde endocannabinoid signaling, whereas binge eating resulted in the downregulation of a gene set enriched for decreased myelination, oligodendrocyte differentiation, and expression.

CONCLUSIONS

We identified Cyfip2 as a major significant genetic factor underlying binge eating and provide a behavioral paradigm for future genome-wide association studies in populations with increased genetic complexity.

Incubation of Methamphetamine but not Heroin Craving After Voluntary Abstinence in Male and Female Rats

We recently introduced an animal model of incubation of methamphetamine craving after choice-based voluntary abstinence in male rats. Here we studied the generality of this phenomenon to (1) female rats, and (2) male and female rats with a history of heroin self-administration. We first trained rats to self-administer palatable food pellets for 6 days (6 h per day) for either methamphetamine (0.1 mg/kg/infusion) or heroin (0.1 mg/kg/infusion) for 12 days (6 h/day). We then assessed relapse to drug seeking under extinction conditions after 1 and 21 abstinence days. Between tests, the rats underwent either voluntary abstinence (achieved via a discrete choice procedure between drug and palatable food; 20 trials/day) or home-cage forced abstinence. We found no sex differences in methamphetamine self-administration or in the strong preference for the palatable food over methamphetamine during the choice-based voluntary abstinence. In both sexes, methamphetamine seeking in the relapse tests was higher after 21 days of either voluntary or forced abstinence than after 1 day (incubation of methamphetamine craving). We also found no sex differences in heroin self-administration or the strong preference for the palatable food over heroin during the choice-based voluntary abstinence. However, male and female rats with a history of heroin self-administration showed incubation of heroin craving after forced but not voluntary abstinence. Our results show that incubation of methamphetamine craving after voluntary abstinence generalizes to female rats. Unexpectedly, prolonged voluntary abstinence prevented the emergence of incubation of heroin craving in both sexes.

Preliminary evidence of sex differences in behavioral and neural responses to palatable food reward in rats

The female bias in eating disorder prevalence is the largest in all of psychiatry. Binge eating on palatable food (PF) is a core, maladaptive symptom that cuts across all major types of eating disorders and can be studied via animal models. Using an individual differences rat model of binge eating that identifies binge eating prone (BEP) and binge eating resistant (BER) phenotypes, we previously showed that, compared with males, females consume more PF and are more likely to be classified as BEP. One potential explanation for this sex difference is that PF is inherently more rewarding to females, leading to higher rates of binge eating. Here we tested the hypothesis that females have more robust behavioral and neural responses to PF reward than males. Adult male (N=18) and female (N=17) Sprague-Dawley rats were exposed to the Conditioned Place Preference paradigm using PF as the unconditioned stimulus. Select males (N=9) and females (N=9) were video-recorded during three of the PF-paired conditioning sessions to score feeding behavior. Following CPP, 13 male and 12 female rats were exposed to PF just prior to sacrifice to induce expression of the neural activation marker Fos, and Fos expression was quantified in mesocorticolimbic, hypothalamic, and amygdalar circuits. In the CPP paradigm, females displayed a more robust shift in preference for the chamber paired with PF compared with males, and behavioral analyses revealed that average duration of individual feeding bouts during pairing sessions was longer in females than in males. Fos expression was significantly higher in females vs. males in select regions of the mesocorticolimbic reward circuit, with no sex differences in hypothalamic or amygdalar regions. These results provide initial evidence that PF may be more rewarding to females than to males, possibly due to heightened responsiveness of neural substrates that mediate the hedonic and motivational responses to PF, which in part, may underlie sex differences in binge eating proneness.

Sex Differences in Binge Eating: Gonadal Hormone Effects Across Development

Eating disorders are highly sexually differentiated disorders that exhibit a female predominance in risk. Most theories focus on psychosocial explanations to the exclusion of biological/genetic influences. The purpose of this descriptive review is to evaluate evidence from animal and human studies in support of gonadal hormone effects on sex differences in binge eating. Although research is in its nascent stages, findings suggest that increased prenatal testosterone exposure in males appears to protect against binge eating. Although pubertal testosterone may exert additional protective effects, the prenatal period is likely critical for the decreased risk observed in males. By contrast, studies indicate that, in females, it is the lack of prenatal testosterone coupled with the organizational effects of pubertal ovarian hormones that may lead to increased binge eating. Finally, twin data suggest that changes in genetic risk may underlie these hormone influences on sex differences across development.

Sex differences in subjective and objective responses to a stimulant medication (methylphenidate): Comparisons between overweight/obese adults with and without binge-eating disorder

OBJECTIVE

The purpose of this study was to examine sex differences in response to a single dose of a psychomotor-stimulant medication (methylphenidate: MP) and to assess whether expected differences were moderated by binge-eating disorder (BED) status. It is anticipated that findings will shed light on factors that contribute to response variation in the use of stimulant pharmacotherapy to treat BED.

METHOD

The study employed a double-blind, drug-placebo, cross-over design in overweight/obese adults with BED (n = 90) and without BED (n = 108). Emotional/mood ratings were assessed every 15 minutes after oral administration of the drug/placebo, and appetite, cravings, and consumption were assessed during a laboratory-based snack-food challenge.

RESULTS

Women reported earlier and more sustained "overall" effects of the drug-including "feeling high"-than the men. There was also a significantly greater suppression in appetite ratings, food cravings, and food consumption from the placebo to the drug condition among the women. Indeed, among men there were no significant differences between the two conditions on any of the food-related variables. BED status also did not moderate any of the drug-placebo differences.

DISCUSSION

These findings are relevant to the use of stimulant pharmacotherapy for BED, and raise the possibility that overweight/obese men may be relatively less responsive to this form of treatment. © 2015 Wiley Periodicals, Inc. (Int J Eat Disord 2016; 49:473-481).

Removal of high-fat diet after chronic exposure drives binge behavior and dopaminergic dysregulation in female mice

A significant contributor to the obesity epidemic is the overconsumption of highly palatable, energy dense foods. Chronic intake of palatable foods is associated with neuroadaptations within the mesocorticolimbic dopamine system adaptations which may lead to behavioral changes, such as overconsumption or bingeing. We examined behavioral and molecular outcomes in mice that were given chronic exposure to a high-fat diet (HFD; 12weeks), with the onset of the diet either in adolescence or adulthood. To examine whether observed effects could be reversed upon removal of the HFD, animals were also studied 4weeks after a return to chow feeding. Most notably, female mice, particularly those exposed to HFD starting in adolescence, demonstrated the emergence of binge-like behavior when given restricted access to a palatable food. Further, changes in dopamine-related gene expression and dopamine content in the prefrontal cortex were observed. Some of these HFD-driven phenotypes reversed upon removal of the diet, whereas others were initiated by removal of the diet. These findings have implications for obesity management and interventions, as both pharmacological and behavioral therapies are often combined with dietary interventions (e.g., reduction in calorie dense foods).

Shared and unique mechanisms underlying binge eating disorder and addictive disorders

Scientific interest in "food addiction" is growing, but the topic remains controversial. One critique of "food addiction" is its high degree of phenotypic overlap with binge eating disorder (BED). In order to examine associations between problematic eating behaviors, such as binge eating and "food addiction," we propose the need to move past examining similarities and differences in symptomology. Instead, focusing on relevant mechanisms may more effectively determine whether "food addiction" contributes to disordered eating behavior for some individuals. This paper reviews the evidence for mechanisms that are shared (i.e., reward dysfunction, impulsivity) and unique for addiction (i.e., withdrawal, tolerance) and eating disorder (i.e., dietary restraint, shape/weight concern) frameworks. This review will provide a guiding framework to outline future areas of research needed to evaluate the validity of the "food addiction" model and to understand its potential contribution to disordered eating.

Behavioral and hormonal responses to stress in binge-like eating prone female rats

Binge eating episodes are frequently stimulated by stress. We developed a model of binge eating proneness based on individual sensitivity of young female Sprague Dawley rats to significantly increase sucrose consumption in response to stress. The rats were subjected to unpredictable intermittent 1-h access to 10% sucrose. After the stabilization of sucrose intake, rats were assessed for consistency of higher (for binge-like eating prone, BEP) or lower (for binge-like eating resistant, BER) sucrose intake in response to unpredictable episodes of foot-shock stress. The objectives of this study included demonstrating face validity of the BEP model and determining if some of the features of this model were pre-existing before exposure to intermittent access to sucrose and repeated stress. The BEP rats consumed a larger (20%>BER) amount of sucrose in a discrete (1-h) period of time compared to the BER phenotype in non-stressful conditions and significantly increased sucrose intake (50%>BER) under stress. Conversely, stress did not affect sucrose intake in BER rats. BEP rats showed higher sucrose intake compared to BER rats at the beginning of darkness as well as during the light period when they were sated and not physically hungry. Analyses of the sucrose licking microstructure revealed that BEP rats had a high motivational drive to consume sucrose in non-stressful condition and an increased hedonic value of sucrose when they were exposed to stressful conditions. BEP rats consumed sucrose much more rapidly under stressful conditions compared to BER rats. Finally, BEP rats demonstrated compulsive-like intake of sucrose (assessed in the light-dark box) and a blunted stress-induced increase in plasma corticosterone levels. Body weight and chow intake were not different between the phenotypes. Before exposure to intermittent access to sucrose and repeated stress, the BEP rats showed no clear evidence for compulsive sucrose intake. However, from the first 1-h access to sucrose, the BEP rats exhibited sucrose overeating; and from the first exposure to stress before intermittent access to sucrose, the BEP rats showed a blunted increase in corticosterone plasma levels. Innate sucrose hyperconsumption and altered reactivity of the hypothalamo-pituitary adrenal (HPA) axis to stress may be involved in the development of binge-like eating. Increased perceived hedonic value of palatable food and an increased motivation to consume this food despite aversive conditions as well as deregulated reactivity of the HPA axis may contribute to stress-induced bingeing on sucrose in BEP rats.

The epidemiology and genetics of binge eating disorder (BED)

This narrative review provides an overview of the epidemiology of binge eating disorder (BED), highlighting the medical history of this disorder and its entry as an independent condition in the Feeding and Eating Disorders section of the recently published Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition. Estimates of prevalence are provided, as well as recognition that the female to male ratio is lower in BED than in other eating disorders. Evidence is also provided of the most common comorbidities of BED, including mood and anxiety disorders and a range of addiction disorders. In addition, discussion of the viewpoint that BED itself may be an addiction - at least in severe cases - is presented. Although the genetic study of BED is still in its infancy, current research is reviewed with a focus on certain neurotransmitter genes that regulate brain reward mechanisms. To date, a focal point of this research has been on the dopamine and the μ-opioid receptor genes. Preliminary evidence suggests that a predisposing risk factor for BED may be a heightened sensitivity to reward, which could manifest as a strong dopamine signal in the brain's striatal region. Caution is encouraged, however, in the interpretation of current findings, since samples are relatively small in much of the research. To date, no genome-wide association studies have focused exclusively on BED.

Role of relaxin-3/RXFP3 system in stress-induced binge-like eating in female rats

Binge eating is frequently stimulated by stress. The neuropeptide relaxin-3 (RLN3) and its native receptor RXFP3 are implicated in stress and appetitive behaviors. We investigated the dynamics of the central RLN3/RXFP3 system in a newly established model of stress-induced binge eating. Female Sprague-Dawley rats were subjected to unpredictable intermittent 1-h access to 10% sucrose. When sucrose intake stabilized, rats were assessed for consistency of higher or lower sucrose intake in response to three unpredictable episodes of foot-shock stress; and assigned as binge-like eating prone (BEP) or binge-like eating resistant (BER). BEP rats displayed elevated consumption of sucrose under non-stressful conditions (30% > BER) and an additional marked increase in sucrose intake (60% > BER) in response to stress. Conversely, sucrose intake in BER rats was unaltered by stress. Chow intake was similar in both phenotypes on 'non-stress' days, but was significantly reduced by stress in BER, but not BEP, rats. After stress, BEP, but not BER, rats displayed a significant increase in RLN3 mRNA levels in the nucleus incertus. In addition, in response to stress, BEP, but not BER, rats had increased RXFP3 mRNA levels in the paraventricular and supraoptic nuclei of the hypothalamus. Intracerebroventricular administration of a selective RXFP3 antagonist, R3(B1-22)R, blocked the stress-induced increase in sucrose intake in BEP rats and had no effect on sucrose intake in BER rats. These results provide important evidence for a role of the central RLN3/RXFP3 system in the regulation of stress-induced binge eating in rats, and have therapeutic implications for eating disorders.

Differential mesocorticolimbic responses to palatable food in binge eating prone and binge eating resistant female rats

Binge eating is a key symptom of many eating disorders (e.g. binge eating disorder, bulimia nervosa, anorexia nervosa binge/purge type), yet the neurobiological underpinnings of binge eating are poorly understood. The mesocorticolimbic reward circuit, including the nucleus accumbens and the medial prefrontal cortex, is likely involved because this circuit mediates the hedonic value and incentive salience of palatable foods (PF). Here we tested the hypothesis that higher propensity for binge eating is associated with a heightened response (i.e., Fos induction) of the nucleus accumbens and medial prefrontal cortex to PF, using an animal model that identifies binge eating prone (BEP) and binge eating resistant (BER) rats. Forty adult female Sprague-Dawley rats were given intermittent access to PF (high fat pellets) 3×/week for 3 weeks. Based on a pattern of either consistently high or consistently low PF consumption across these feeding tests, 8 rats met criteria for categorization as BEP, and 11 rats met criteria for categorization as BER. One week after the final feeding test, BEP and BER rats were either exposed to PF in their home cages or were given no PF in their home cages for 1h prior to perfusion, leading to three experimental groups for the Fos analysis: BEPs given PF, BERs given PF, and a No PF control group. The total number of Fos-immunoreactive (Fos-ir) cells in the nucleus accumbens core and shell, and the cingulate, prelimbic, and infralimbic regions of the medial prefrontal cortex was estimated by stereological analysis. PF induced higher Fos expression in the nucleus accumbens shell and core and in the prelimbic and infralimbic cortex of BEP rats compared to No PF controls. Throughout the nucleus accumbens and medial prefrontal cortex, PF induced higher Fos expression in BEP than in BER rats, even after adjusting for differences in PF intake. Differences in the neural activation pattern between BEP and BER rats were more robust in prefrontal cortex than in nucleus accumbens. These data confirm that PF activates brain regions responsible for encoding the incentive salience and hedonic properties of PF, and suggest that binge eating proneness is associated with enhanced responses to PF in brain regions that exert executive control over food reward.

Developmental emergence of an obsessive-compulsive phenotype and binge behavior in rats

RATIONALE

Obsessive-compulsive disorder (OCD) gradually emerges and reaches clinical significance during early adulthood. Whether a predisposition for OCD manifests as binge eating disorder earlier during adolescence is proposed.

OBJECTIVES

To further characterize how OCD-like behaviors increase across maturation and to determine whether an OCD-like predisposition increases the likelihood of binge eating during adolescence.

METHODS

Male and female Sprague-Dawley rats were injected with the tricyclic antidepressant clomipramine (CMI, 15 mg/kg) or saline vehicle twice daily between postnatal days 9-15. Both groups were tested for perseverative (spontaneous alternation) and anxiety-like (elevated plus maze; marble burying) behaviors during juvenility (day 28), adolescence (day 60), and adulthood (day 90). Both motivations to eat sucrose pellets and binge eating on fat were investigated.

RESULTS

Sex- and age-dependent increases in anxiety-like and perseverative behavior were observed in CMI subjects. Differences in consummatory behaviors emerged during late adolescence, while no significant differences in alternation or anxiety-like behaviors were detected between CMI and vehicle animals until adulthood. Adolescent CMI females consumed more sucrose pellets in 30 min relative to vehicle females, whereas adolescent CMI males consumed approximately half as much as vehicle males. Sucrose consumption did not differ between groups in adulthood. Adolescent CMI rats demonstrated more fat bingeing than vehicles, independent of sex.

CONCLUSIONS

OCD-like behaviors are emerging during adolescence, but sucrose consumption and fat bingeing in CMI-treated animals significantly precedes the appearance of anxiety and perseveration. This OCD-like phenotype emerges fully during adulthood, suggesting that eating may likely serve as a coping strategy in these animals.

Mice that gorged during dietary restriction increased foraging related behaviors and differed in their macronutrient preference when released from restriction

Caloric restriction (CR) can trigger gorging behavior. We examined macronutrient choice and behavior in mice that gorged during restriction compared to restricted non-gorgers and controls. Fifty MF1 male mice were restricted to 75% of ad-libitum food intake (FI), while ten controls were fed ad-lib. Body mass (BM) and FI were measured two and 24-h after food inclusion over 14-days. ‘Gorging’ mice were defined as those which ate over 25% of their daily FI in 2-h. The top 11 gorgers and the lowest 9 gorgers, along with 10 controls, had their behavior analysed during restriction, and were then provided with an unrestricted food choice, consisting of three diets that were high in fat, protein or carbohydrate. During restriction gorgers ate on average 51% of their daily FI in the 2-h following food introduction while the non-gorgers ate only 16%. Gorgers lost significantly more BM than non-gorgers possibly due to an increased physical activity linked to anticipation of daily food provision. Controls and non-gorgers spent most of their time sleeping. After restriction, both gorgers and non-gorgers were hyperphagic until their lost weight was regained. All 3 groups favoured high fat food. Gorgers and non-gorgers had a significantly greater high carbohydrate diet intake than controls, and gorgers also had a significantly greater high protein diet intake than non-gorgers and controls. On unrestricted food, they did not continue to gorge, although they still had a significantly greater 2-h FI than the other groups. Elevated protein intake may play an important role in the recovery of the lost lean tissue of gorgers after restriction.

Current considerations regarding food addiction

"Food addiction" is an emerging area, and behavioral and biological overlaps have been observed between eating and addictive disorders. Potential misconceptions about applying an addiction framework to problematic eating behavior may inhibit scientific progress. Critiques of "food addiction" that focus on descriptive differences between overeating and illicit drugs are similar to early criticisms of the addictiveness of tobacco. Although food is necessary for survival, the highly processed foods associated with addictive-like eating may provide little health benefit. Individual differences are important in determining who develops an addiction. If certain foods are addictive, the identification of possible risk factors for "food addiction" is an important next step. Not all treatments for addiction require abstinence. Addiction interventions that focus on moderation or controlled use may lead to novel approaches to treating eating-related problems. Finally, addiction-related policies that focus on environmental (instead of educational) targets may have a larger public health impact in reducing overeating.

Which foods may be addictive? The roles of processing, fat content, and glycemic load

Objectives

We propose that highly processed foods share pharmacokinetic properties (e.g. concentrated dose, rapid rate of absorption) with drugs of abuse, due to the addition of fat and/or refined carbohydrates and the rapid rate the refined carbohydrates are absorbed into the system, indicated by glycemic load (GL). The current study provides preliminary evidence for the foods and food attributes implicated in addictive-like eating.

Design

Cross-sectional.

Setting

University (Study One) and community (Study Two).

Participants

120 undergraduates participated in Study One and 384 participants recruited through Amazon MTurk participated in Study Two.

Measurements

In Study One, participants (n = 120) completed the Yale Food Addiction Scale (YFAS) followed by a forced-choice task to indicate which foods, out of 35 foods varying in nutritional composition, were most associated with addictive-like eating behaviors. Using the same 35 foods, Study Two utilized hierarchical linear modeling to investigate which food attributes (e.g., fat grams) were related to addictive-like eating behavior (at level one) and explored the influence of individual differences for this association (at level two).

Results

In Study One, processed foods, higher in fat and GL, were most frequently associated with addictive-like eating behaviors. In Study Two, processing was a large, positive predictor for whether a food was associated with problematic, addictive-like eating behaviors. BMI and YFAS symptom count were small-to-moderate, positive predictors for this association. In a separate model, fat and GL were large, positive predictors of problematic food ratings. YFAS symptom count was a small, positive predictor of the relationship between GL and food ratings.

Conclusion

The current study provides preliminary evidence that not all foods are equally implicated in addictive-like eating behavior, and highly processed foods, which may share characteristics with drugs of abuse (e.g. high dose, rapid rate of absorption) appear to be particularly associated with “food addiction.”

Our stolen figures: the interface of sexual differentiation, endocrine disruptors, maternal programming, and energy balance

This article is part of a Special Issue "Energy Balance". The prevalence of adult obesity has risen markedly in the last quarter of the 20th century and has not been reversed in this century. Less well known is the fact that obesity prevalence has risen in domestic, laboratory, and feral animals, suggesting that all of these species have been exposed to obesogenic factors present in the environment. This review emphasizes interactions among three biological processes known to influence energy balance: Sexual differentiation, endocrine disruption, and maternal programming. Sexual dimorphisms include differences between males and females in body weight, adiposity, adipose tissue distribution, ingestive behavior, and the underlying neural circuits. These sexual dimorphisms are controlled by sex chromosomes, hormones that masculinize or feminize adult body weight during perinatal development, and hormones that act during later periods of development, such as puberty. Endocrine disruptors are natural and synthetic molecules that attenuate or block normal hormonal action during these same developmental periods. A growing body of research documents effects of endocrine disruptors on the differentiation of adipocytes and the central nervous system circuits that control food intake, energy expenditure, and adipose tissue storage. In parallel, interest has grown in epigenetic influences, including maternal programming, the process by which the mother's experience has permanent effects on energy-balancing traits in the offspring. This review highlights the points at which maternal programming, sexual differentiation, and endocrine disruption might dovetail to influence global changes in energy balancing traits.

Binge-like consumption of a palatable food accelerates habitual control of behavior and is dependent on activation of the dorsolateral striatum

Access to highly palatable and calorically dense foods contributes to increasing rates of obesity worldwide. Some have made the controversial argument that consumption of such foods can lead to "food addiction," yet little is known about how long-term access to highly palatable foods might alter goal-directed learning and decision making. In the following experiments, rats were given 5 weeks of continuous or restricted daily access to sweetened condensed milk (SCM) before instrumental training for food reward. Subsequently we examined whether goal-directed performance was impaired in these groups using the outcome-devaluation task. Control rats reduced responding following devaluation of the earned outcome as did those with previous continuous access to SCM. Of interest, rats with previous restricted access to SCM responded similarly under the devalued and nondevalued conditions, indicating loss of goal-directed control of responding. To identify whether the loss of goal-directed control was accompanied by differences in neuronal activity, we used c-Fos immunohistochemistry to examine the patterns of activation during devaluation testing. We observed greater c-Fos immunoreactivity in the dorsolateral striatum (DLS) and associated cortical regions in the group that received previous restricted access to SCM and demonstrated a lack of sensitivity to outcome devaluation. Infusion of the AMPA-receptor antagonist CNQX or dopamine D1-receptor antagonist SCH-23390 into the DLS before testing restored goal-directed performance in the restricted SCM group, confirming that this region is essential for habit-based performance. These results indicate that previous diet can alter subsequent learning and activity in the neural circuits that support performance.

Ovarian Hormone Influences on Dysregulated Eating: A Comparison of Associations in Women with versus without Binge Episodes

Changes in ovarian hormones predict changes in emotional eating across the menstrual cycle. However, prior studies have not examined whether the nature of associations varies across dysregulated eating severity. The current study determined whether the strength and/or nature of hormone/dysregulated eating associations differ based on the presence of clinically diagnosed binge episodes (BEs). Participants included 28 women with BEs and 417 women without BEs who provided salivary hormone samples, ratings of emotional eating, and BE frequency for 45 days. Results revealed stronger associations between dysregulated eating and ovarian hormones in women with BEs as compared to women without BEs. The nature of associations also differed, as progesterone moderated the effects of lower estradiol levels on dysregulated eating in women with BEs only. Although hormone/dysregulated eating associations are present across the spectrum of pathology, the nature of associations may vary in ways that have implications for etiological models and treatment.

Gender differences in food craving among overweight and obese patients attending low energy diet therapy: a matched case-control study

This case-control study examined gender differences in food craving among a sample of overweight and obese patients attending low energy diet therapy. To disentangle the specific role of gender from the role of confounders, we paired groups for BMI, age and severity of binge eating as assed by the Binge Eating Scale (BES). The participants were 73 pairs of patients who were attending low energy diet therapy. All the participants were administered the State and Trait Food Cravings Questionnaire, trait version (FCQ-T) and the BES. Female patients had higher mean scores on six out of nine dimensions of the FCQ-T. When controlling for the effect of other variables, obese and overweight female patients were 1.1 times more likely to report higher anticipation of relief of negative states and feelings from eating than their male pairs. Obese and overweight female patients experience more cravings for food than their male pairs despite comparable severity of binge eating and obesity suggesting the need for tailored interventions.

The effects of ovariectomy on binge eating proneness in adult female rats

Ovarian hormones are associated with binge eating in women, however findings are limited by the lack of experimental control inherent in human studies. Animal research that manipulates ovarian hormone status and examines individual differences in extreme binge eating proneness is needed to model clinical phenotypes in humans and to confirm causal effects. The purpose of this study was to examine the effects of adult ovariectomy on overall binge eating risk and extreme binge eating phenotypes using the binge eating resistant (BER)/binge eating prone (BEP) rat model. We predicted that palatable food consumption would significantly increase after ovariectomy in all rats because ovarian hormones generally suppress food intake. If differences in responsiveness to ovarian hormones underlie BER/BEP phenotypes, then differences in binge eating between BER and BEP rats would be eliminated or diminished after ovariectomy. Changes in palatable food (PF) intake were compared in BER and BEP rats before and after ovariectomy in two samples of adult females. Findings were highly similar in the two samples. PF intake increased significantly following ovariectomy in all rats. However, BEP rats consistently consumed larger amounts of PF than BER rats, both before and after ovariectomy. The consistency of findings across two samples of rats provides strong support for activational effects of ovarian hormones on binge eating. However, the immunity of extreme binge eating phenotypes to ovarian hormone ablation suggests that other, earlier mechanisms (e.g., organizational hormone effects or hormone-independent effects) determine the expression of binge eating phenotypes.