Association of body mass and brain activation during gastric distention: implications for obesity

Neuroimaging model of visceral manipulation in awake rat

Reciprocal neuronal connections exist between the internal organs of the body and the nervous system. These projections to and from the viscera play an essential role in maintaining and finetuning organ responses in order to sustain homeostasis and allostasis. Functional maps of brain regions participating in this bidirectional communication have been previously studied in awake humans and anesthetized rodents. To further refine the mechanistic understanding of visceral influence on brain states, however, new paradigms that allow for more invasive, and ultimately more informative, measurements and perturbations must be explored. Further, such paradigms should prioritize human translatability. In the current paper, we address these issues by demonstrating the feasibility of non-anesthetized animal imaging during visceral manipulation. More specifically, we used a barostat interfaced with an implanted gastric balloon to cyclically induce distension of a non-anesthetized rat's stomach during simultaneous BOLD fMRI. General linear modeling and spatial independent component analysis revealed several regions with BOLD activation temporally coincident with the gastric distension stimulus. The ON-OFF (20 mmHg - 0 mmHg) barostat-balloon pressure cycle resulted in widespread BOLD activation of the inferior colliculus, cerebellum, ventral midbrain, and a variety of hippocampal structures. These results suggest that neuroimaging models of gastric manipulation in the non-anesthetized rat are achievable and provide an avenue for more comprehensive studies involving the integration of other neuroscience techniques like electrophysiology.

Significance Statement

It is unclear to what extent measurements of brain activity are affected by background, and experimentally unrelated, interoceptive processes. To advance our understanding of ongoing visceral activity's influence on brain states, here we provide a proof of concept, anesthesia-free animal model of visceral manipulation during simultaneous BOLD fMRI. We successfully demonstrated BOLD activation during gastric distension of the unanesthetized rat in both classically reported (cerebellum, hippocampus) and novel (inferior colliculus) regions. This paradigm establishes an important foundation for further interrogation of viscera-brain interactions.

Structural Connectivity between Olfactory Tubercle and Ventrolateral Periaqueductal Gray Implicated in Human Feeding Behavior

The olfactory tubercle (TUB), also called the tubular striatum, receives direct input from the olfactory bulb and, along with the nucleus accumbens, is one of the two principal components of the ventral striatum. As a key component of the reward system, the ventral striatum is involved in feeding behavior, but the vast majority of research on this structure has focused on the nucleus accumbens, leaving the TUB's role in feeding behavior understudied. Given the importance of olfaction in food seeking and consumption, olfactory input to the striatum should be an important contributor to motivated feeding behavior. Yet the TUB is vastly understudied in humans, with very little understanding of its structural organization and connectivity. In this study, we analyzed macrostructural variations between the TUB and the whole brain and explored the relationship between TUB structural pathways and feeding behavior, using body mass index (BMI) as a proxy in females and males. We identified a unique structural covariance between the TUB and the periaqueductal gray (PAG), which has recently been implicated in the suppression of feeding. We further show that the integrity of the white matter tract between the two regions is negatively correlated with BMI. Our findings highlight a potential role for the TUB-PAG pathway in the regulation of feeding behavior in humans.

The Cerebellar Response to Visual Portion Size Cues Is Associated with the Portion Size Effect in Children

The neural mechanisms underlying susceptibility to eating more in response to large portions (i.e., the portion size effect) remain unclear. Thus, the present study examined how neural responses to portion size relate to changes in weight and energy consumed as portions increase. Associations were examined across brain regions traditionally implicated in appetite control (i.e., an appetitive network) as well as the cerebellum, which has recently been implicated in appetite-related processes. Children without obesity (i.e., BMI-for-age-and-sex percentile < 90; N = 63; 55% female) viewed images of larger and smaller portions of food during fMRI and, in separate sessions, ate four meals that varied in portion size. Individual-level linear and quadratic associations between intake (kcal, grams) and portion size (i.e., portion size slopes) were estimated. The response to portion size in cerebellar lobules IV-VI was associated with the quadratic portion size slope estimated from gram intake; a greater response to images depicting smaller compared to larger portions was associated with steeper increases in intake with increasing portion sizes. Within the appetitive network, neural responses were not associated with portion size slopes. A decreased cerebellar response to larger amounts of food may increase children's susceptibility to overeating when excessively large portions are served.

Cerebellar Prediction and Feeding Behaviour

Given the importance of the cerebellum in controlling movements, it might be expected that its main role in eating would be the control of motor elements such as chewing and swallowing. Whilst such functions are clearly important, there is more to eating than these actions, and more to the cerebellum than motor control. This review will present evidence that the cerebellum contributes to homeostatic, motor, rewarding and affective aspects of food consumption.Prediction and feedback underlie many elements of eating, as food consumption is influenced by expectation. For example, circadian clocks cause hunger in anticipation of a meal, and food consumption causes feedback signals which induce satiety. Similarly, the sight and smell of food generate an expectation of what that food will taste like, and its actual taste will generate an internal reward value which will be compared to that expectation. Cerebellar learning is widely thought to involve feed-forward predictions to compare expected outcomes to sensory feedback. We therefore propose that the overarching role of the cerebellum in eating is to respond to prediction errors arising across the homeostatic, motor, cognitive, and affective domains.

Fetal programming of human energy homeostasis brain networks: Issues and considerations

In this paper, we present a transdisciplinary framework and testable hypotheses regarding the process of fetal programming of energy homeostasis brain circuitry. Our model proposes that key aspects of energy homeostasis brain circuitry already are functional by the time of birth (with substantial interindividual variation); that this phenotypic variation at birth is an important determinant of subsequent susceptibility for energy imbalance and childhood obesity risk; and that this brain circuitry exhibits developmental plasticity, in that it is influenced by conditions during intrauterine life, particularly maternal-placental-fetal endocrine, immune/inflammatory, and metabolic processes and their upstream determinants. We review evidence that supports the scientific premise for each element of this formulation, identify future research directions, particularly recent advances that may facilitate a better quantification of the ontogeny of energy homeostasis brain networks, highlight animal and in vitro-based approaches that may better address the determinants of interindividual variation in energy homeostasis brain networks, and discuss the implications of this formulation for the development of strategies targeted towards the primary prevention of childhood obesity.

Of Mice and Men: Impacts of Calorie Restriction on Metabolomics of the Cerebellum

The main purpose of research in mice is to explore metabolic changes in animal models and then predict or propose potential translational benefits in humans. Although some researchers in the brain research field have mentioned that the mouse experiments results still lack the complex neuroanatomy of humans, caution is required to interpret the findings. In mice, we observed in article seventeenth of the series of the effects of graded levels of calorie restriction, metabolomic changes in the cerebellum indicated activation of hypothalamocerebellar connections driven by hunger responses. Therefore, the purpose of the current perspective is to set this latest paper into a wider context of the physiological, behavioral, and molecular changes seen in these mice and to compare and contrast them with previous human studies.

Gastrointestinal Vagal Afferents and Food Intake: Relevance of Circadian Rhythms

Gastrointestinal vagal afferents (VAs) play an important role in food intake regulation, providing the brain with information on the amount and nutrient composition of a meal. This is processed, eventually leading to meal termination. The response of gastric VAs, to food-related stimuli, is under circadian control and fluctuates depending on the time of day. These rhythms are highly correlated with meal size, with a nadir in VA sensitivity and increase in meal size during the dark phase and a peak in sensitivity and decrease in meal size during the light phase in mice. These rhythms are disrupted in diet-induced obesity and simulated shift work conditions and associated with disrupted food intake patterns. In diet-induced obesity the dampened responses during the light phase are not simply reversed by reverting back to a normal diet. However, time restricted feeding prevents loss of diurnal rhythms in VA signalling in high fat diet-fed mice and, therefore, provides a potential strategy to reset diurnal rhythms in VA signalling to a pre-obese phenotype. This review discusses the role of the circadian system in the regulation of gastrointestinal VA signals and the impact of factors, such as diet-induced obesity and shift work, on these rhythms.

Neural correlates of future weight loss reveal a possible role for brain-gastric interactions

Lifestyle dietary interventions are an essential practice in treating obesity, hence neural factors that may assist in predicting individual treatment success are of great significance. Here, in a prospective, open-label, three arms study, we examined the correlation between brain resting-state functional connectivity measured at baseline and weight loss following 6 months of lifestyle intervention in 92 overweight participants. We report a robust subnetwork composed mainly of sensory and motor cortical regions, whose edges correlated with future weight loss. This effect was found regardless of intervention group. Importantly, this main finding was further corroborated using a stringent connectivity-based prediction model assessed with cross-validation thus attesting to its robustness. The engagement of senso-motor regions in this subnetwork is consistent with the over-sensitivity to food cues theory of weight regulation. Finally, we tested an additional hypothesis regarding the role of brain-gastric interaction in this subnetwork, considering recent findings of a cortical network synchronized with gastric activity. Accordingly, we found a significant spatial overlap with the subnetwork reported in the present study. Moreover, power in the gastric basal electric frequency within our reported subnetwork negatively correlated with future weight loss. This finding was specific to the weight loss related subnetwork and to the gastric basal frequency. These findings should be further corroborated by combining direct recordings of gastric activity in future studies. Taken together, these intriguing results may have important implications for our understanding of the etiology of obesity and the mechanism of response to dietary intervention.

Is obesity related to enhanced neural reactivity to visual food cues? A review and meta-analysis

Theoretical work suggests that obesity is related to enhanced incentive salience of food cues. However, evidence from both behavioral and neuroimaging studies on the topic is mixed. In this work we review the literature on cue reactivity in obesity and perform a preregistered meta-analysis of studies investigating effects of obesity on brain responses to passive food pictures viewing. Further, we examine whether age influences brain responses to food cues in obesity. In the meta-analysis we included 13 studies of children and adults that investigated group differences (obese vs. lean) in responses to food vs. non-food pictures viewing. While we found no significant differences in the overall meta-analysis, we show that age significantly influences brain response differences to food cues in the left insula and the left fusiform gyrus. In the left insula, obese vs. lean brain differences in response to food cues decreased with age, while in the left fusiform gyrus the pattern was opposite. Our results suggest that there is little evidence for obesity-related differences in responses to food cues and that such differences might be mediated by additional factors that are often not considered.

Differences in brain structure and function in children with the FTO obesity-risk allele

OBJECTIVE

Noncoding alleles of the fat mass and obesity-associated (FTO) gene have been associated with obesity risk, yet the underlying mechanisms remain unknown. Risk allele carriers show alterations in brain structure and function, but previous studies have not disassociated the effects of genotype from those of body mass index (BMI).

METHODS

Differences in brain structure and function were examined in children without obesity grouped by their number of copies (0,1,2) of the FTO obesity-risk single-nucleotide polymorphism (SNP) rs1421085. One hundred five 5- to 10-year-olds (5th-95th percentile body fat) were eligible to participate. Usable scans were obtained from 93 participants (15 CC [homozygous risk], 31 CT [heterozygous] and 47 TT [homozygous low risk]).

RESULTS

Homozygous C allele carriers (CCs) showed greater grey matter volume in the cerebellum and temporal fusiform gyrus. CCs also demonstrated increased bilateral cerebellar white matter fibre density and increased resting-state functional connectivity between the bilateral cerebellum and regions in the frontotemporal cortices.

CONCLUSIONS

This is the first study to examine brain structure and function related to FTO alleles in young children not yet manifesting obesity. This study lends support to the notion that the cerebellum may be involved in FTO-related risk for obesity, yet replication and further longitudinal study are required.

Implementation of a New Food Picture Database in the Context of fMRI and Visual Cognitive Food-Choice Task in Healthy Volunteers

This pilot study aimed at implementing a new food picture database in the context of functional magnetic resonance imaging (fMRI) cognitive food-choice task, with an internal conflict or not, in healthy normal-weight adults. The database contains 170 photographs including starters, main courses, and desserts; it presents a broad-spectrum of energy content and is provided with portion weight and nutritional information. It was tested in 16 participants who evaluated the energy density and gave a liking score for all food pictures via numerical scales. First, volunteers were segregated into two groups according to their eating habits according to a food consumption frequency questionnaire (FCFQ) to assess whether the database might elicit different appreciations according to individual eating habits. Second, participants underwent fMRI cognitive food-choice task (van der Laan et al., 2014), using our picture database, in which they had to choose between high-energy (HE) and low-energy (LE) foods, under a similar liking (SL, foods with similar hedonic appraisals) condition or a different liking (DL, foods with different hedonic appraisals) condition. Participants evaluated correctly the caloric content of dishes (from r = 0.72 to r = 0.79, P < 0.001), confirming a good perception of the caloric discrepancies between food pictures. Two subgroups based on FCFQ followed by a principal component analysis (PCA) and a hierarchical ascendant classification (HAC) were defined, that is, Prudent-type (PTc, N = 9) versus Western-type (WTc, N = 7) consumers, where the WTc group showed higher consumption of HE palatable foods than PTc (P < 0.05). The WTc group showed a higher correlation between liking and caloric evaluation of the food pictures as compared to PTc (r = 0.77 and r = 0.36, respectively, P < 0.001), confirming that food pictures elicited variable responses according to contrasted individual eating habits. The fMRI analyses showed that the DL condition elicited the activation of dorsal anterior cingulate cortex (dACC), involved in internal conflict monitoring, whereas SL condition did not, and that LE food choice involved high-level cognitive processes with higher activation of the hippocampus (HPC) and fusiform gyrus compared to HE food choice. Overall, this pilot study validated the use of the food picture database and fMRI-based procedure assessing decision-making processing during a food choice cognitive task with and without internal conflict.

Prefronto-cerebellar neuromodulation affects appetite in obesity

Human neuroimaging studies have consistently reported changes in cerebellar function and integrity in association with obesity. To date, however, the nature of this link has not been studied directly. Emerging evidence suggests a role for the cerebellum in higher cognitive functions through reciprocal connections with the prefrontal cortex. The purpose of this exploratory study was to examine appetite changes associated with noninvasive prefronto-cerebellar neuromodulation in obesity. 12 subjects with class I obesity (mean BMI 32.9 kg/m²) underwent a randomized, single-blinded, sham-controlled, crossover study, during which they received transcranial direct current stimulation (tDCS; active/sham) aimed at simultaneously enhancing the activity of the prefrontal cortex and decreasing the activity of the cerebellum. Changes in appetite (state and food-cue-triggered) and performance in a food-modified working memory task were evaluated. We found that active tDCS caused an increase in hunger and desire to eat following food-cue exposure. In line with these data, subjects also tended to make more errors during the working memory task. No changes in basic motor performance occurred. This study represents the first demonstration that prefronto-cerebellar neuromodulation can influence appetite in individuals with obesity. While preliminary, our findings support a potential role for prefronto-cerebellar pathways in the behavioral manifestations of obesity.

Body mass variability is represented by distinct functional connectivity patterns

Understanding weight-related differences in functional connectivity provides key insight into neurocognitive factors implicated in obesity. Here, we sampled three groups from human connectome project data: 1) 47 pairs of BMI-discordant twins (n = 94; average BMI-discordancy 6.7 ± 3.1 kg/m2), 2) 47 pairs of gender and BMI matched BMI-discordant, unrelated individuals, and 3) 47 pairs of BMI-similar twins, to test for body mass dependent differences in between network functional connectivity. Across BMI discordant samples, three networks appeared to be highly sensitive to weight status; specifically, a network comprised of gustatory processing regions, a visual processing network, and the default mode network (DMN). Further, in the BMI-discordant twin sample, twins with lower BMI had stronger connectivity between striatal/thalamic and prefrontal networks (pFWE = 0.04). We also observed that individuals with a higher BMI than their twin had stronger connectivity between cerebellar and insular networks (pFWE = 0.04). Connectivity patterns observed in the BMI-discordant twin sample were not seen in a BMI-similar sample, providing evidence that the results are specific to BMI discordance. Beyond the involvement of gustatory and visual networks and the DMN, little overlap in results were seen between the two BMI-discordant samples. In concordance with previous findings, we hypothesize that stronger cortical-striatal-thalamic connectivity associated with lower body mass in twins may facilitate increased regulation of hedonically motivated behaviors. In twins with higher body mass, increased cerebellar-insula connectivity may be associated with compromised satiation signaling, an interpretation dovetailing prior research. The lack of overlapping results between the two BMI discordant samples may be a function of higher study design sensitivity in the BMI-discordant twin sample, relative to the more generalizable results in the unrelated sample. These findings demonstrate that distinct connectivity patterns can represent weight variability, adding to mounting evidence that implicates atypical brain functioning with the accumulation and/or maintenance of elevated weight.

Visceral adiposity and insular networks: associations with food craving

BACKGROUND/OBJECTIVES

Accumulation of visceral adiposity can disrupt the brain's sensitivity to interoceptive feedback, which is coded in the insula. This study aimed to test the link between visceral fat and the functional connectivity of two insulae regions relevant for eating behavior: the middle-dorsal insula (mIns), which codes homeostatic changes, and the rostral insula (rIns), which codes stable representations of food properties. We also assessed the impact of visceral adiposity-associated insulae networks on food craving.

SUBJECTS/METHODS

Seventy-five adults ranging in weight status (normal and excess weight) underwent resting-state functional magnetic resonance imaging (fMRI) and subjective food craving measures. We examined the association between visceral fat and seed-based functional connectivity of the mIns and the rIns, controlling for BMI, age, and sex, using multiple regressions in SPM8. We also tested if visceral fat mediated the association between insulae connectivity and food craving.

RESULTS

Higher visceral adiposity was associated with decreased connectivity between the mIns and a cluster involving the hypothalamus and the bed nucleus of the stria terminalis. Decreased connectivity in this network was associated with greater food craving, a relation mediated by visceral adiposity. Visceral adiposity was also associated with increased connectivity between the mIns and the middle frontal gyri and the right intraparietal cortex, and between the rIns and the right amygdala.

CONCLUSIONS

Accumulation of visceral adiposity is linked to disrupted functional connectivity within the mIns and rIns networks. Furthermore, the link between the mIns network and food craving is mediated by visceral fat. Findings suggest that visceral fat disrupts insula coding of bodily homeostatic signals, which may boost externally driven food cravings.

Early Life Stress Associated With Increased Striatal N-Acetyl-Aspartate: Cerebrospinal Fluid Corticotropin-Releasing Factor Concentrations, Hippocampal Volume, Body Mass and Behavioral Correlates

Introduction

Using proton magnetic resonance spectroscopy imaging, the effects of early life stress on nonhuman primate striatal neuronal integrity were examined as reflected by N-acetyl aspartate (NAA) concentrations. NAA measures were interrogated through examining their relationship to previously documented early life stress markers—cerebrospinal fluid corticotropin-releasing factor concentrations, hippocampal volume, body mass, and behavioral timidity. Rodent models of depression exhibit increases in neurotrophic effects in the nucleus accumbens. We hypothesized that rearing under conditions of early life stress (variable foraging demand, VFD) would produce persistent elevations of NAA concentrations (in absolute or ratio form) in ventral striatum/caudate nucleus (VS/CN) with altered correlation to early life stress markers.

Methods

Eleven bonnet macaque males reared under VFD conditions and seven age-matched control subjects underwent proton magnetic resonance spectroscopy imaging during young adulthood. Voxels were placed over VS/CN to capture nucleus accumbens. Cisternal cerebrospinal fluid corticotropin-releasing factor concentrations, hippocampal volume, body mass, and response to a human intruder had been previously determined.

Results

VFD-reared monkeys exhibited significantly increased NAA/creatine concentrations in right VS/CN in comparison to normally reared controls, controlling for multiple comparisons. In comparison to controls, VFD cerebrospinal fluid corticotropin-releasing factor concentrations were directly associated with right VS/CN absolute NAA. Left hippocampal volume was inversely associated with left VS/CN NAA/creatine in VFD reared but not in controls. Disruption of a normative inverse correlation between left VS/CN NAA and body mass was noted in VFD. Only non-VFD subjects exhibited a direct relationship between timidity response to an intruder and right VS/CN NAA.

Conclusion

Early life stress produced persistent increases in VS/CN NAA, which demonstrated specific patterns of association (or lack thereof) to early life stress markers in comparison to non-VFD subjects. The data are broadly consistent with a stable nonhuman primate phenotype of anxiety and mood disorder vulnerability whereby in vivo indicators of neuronal integrity, although reduced in hippocampus, are increased in striatum. The findings may provide a catalyst for further studies in humans and other species regarding a reciprocal hippocampal/nucleus accumbens relationship in affective disorders.

Intrinsic brain subsystem associated with dietary restraint, disinhibition and hunger: an fMRI study

Eating behaviors are closely related to body weight, and eating traits are depicted in three dimensions: dietary restraint, disinhibition, and hunger. The current study aims to explore whether these aspects of eating behaviors are related to intrinsic brain activation, and to further investigate the relationship between the brain activation relating to these eating traits and body weight, as well as the link between function connectivity (FC) of the correlative brain regions and body weight. Our results demonstrated positive associations between dietary restraint and baseline activation of the frontal and the temporal regions (i.e., food reward encoding) and the limbic regions (i.e., homeostatic control, including the hypothalamus). Disinhibition was positively associated with the activation of the frontal motivational system (i.e., OFC) and the premotor cortex. Hunger was positively related to extensive activations in the prefrontal, temporal, and limbic, as well as in the cerebellum. Within the brain regions relating to dietary restraint, weight status was negatively correlated with FC of the left middle temporal gyrus and left inferior temporal gyrus, and was positively associated with the FC of regions in the anterior temporal gyrus and fusiform visual cortex. Weight status was positively associated with the FC within regions in the prefrontal motor cortex and the right ACC serving inhibition, and was negatively related with the FC of regions in the frontal cortical-basal ganglia-thalamic circuits responding to hunger control. Our data depicted an association between intrinsic brain activation and dietary restraint, disinhibition, and hunger, and presented the links of their activations and FCs with weight status.

Newborn insula gray matter volume is prospectively associated with early life adiposity gain

Background

The importance of energy homeostasis brain circuitry in the context of obesity is well established, however, the developmental ontogeny of this circuitry in humans is currently unknown. Here, we investigate the prospective association between newborn gray matter (GM) volume in the insula, a key brain region underlying energy homeostasis, and change in percent body fat accrual over the first six months of postnatal life, an outcome that represents among the most reliable infant predictors of childhood obesity risk.

Methods

52 infants (29 male, 23 female, gestational age at birth=39[1.5] weeks) were assessed using structural MRI shortly after birth (postnatal age at MRI scan=25.9[12.2] days), and serial Dual X-Ray Absorptiometry shortly after birth (postnatal age at DXA scan 1=24.6[11.4] days) and at six months of age (postnatal age at DXA scan 2=26.7[3.3] weeks).

Results

Insula GM volume was inversely associated with change in percent body fat from birth to six-months postnatal age and accounted for 19% of its variance (β=-3.6%/S.D., p=0.001). This association was driven by the central-posterior portion of the insula, a region of particular importance for gustation and interoception. The direction of this effect is in concordance with observations in adults, and the results remained statistically significant after adjusting for relevant covariates and potential confounding variables.

Conclusions

Together, these findings suggest an underlying neural basis of childhood obesity that precedes the influence of the postnatal environment. The identification of plausible brain-related biomarkers of childhood obesity risk that predate the influence of the postnatal obesogenic environment may contribute to an improved understanding of propensity for obesity, early identification of at-risk individuals, and intervention targets for primary prevention.

Altered microstructure of brain white matter in females with anorexia nervosa: a diffusion tensor imaging study

OBJECTIVE

Structural studies have reported anorexia nervosa (AN) patients with abnormal gray matter in several brain regions and dysfunction in some connected neural circuits. However, the role of white matter (WM) in AN patients has rarely been investigated. The present study aimed to assess alterations in WM microstructure of the entire brain in females with AN using a voxel-based method on diffusion tensor imaging (DTI) data.

MATERIALS AND METHODS

The study enrolled 8 female patients with AN and 14 age-matched females as controls (CW). The DTI data was collected from each subject to calculate the fractional anisotropy (FA) maps of the whole brain by the DTI-Studio software. Subsequently, a 2-sample t-test (P<0.05, corrected) was performed to detect the difference in FA maps of AN and CW group, and a Pearson's correlation analyzed the relationship between mean FA value of brain regions and body mass index (BMI).

RESULTS

Compared with CW, AN patients revealed a significant decrease in FA maps in the left superior frontal gyrus, medial frontal gyrus, anterior cingulate cortex, middle frontal gyrus, inferior frontal gyrus, thalamus, and bilateral insula. Moreover, significantly positive correlations were established between the mean FA value of the left inferior frontal gyrus, insula as well as thalamus and BMI in AN patients.

CONCLUSIONS

Our findings supported the presence of WM abnormality in patients with AN. The significant differences of FA maps, in patients with AN, were associated with their aberrant BMI. The results further improved our understanding of the pathophysiological mechanisms underlying AN.

The obese brain as a heritable phenotype: a combined morphometry and twin study

Background

Body weight and adiposity are heritable traits. To date it remains unknown whether obesity-associated brain structural alterations are under a similar level of genetic control.

Methods

For this study we utilized magnetic resonance imaging (MRI) data from the Human Connectome Project. Voxel based morphometry (VBM) was used to investigate associations between body mass index (BMI) and regional gray matter volume (GMV) in a sample of 875 young adults with a wide BMI range (386m/489f; age 28.8 ± 3.7y; BMI 26.6 ± 5.3 kg*m-2), that included 86 pairs of monozygotic twins and 82 pairs of dizygotic twins. Twin data were analyzed by applying the additive genetic, common environmental and residual effects (ACE) model to determine heritability of brain regions that were associated with BMI.

Results

We observed positive associations between BMI and GMV in the ventromedial prefrontal cortex and the right cerebellum and widespread negative associations within the prefrontal cortex, cerebellum, temporal lobes and distinct subcortical structures. Varying degrees of heritability were found for BMI-associated brain regions, with highest heritability estimates for cerebellar GMV and subcortical structures.

Conclusions

These data indicate that brain regions associated with obesity are subject to differing levels of genetic control and environmental influences. Specific brain regions with high heritability might represent an inherent vulnerability factor for obesity.

Endogenous GLP-1 mediates postprandial reductions in activation in central reward and satiety areas in patients with type 2 diabetes

AIMS/HYPOTHESIS

The central nervous system (CNS) is a major player in the regulation of food intake. The gut hormone glucagon-like peptide-1 (GLP-1) has been proposed to have an important role in this regulation by relaying information about nutritional status to the CNS. We hypothesised that endogenous GLP-1 has effects on CNS reward and satiety circuits.

METHODS

This was a randomised, crossover, placebo-controlled intervention study, performed in a university medical centre in the Netherlands. We included patients with type 2 diabetes and healthy lean control subjects. Individuals were eligible if they were 40-65 years. Inclusion criteria for the healthy lean individuals included a BMI <25 kg/m(2) and normoglycaemia. Inclusion criteria for the patients with type 2 diabetes included BMI >26 kg/m(2), HbA1c levels between 42 and 69 mmol/mol (6.0-8.5%) and treatment for diabetes with only oral glucose-lowering agents. We assessed CNS activation, defined as blood oxygen level dependent (BOLD) signal, in response to food pictures in obese patients with type 2 diabetes (n = 20) and healthy lean individuals (n = 20) using functional magnetic resonance imaging (fMRI). fMRI was performed in the fasted state and after meal intake on two occasions, once during infusion of the GLP-1 receptor antagonist exendin 9-39, which was administered to block actions of endogenous GLP-1, and on the other occasion during saline (placebo) infusion. Participants were blinded for the type of infusion. The order of infusion was determined by block randomisation. The primary outcome was the difference in BOLD signal, i.e. in CNS activation, in predefined regions in the CNS in response to viewing food pictures.

RESULTS

All patients were included in the analyses. Patients with type 2 diabetes showed increased CNS activation in CNS areas involved in the regulation of feeding (insula, amygdala and orbitofrontal cortex) in response to food pictures compared with lean individuals (p ≤ 0.04). Meal intake reduced activation in the insula in response to food pictures in both groups (p ≤ 0.05), but this was more pronounced in patients with type 2 diabetes. Blocking actions of endogenous GLP-1 significantly prevented meal-induced reductions in bilateral insula activation in response to food pictures in patients with type 2 diabetes (p ≤ 0.03).

CONCLUSIONS/INTERPRETATION

Our findings support the hypothesis that endogenous GLP-1 is involved in postprandial satiating effects in the CNS of obese patients with type 2 diabetes.

TRIAL REGISTRATION

ClinicalTrials.gov NCT 01363609. Funding The study was funded in part by a grant from Novo Nordisk.

Advances from neuroimaging studies in eating disorders

Over the past decade, brain imaging has helped to better define eating disorder-related brain circuitry. Brain research on gray matter (GM) and white matter (WM) volumes had been inconsistent, possibly due to the effects of acute starvation, exercise, medication, and comorbidity, but newer studies have controlled for such effects. Those studies suggest larger left medial orbitofrontal gyrus rectus volume in ill adult and adolescent anorexia nervosa after recovery from anorexia nervosa, and in adult bulimia nervosa. The orbitofrontal cortex is important in terminating food intake, and altered function could contribute to self-starvation. The right insula, which processes taste but also interoception, was enlarged in ill adult and adolescent anorexia nervosa, as well as adults recovered from the illness. The fixed perception of being fat in anorexia nervosa could be related to altered insula function. A few studies investigated WM integrity, with the most consistent finding of reduced fornix integrity in anorexia and bulimia nervosa-a limbic pathway that is important in emotion but also food intake regulation. Functional brain imaging using basic sweet taste stimuli in eating disorders during the ill state or after recovery implicated repeatedly reward pathways, including insula and striatum. Brain imaging that targeted dopamine-related brain activity using taste-reward conditioning tasks suggested that this circuitry is hypersensitive in anorexia nervosa, but hyporesponsive in bulimia nervosa and obesity. Those results are in line with basic research and suggest adaptive reward system changes in the human brain in response to extremes of food intake-changes that could interfere with normalization of eating behavior.

Roux-en-Y Gastric Bypass Alters Brain Activity in Regions that Underlie Reward and Taste Perception

BACKGROUND

Roux-en-Y gastric bypass (RYGB) surgery is a very effective bariatric procedure to achieve significant and sustained weight loss, yet little is known about the procedure's impact on the brain. This study examined the effects of RYGB on the brain's response to the anticipation of highly palatable versus regular food.

METHODS

High fat diet-induced obese rats underwent RYGB or sham operation and were then tested for conditioned place preference (CPP) for the bacon-paired chamber, relative to the chow-paired chamber. After CPP, animals were placed in either chamber without the food stimulus, and brain-glucose metabolism (BGluM) was measured using positron emission tomography (μPET).

RESULTS

Bacon CPP was only observed in RYGB rats that had stable weight loss following surgery. BGluM assessment revealed that RYGB selectively activated regions of the right and midline cerebellum (Lob 8) involved in subjective processes related to reward or expectation. Also, bacon anticipation led to significant activation in the medial parabrachial nuclei (important in gustatory processing) and dorsomedial tegmental area (key to reward, motivation, cognition and addiction) in RYGB rats; and activation in the retrosplenial cortex (default mode network), and the primary visual cortex in control rats.

CONCLUSIONS

RYGB alters brain activity in areas involved in reward expectation and sensory (taste) processing when anticipating a palatable fatty food. Thus, RYGB may lead to changes in brain activity in regions that process reward and taste-related behaviors. Specific cerebellar regions with altered metabolism following RYGB may help identify novel therapeutic targets for treatment of obesity.

A potential role for the midbrain in integrating fat-free mass determined energy needs: An H2 (15) O PET study

Little is known on how sensing of energy needs is centrally represented, integrated, and translated into the behavioral aspects of energy homeostasis. Fat free mass (FFM) is the major determinant of energy expenditure. We investigated how interindividual variances in FFM relate to neuronal activity in humans. Healthy adults (n = 64, 21F/43M; age 31.3 ± 9.1y; percentage of body fat [PFAT] 25.6 ± 10.7%; BMI 30.4 ± 9) underwent a 36h fast and subsequent H(2) (15) O positron emission tomographic (PET) measurement of regional cerebral blood flow (rCBF). Multiple variable regression analysis revealed significant associations of FFM with rCBF within the midbrain [including parts of the periaqueductal gray (PAG), ventral tegmental area (VTA), thalamic and hypothalamic regions], the bilateral parahippocampal region, left anterior cingulate, left insular cortex, right cerebellum, and distinct regions within the temporal and occipital cortex. In contrast, no significant associations were found for fat mass (FM). We investigated the potential functional-anatomical link between FFM and central regulation of food intake by performing a conjunction analysis of FFM and the perceived hunger feelings. This showed a significant overlap within the midbrain PAG. Mediation analysis demonstrated a significant indirect effect of FFM on hunger with PAG rCBF as mediator. Most regions we found to be associated with FFM form part in ascending homeostatic pathways and cortical circuitries implicated in the regulation of basic bodily functions indicating a potential role of these central networks in the integration of FFM determined energy needs.

The role of glucagon-like peptide-1 impairment in obesity and potential therapeutic implications

The hormone glucagon-like peptide-1 (GLP-1) is released from the gut in response to food intake. It acts as a satiety signal, leading to reduced food intake, and also as a regulator of gastric emptying. Furthermore, GLP-1 functions as an incretin hormone, stimulating insulin release and inhibiting glucagon secretion from the pancreas in response to food ingestion. Evidence suggests that the action or effect of GLP-1 may be impaired in obese subjects, even in those with normal glucose tolerance. GLP-1 impairment may help explain the increased gastric emptying and decreased satiety signalling seen in obesity. Incretin impairment, probably associated with reduced insulinotropic potency of GLP-1, is also characteristic of type 2 diabetes (T2D). Therefore, it is possible that incretin impairment may contribute to the pathophysiological bridge between obesity and T2D. This review summarises current knowledge about the pathophysiology and consequences of GLP-1 and incretin impairment in obesity, and examines the evidence for an incretin-related link between obesity and T2D. It also considers the current literature surrounding the novel use of GLP-1 receptor agonists as a treatment for obesity in patients with normoglycaemia, prediabetes and T2D.

Altered gastric vagal mechanosensitivity in diet-induced obesity persists on return to normal chow and is accompanied by increased food intake

BACKGROUND AND AIMS

Gastric vagal afferents convey satiety signals in response to mechanical stimuli. The sensitivity of these afferents is decreased in diet-induced obesity. Leptin, secreted from gastric epithelial cells, potentiates the response of vagal afferents to mechanical stimuli in lean mice, but has an inhibitory effect in high-fat diet (HFD)-induced obese mice. We sought to determine whether changes in vagal afferent function and response to leptin in obesity were reversible by returning obese mice consuming a HFD to standard laboratory chow diet (SLD).

METHODS

Eight-week-old female C57BL/6 mice were either fed a SLD (N=20) or HFD (N=20) for 24 weeks. A third group was fed a HFD for 12 weeks and then a SLD for a further 12 weeks (RFD, N=18). An in vitro gastro-oesophageal vagal afferent preparation was used to determine the mechanosensitivity of gastric vagal afferents and the modulatory effect of leptin (0.1-10 nM) was examined. Retrograde tracing and quantitative RT-PCR were used to determine the expression of leptin receptor (LepR) messenger RNA (mRNA) in whole nodose and specific cell bodies traced from the stomach.

RESULTS

After 24 weeks, both the HFD and RFD mice had increased body weight, gonadal fat mass, plasma leptin, plasma insulin and daily energy consumption compared with the SLD mice. The HFD and RFD mice had reduced tension receptor mechanosensitivity and leptin further inhibited responses to tension in HFD, RFD but not SLD mice. Mucosal receptors from both the SLD and RFD mice were potentiated by leptin, an effect not seen in HFD mice. LepR expression was unchanged in the whole nodose, but was reduced in the mucosal afferents of the HFD and RFD mice.

CONCLUSION

Disruption of gastric vagal afferent function by HFD-induced obesity is only partially reversible by dietary change, which provides a potential mechanism preventing maintenance of weight loss.

Hypothalamic metabolic compartmentation during appetite regulation as revealed by magnetic resonance imaging and spectroscopy methods

We review the role of neuroglial compartmentation and transcellular neurotransmitter cycling during hypothalamic appetite regulation as detected by Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) methods. We address first the neurochemical basis of neuroendocrine regulation in the hypothalamus and the orexigenic and anorexigenic feed-back loops that control appetite. Then we examine the main MRI and MRS strategies that have been used to investigate appetite regulation. Manganese-enhanced magnetic resonance imaging (MEMRI), Blood oxygenation level-dependent contrast (BOLD), and Diffusion-weighted magnetic resonance imaging (DWI) have revealed Mn²⁺ accumulations, augmented oxygen consumptions, and astrocytic swelling in the hypothalamus under fasting conditions, respectively. High field ¹H magnetic resonance in vivo, showed increased hypothalamic myo-inositol concentrations as compared to other cerebral structures. ¹H and ¹³C high resolution magic angle spinning (HRMAS) revealed increased neuroglial oxidative and glycolytic metabolism, as well as increased hypothalamic glutamatergic and GABAergic neurotransmissions under orexigenic stimulation. We propose here an integrative interpretation of all these findings suggesting that the neuroendocrine regulation of appetite is supported by important ionic and metabolic transcellular fluxes which begin at the tripartite orexigenic clefts and become extended spatially in the hypothalamus through astrocytic networks becoming eventually MRI and MRS detectable.

The addictive dimensionality of obesity

Our brains are hardwired to respond and seek immediate rewards. Thus, it is not surprising that many people overeat, which in some can result in obesity, whereas others take drugs, which in some can result in addiction. Though food intake and body weight are under homeostatic regulation, when highly palatable food is available, the ability to resist the urge to eat hinges on self-control. There is no homeostatic regulator to check the intake of drugs (including alcohol); thus, regulation of drug consumption is mostly driven by self-control or unwanted effects (i.e., sedation for alcohol). Disruption in both the neurobiological processes that underlie sensitivity to reward and those that underlie inhibitory control can lead to compulsive food intake in some individuals and compulsive drug intake in others. There is increasing evidence that disruption of energy homeostasis can affect the reward circuitry and that overconsumption of rewarding food can lead to changes in the reward circuitry that result in compulsive food intake akin to the phenotype seen with addiction. Addiction research has produced new evidence that hints at significant commonalities between the neural substrates underlying the disease of addiction and at least some forms of obesity. This recognition has spurred a healthy debate to try and ascertain the extent to which these complex and dimensional disorders overlap and whether or not a deeper understanding of the crosstalk between the homeostatic and reward systems will usher in unique opportunities for prevention and treatment of both obesity and drug addiction.

Striatocortical pathway dysfunction in addiction and obesity: differences and similarities

Neuroimaging techniques are starting to reveal significant overlap in the brain circuitry underlying addiction and disorders of dyscontrol over rewarding behaviors (such as binge eating disorder and obesity). Positron emission tomography (PET) has demonstrated impaired striatal dopamine (DA) signaling (decreased D2 receptors) in drug addiction and obesity that is associated with reduced baseline glucose metabolism in medial and ventral prefrontal brain regions. Functional magnetic resonance imaging (fMRI) has documented brain activation abnormalities that also implicate DA-modulated striato-cortical pathways. In this review we map findings from recent neuroimaging studies that differentiate brain activation in drug/food addiction from those in controls within brain networks functionally connected with ventral and dorsal striatum. We show that regions found to be abnormal in addiction and obesity frequently emerge at the overlap of the dorsal and the ventral striatal networks. Medial temporal and superior frontal regions functionally connected with dorsal striatum display greater vulnerability in obesity and eating disorders than in drug addictions, indicating more widespread abnormalities for obesity and eating disorders than for addictions. This corroborates involvement of both ventral striatal (predominantly associated with reward and motivation) and dorsal striatal networks (associated with habits or stimulus response learning) in addiction and obesity but also identify distinct patterns between these two disorders.

Obesity and addiction: neurobiological overlaps

Drug addiction and obesity appear to share several properties. Both can be defined as disorders in which the saliency of a specific type of reward (food or drug) becomes exaggerated relative to, and at the expense of others rewards. Both drugs and food have powerful reinforcing effects, which are in part mediated by abrupt dopamine increases in the brain reward centres. The abrupt dopamine increases, in vulnerable individuals, can override the brain's homeostatic control mechanisms. These parallels have generated interest in understanding the shared vulnerabilities between addiction and obesity. Predictably, they also engendered a heated debate. Specifically, brain imaging studies are beginning to uncover common features between these two conditions and delineate some of the overlapping brain circuits whose dysfunctions may underlie the observed deficits. The combined results suggest that both obese and drug-addicted individuals suffer from impairments in dopaminergic pathways that regulate neuronal systems associated not only with reward sensitivity and incentive motivation, but also with conditioning, self-control, stress reactivity and interoceptive awareness. In parallel, studies are also delineating differences between them that centre on the key role that peripheral signals involved with homeostatic control exert on food intake. Here, we focus on the shared neurobiological substrates of obesity and addiction.

Fat-free body mass but not fat mass is associated with reduced gray matter volume of cortical brain regions implicated in autonomic and homeostatic regulation

Obesity has been associated with alterations of both functional and structural aspects of the human central nervous system. In obese individuals both fat mass (FM; primarily consisting of adipose tissue) and fat-free mass (FFM; all non-adipose tissues) are increased and it remains unknown whether these compartments have separate effects on human brain morphology. We used voxel-based morphometry to investigate the relationships between measures of body composition and regional gray matter volume (GMV) in 76 healthy adults with a wide range of adiposity (24 F/52 M; age 32.1 ± 8.8 years; percentage of body fat [PFAT%] 25.5 ± 10.9%; BMI 29.8 ± 8.9). Fat-free mass index (FFMI kg × m(-2)) showed negative associations in bilateral temporal regions, the bilateral medial and caudolateral OFC, and the left insula. Fat mass index (FMI kg × m(-2)) showed similar, but less extensive negative associations within temporal cortical regions and the left caudolateral orbitofrontal cortex (OFC). In addition, negative associations were seen for FMI with GMV of the cerebellum. Associations of FFMI with temporal and medial orbitofrontal GMV appeared to be independent of adiposity. No associations were seen between measures of adiposity (i.e. FM and PFAT) and GMV when adjusted for FFM. The majority of regions that we find associated with FFM have been implicated in the regulation of eating behavior and show extensive projections to central autonomic and homeostatic core structures. These data indicate that not adipose tissue or relative adiposity itself, but obesity related increases in absolute tissue mass and particularly FFM may have a more predominant effect on the human brain. This might be explained by the high metabolic demand of FFM and related increases in total energy needs.

Beyond pharmacotherapy: understanding the links between obesity and chronic mental illness

While differences in weight-gain potential exist, both between and within classes of psychiatry medications, most commonly used atypical antipsychotics, mood stabilizers, and antidepressants result in some degree of weight gain. This is not new information and it requires an understanding of the tolerability profiles of different treatments and their goodness of fit with specific patient phenotypes. However, this iatrogenic association represents only a piece of this obesity-mental illness dyad. The complex interplay between psychiatric illness and weight involves neurobiology, psychology, and sociological factors. Parsing the salient variables in people with mental illness is an urgent need insofar as mortality from physical health causes is the most common cause of premature mortality in people with chronic mental illness. Our review examines issues associated with common chronic mental illnesses that may underlie this association and warrant further study if we hope to clinically intervene to control this life-threatening comorbidity.

Food and drug reward: overlapping circuits in human obesity and addiction

Both drug addiction and obesity can be defined as disorders in which the saliency value of one type of reward (drugs and food, respectively) becomes abnormally enhanced relative to, and at the expense of others. This model is consistent with the fact that both drugs and food have powerful reinforcing effects-partly mediated by dopamine increases in the limbic system-that, under certain circumstances or in vulnerable individuals, could overwhelm the brain's homeostatic control mechanisms. Such parallels have generated significant interest in understanding the shared vulnerabilities and trajectories between addiction and obesity. Now, brain imaging discoveries have started to uncover common features between these two conditions and to delineate some of the overlapping brain circuits whose dysfunctions may explain stereotypic and related behavioral deficits in human subjects. These results suggest that both obese and drug-addicted individuals suffer from impairments in dopaminergic pathways that regulate neuronal systems associated not only with reward sensitivity and incentive motivation, but also with conditioning (memory/learning), impulse control (behavioural inhibition), stress reactivity, and interoceptive awareness. Here, we integrate findings predominantly derived from positron emission tomography that shed light on the role of dopamine in drug addiction and in obesity, and propose an updated working model to help identify treatment strategies that may benefit both of these conditions.

Reduced nucleus accumbens and caudate nucleus activation to a pleasant taste is associated with obesity in older adults

Although obesity is recognized as a global health epidemic, insufficient research has been directed to understanding the rising prevalence of obesity in the fastest growing segment of the population, older adults. Late-life obesity has been linked to declines in physical health and cognitive function, with implications not only for the individual, but also for society. We investigated the hypothesis that altered brain responses to food reward is associated with obesity, using fMRI of response to pleasant and aversive taste stimuli in young and older adults performing a hedonic evaluation task. Correlations between higher levels of abdominal fat/body mass index and reduced fMRI activation to sucrose in dopamine-related brain regions (caudate, nucleus accumbens) were large in older adults. Significant associations between a hypofunctioning reward response and obesity suggest the hypothesis that decreased dopamine functioning may be a plausible mechanism for weight gain in older adults.

Gastric distention induced functional magnetic resonance signal changes in the rodent brain

Investigating the localization of gastric sensation within the brain is important for understanding the neural correlates of satiety. Previous rodent studies have identified the brain-stem and hypothalamus as key mediators of gastric distention-induced satiation. Although, recent blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) studies in humans have identified a role for higher cortico-limbic structures in mediating the satiation effects of gastric distention, the role of these regions in rodents remains to be characterized. We determined the effects of gastric distention on global spatio-temporal BOLD fMRI signal changes in the rodent brain. Brain images were acquired with a high resolution 9.4 T magnet during gastric distention with continuous monitoring of blood pressure in adult male Sprague Dawley rats (n=8-10). Distention of the stomach with an intragastric balloon, at rates which mimicked the rate of consumption and emptying of a mixed nutrient liquid meal, resulted in robust reduction in food intake and increase in blood pressure. Gastric distention increased BOLD fMRI activity within homeostatic regions such as the hypothalamus and nucleus tractus solitarius, as well as non homeostatic regions including the hippocampus, amygdala, thalamus, cerebellum and the cortex (cingulate, insular, motor and sensory cortices). Further, the increase in BOLD fMRI activity following distention was strongly correlated to an increase in blood pressure. These results indicate that gastric distention, mimicking the rate of intake and emptying of a liquid meal, increases BOLD fMRI activity in both homeostatic and non homeostatic brain circuits which regulate food intake, and that these BOLD fMRI signal changes may in part be attributable to transient increases in blood pressure.

Effects of insulin on brain glucose metabolism in impaired glucose tolerance

OBJECTIVE

Insulin stimulates brain glucose metabolism, but this effect of insulin is already maximal at fasting concentrations in healthy subjects. It is not known whether insulin is able to stimulate glucose metabolism above fasting concentrations in patients with impaired glucose tolerance.

RESEARCH DESIGN AND METHODS

We studied the effects of insulin on brain glucose metabolism and cerebral blood flow in 13 patients with impaired glucose tolerance and nine healthy subjects using positron emission tomography (PET). All subjects underwent PET with both [(18)F]fluorodeoxyglucose (for brain glucose metabolism) and [(15)O]H(2)O (for cerebral blood flow) in two separate conditions (in the fasting state and during a euglycemic-hyperinsulinemic clamp). Arterial blood samples were acquired during the PET scans to allow fully quantitative modeling.

RESULTS

The hyperinsulinemic clamp increased brain glucose metabolism only in patients with impaired glucose tolerance (whole brain: +18%, P = 0.001) but not in healthy subjects (whole brain: +3.9%, P = 0.373). The hyperinsulinemic clamp did not alter cerebral blood flow in either group.

CONCLUSIONS

We found that insulin stimulates brain glucose metabolism at physiological postprandial levels in patients with impaired glucose tolerance but not in healthy subjects. These results suggest that insulin stimulation of brain glucose metabolism is maximal at fasting concentrations in healthy subjects but not in patients with impaired glucose tolerance.

Mood disorders and obesity: understanding inflammation as a pathophysiological nexus

The aim of this review is to evaluate the evidentiary base supporting the hypothesis that the increased hazard for obesity in mood disorder populations (and vice versa) is a consequence of shared pathophysiological pathways. We conducted a PubMed search of all English-language articles with the following search terms: obesity, inflammation, hypothalamic-pituitary-adrenal axis, insulin, cognition, CNS, and neurotransmitters, cross-referenced with major depressive disorder and bipolar disorder. The frequent co-occurrence of mood disorders and obesity may be characterized by interconnected pathophysiology. Both conditions are marked by structural and functional abnormalities in multiple cortical and subcortical brain regions that subserve cognitive and/or affective processing. Abnormalities in several interacting biological networks (e.g. immuno-inflammatory, insulin signaling, and counterregulatory hormones) contribute to the co-occurence of mood disorders and obesity. Unequivocal evidence now indicates that obesity and mood disorders are chronic low-grade pro-inflammatory states that result in a gradual accumulation of allostatic load. Abnormalities in key effector proteins of the pro-inflammatory cascade include, but are not limited to, cytokines/adipokines such as adiponectin, leptin, and resistin as well as tumor necrosis factor alpha and interleukin-6. Taken together, the bidirectional relationship between obesity and mood disorders may represent an exophenotypic manifestation of aberrant neural and inflammatory networks. The clinical implications of these observations are that, practitioners should screen individuals with obesity for the presence of clinically significant depressive symptoms (and vice versa). This clinical recommendation is amplified in individuals presenting with biochemical indicators of insulin resistance and other concurrent conditions associated with abnormal inflammatory signaling (e.g. cardiovascular disease).

The first taste is always with the eyes: a meta-analysis on the neural correlates of processing visual food cues

Food selection is primarily guided by the visual system. Multiple functional neuro-imaging studies have examined the brain responses to visual food stimuli. However, the results of these studies are heterogeneous and there still is uncertainty about the core brain regions involved in the neural processing of viewing food pictures. The aims of the present study were to determine the concurrence in the brain regions activated in response to viewing pictures of food and to assess the modulating effects of hunger state and the food's energy content. We performed three Activation Likelihood Estimation (ALE) meta-analyses on data from healthy normal weight subjects in which we examined: 1) the contrast between viewing food and nonfood pictures (17 studies, 189 foci), 2) the modulation by hunger state (five studies, 48 foci) and 3) the modulation by energy content (seven studies, 86 foci). The most concurrent brain regions activated in response to viewing food pictures, both in terms of ALE values and the number of contributing experiments, were the bilateral posterior fusiform gyrus, the left lateral orbitofrontal cortex (OFC) and the left middle insula. Hunger modulated the response to food pictures in the right amygdala and left lateral OFC, and energy content modulated the response in the hypothalamus/ventral striatum. Overall, the concurrence between studies was moderate: at best 41% of the experiments contributed to the clusters for the contrast between food and nonfood. Therefore, future research should further elucidate the separate effects of methodological and physiological factors on between-study variations.

Reduced accumbens dopamine in Sprague-Dawley rats prone to overeating a fat-rich diet

Obese humans and animals exhibit reduced functioning of the dopamine (DA) system in the nucleus accumbens (NAc). The question addressed here is whether this change in NAc DA can be detected in Sprague-Dawley rats that are prone to obesity on a fat-rich diet but still at normal body weight. Rats were subgrouped as "obesity-prone" (OP) or "obesity-resistant" (OR), based on their weight gain during 5days of access to a high-fat diet, and were then shifted to a lower-fat chow diet before microdialysis testing was performed. The OP rats compared to OR rats exhibited markedly reduced basal levels of DA in the NAc. After a high-fat challenge meal, both OP and OR rats showed a significant increase in extracellular DA and its metabolites; however, the NAc DA of the OP rats still remained at reduced levels. Also, the increase in DA and metabolite levels observed in OR rats after systemic administration of a fat emulsion was not evident in the OP rats, which instead showed no change in DA and a decrease in its metabolites. These results demonstrate, first, that fat can stimulate accumbal DA release and, second, that outbred rats prone to overeating and becoming obese on a palatable, fat-rich diet exhibit reduced signaling in the mesolimbic DA system while still at normal weight, suggesting that it may be causally related to their excess consummatory behavior.

Neurobiology of food addiction

PURPOSE OF REVIEW

To review recent work on disorders related to food use, including food addiction, and to highlight the similarities and differences between food and drugs of abuse.

RECENT FINDINGS

Recent work on food use disorders has demonstrated that the same neurobiological pathways that are implicated in drug abuse also modulate food consumption, and that the body's regulation of food intake involves a complex set of peripheral and central signaling networks. Moreover, new research indicates that rats can become addicted to certain foods, that men and women may respond differently to external food cues, and that the intrauterine environment may significantly impact a child's subsequent risk of developing obesity, diabetes, and hypercholesterolemia.

SUMMARY

First, work presented in this review strongly supports the notion that food addiction is a real phenomenon. Second, although food and drugs of abuse act on the same central networks, food consumption is also regulated by peripheral signaling systems, which adds to the complexity of understanding how the body regulates eating, and of treating pathological eating habits. Third, neurobiological research reviewed here indicates that traditional pharmacological and behavioral interventions for other substance-use disorders may prove useful in treating obesity.