Functional neuroimaging: a new generation of human brain studies in obesity research

Food addiction and obesity: evidence from bench to bedside

Obesity has become a major health problem and epidemic. However, much of the current debate has been fractious and etiologies of obesity have been attributed to eating behavior or fast food, personality issues, depression, addiction, or genetics. One of the interesting new hypotheses for epidemic obesity is food addiction, which is associated with both substance-related disorder and eating disorder. Accumulating evidences have shown that there are many shared neural and hormonal pathways as well as distinct differences that may help researchers find why certain individuals overeat and become obese. Functional neuroimaging studies have further revealed that good or great smelling, looking, tasting, and reinforcing food has characteristics similar to that of drugs of abuse. Many of the brain changes reported for hedonic eating and obesity are also seen in various forms of addictions. Most importantly, overeating and obesity may have an acquired drive like drug addiction with respect to motivation and incentive; craving, wanting, and liking occur after early and repeated exposures to stimuli. The acquired drive for great food and relative weakness of the satiety signal would cause an imbalance between the drive and hunger/reward centers in the brain and their regulation.

Neuroimaging, gut peptides and obesity: novel studies of the neurobiology of appetite

Two major biological players in the regulation of body weight are the gut and the brain. Peptides released from the gut convey information about energy needs to areas of the brain involved in homeostatic control of food intake. There is emerging evidence that human food intake is also under the control of cortical and subcortical areas related to reward and cognition. The extent to which gut hormones influence these brain areas is not fully understood. Novel methods combining the study of neural activity and hormonal signalling promise to advance our understanding of gut-brain interactions. Here, we review a growing number of animal and human studies using neuroimaging methods (functional magnetic resonance imaging, positron emission tomography) to measure brain activation in relation to nutrient loads and infusion of gut peptides. Implications for current and future pharmacological treatments for obesity are discussed.

Individual differences in the neurophysiology of reward and the obesity epidemic

The obesity epidemic has unfolded in a matter of decades, not millennia, and therefore cannot be attributed to a drift in the genome. Rather, the temporal characteristics of the epidemic more closely track environmental and lifestyle changes, such as reduced physical activity, increased availability of palatable and energy-dense foods and drinks, and increased acceptance of eating outside of meal time (among others). One important observation is that not everyone is becoming obese. This suggests that individual factors interact with recent environmental changes to predispose some to overeat. One hypothesis that has been gaining traction in the neuroscience community is that individual differences in the neural encoding of foods may predispose some to overeat in the presence of a surplus of energy-dense, palatable foods and drinks. The aim of this review is to highlight several possible ways by which individual differences in the neurophysiology of food reward may lead to overeating.

The neurohormonal regulation of energy intake in relation to bariatric surgery for obesity

Obesity has reached pandemic proportions, with bariatric surgery representing the only currently available treatment demonstrating long-term effectiveness. Over 200,000 bariatric procedures are performed each year in the US alone. Given the reliable and singular success of bariatric procedures, increased attention is being paid to identifying the accompanying neurohormonal changes that may contribute to the resulting decrease in energy intake. Numerous investigations of postsurgical changes in gut peptides have been conducted, suggesting greater alterations in endocrine function in combination restrictive and malabsorptive procedures (e.g., Roux-en-Y gastric bypass) as compared to purely restrictive procedures (e.g., gastric banding), which may contribute to the increased effectiveness of combination procedures. However, very few studies have been performed and relatively little is known about changes in neural activation that may result from bariatric procedures, which likely interact with changes in gut peptides to influence postsurgical caloric intake. This review provides a background in the neurohormonal regulation of energy intake and discusses how differing forms of bariatric surgery may affect the neurohormonal network, with emphasis on Roux-en-Y gastric bypass, the most commonly performed procedure worldwide. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Eigenvector centrality mapping for analyzing connectivity patterns in fMRI data of the human brain

Functional magnetic resonance data acquired in a task-absent condition ("resting state") require new data analysis techniques that do not depend on an activation model. In this work, we introduce an alternative assumption- and parameter-free method based on a particular form of node centrality called eigenvector centrality. Eigenvector centrality attributes a value to each voxel in the brain such that a voxel receives a large value if it is strongly correlated with many other nodes that are themselves central within the network. Google's PageRank algorithm is a variant of eigenvector centrality. Thus far, other centrality measures - in particular "betweenness centrality" - have been applied to fMRI data using a pre-selected set of nodes consisting of several hundred elements. Eigenvector centrality is computationally much more efficient than betweenness centrality and does not require thresholding of similarity values so that it can be applied to thousands of voxels in a region of interest covering the entire cerebrum which would have been infeasible using betweenness centrality. Eigenvector centrality can be used on a variety of different similarity metrics. Here, we present applications based on linear correlations and on spectral coherences between fMRI times series. This latter approach allows us to draw conclusions of connectivity patterns in different spectral bands. We apply this method to fMRI data in task-absent conditions where subjects were in states of hunger or satiety. We show that eigenvector centrality is modulated by the state that the subjects were in. Our analyses demonstrate that eigenvector centrality is a computationally efficient tool for capturing intrinsic neural architecture on a voxel-wise level.

Bariatric surgery and taste: novel mechanisms of weight loss

PURPOSE OF REVIEW

The mechanisms by which obesity surgery and in particular gastric bypass cause weight loss are unclear. The review will focus on the concept of alterations in the sense of taste after obesity surgery.

RECENT FINDINGS

Patients after obesity surgery and gastric bypass in particular change their eating behaviour and adopt healthier food preferences by avoiding high-calorie and high-fat foods. Patients find sweet and fatty meals less pleasant not due to postingestive side effects but through changes in the sense of taste. The acuity for sweet taste increases after gastric bypass potentially leading to increased intensity of perception. Obese patients experience higher activation of their brain taste reward and addiction centres in response to high calorie and fat tasting. Gastric bypass may reverse these taste hedonics, perhaps through the influence on gustatory pathways caused by enhanced gut hormone responses after surgery.

SUMMARY

Elucidation of the metabolic mechanism behind the alterations in taste after obesity surgery could lead to the development of novel surgical and nonsurgical procedures for the treatment of obesity.

Peripherally injected cholecystokinin-induced neuronal activation is modified by dietary composition in mice

The aim of this study was to investigate the effect of long-term nutrient intake on the central response to the anorexigenic gut hormone CCK. C57BL/6 mice were fed one of three diets for 6 weeks: standard high carbohydrate (HC), high fat (HF), or high protein (HP). Assessment of brain response to cholecystokinin (CCK) by manganese-enhanced MRI (MEMRI) showed a reduction in neuronal activity both in an appetite-related area (ventromedial nucleus of the hypothalamus) and areas associated with reward (nucleus accumbens and striatum) regardless of diet. When comparing diet effects, while the HF diet did not induce any change in activity, reductions in MEMRI-associated signal were found in the paraventricular nucleus (PVN) and lateral hypothalamic area (LHA) when comparing the HP to the HC diet. In addition, a significant interaction was found between CCK administration and the HF diet, shown by an increased activation in the PVN, which suggests a decrease the inhibiting action of CCK. Our results put forward that the long-term intake of an HP diet leads to a reduction in basal hypothalamic activation while a high-fat diet leads to desensitization to CCK-induced effects in the hypothalamus.

Increased dorsolateral prefrontal cortex activation in obese children during observation of food stimuli

OBJECTIVE

Food cues yield different patterns of brain activation in obese compared with normal-weight adults in prefrontal and limbic/paralimbic areas. For children, no mapping studies comparing representation sites for food and other stimuli between obese and normal-weight subjects are available.

DESIGN

We used a cross-sectional design of two age-matched subject groups to investigate differences in brain activation in response to visually presented food, pleasant, and neutral pictures between obese/overweight and normal children.

SUBJECTS

22 overweight/obese children were compared with 22 normal-weight children.

MEASUREMENTS

Functional magnetic resonance imaging (of the whole head during perception of visually presented stimuli), psychological testing, and psychophysical measures of heart rate deceleration were assessed.

RESULTS

Obese children showed higher activation of the dorsolateral prefrontal cortex (DLPFC) in response to food pictures. In addition, DLPFC activation was negatively correlated with self-esteem. In contrast, normal-weight children showed higher activation of the caudate and hippocampus specific to food pictures, and of the anterior cingulate cortex and thalamus to visual cues in general. In response to food stimuli, obese children showed a heart rate deceleration correlating positively with activation of the ventrolateral prefrontal cortex.

CONCLUSION

Obese children react to food stimuli with increased prefrontal activation, which might be associated with increased inhibitory control.

Targeting the CNS to treat type 2 diabetes

Research on the role of peripheral organs in the regulation of glucose homeostasis has led to the development of various monotherapies that aim to improve glucose uptake and insulin action in these organs for the treatment of type 2 diabetes. It is now clear that the central nervous system (CNS) also plays an important part in orchestrating appropriate glucose metabolism, with accumulating evidence linking dysregulated CNS circuits to the failure of normal glucoregulatory mechanisms. There is evidence that there is substantial overlap between the CNS circuits that regulate energy balance and those that regulate glucose levels, suggesting that their dysregulation could link obesity and diabetes. These findings present new targets for therapies that may be capable of both inducing weight loss and improving glucose regulation.

Asymmetric prefrontal cortex activation in relation to markers of overeating in obese humans

Dietary restraint is heavily influenced by affect, which has been independently related to asymmetrical activation in the prefrontal cortex (prefrontal asymmetry) in electroencephalograph (EEG) studies. In normal weight individuals, dietary restraint has been related to prefrontal asymmetry; however, this relationship was not mediated by affect. This study was designed to test the hypotheses that, in an overweight and obese sample, dietary restraint as well as binge eating, disinhibition, hunger, and appetitive responsivity would be related to prefrontal asymmetry independent of affect at the time of assessment. Resting EEG recordings and self-report measures of overeating and affect were collected in 28 overweight and obese adults. Linear regression analyses were used to predict prefrontal asymmetry from appetitive measures while controlling for affect. Cognitive restraint and binge eating were not associated with prefrontal asymmetry. However, disinhibition, hunger, and appetitive responsivity predicted left-, greater than right-, sided prefrontal cortex activation independent of affect. Findings in this study add to a growing literature implicating the prefrontal cortex in the cognitive control of dietary intake. Further research to specify the precise role of prefrontal asymmetry in the motivation toward, and cessation of, feeding in obese individuals is encouraged.

Neural correlates of individual differences related to appetite

Using neuroimaging technologies to compare normal weight and obese individuals can reveal much about the pathophysiological state of obesity but such comparisons tell us little about what makes some normal weight individuals susceptible to obesity or about important individual differences amongst obese individuals. The current review therefore reviews neuroimaging research on individual difference measures that can illuminate these important topics. After introducing three neuropsychological models of the nature of motivation to approach rewarding stimuli, neuroimaging research on measures of impulsivity, craving, binge eating, restrained eating and disinhibited eating is reviewed. Although neuroimaging research on individual differences measures of brain activity related to appetite is in its infancy, existing studies suggest that such research could enrich the understanding, prevention and treatment of disordered eating and obesity.

Neuroimaging in eating disorders and obesity: implications for research

Medicine and psychiatry have benefited from developments in investigational techniques. Neuroimaging is one such domain that has technically progressed enormously in recent years, resulting in, for example, higher temporal and spatial resolution. Neuroimaging techniques have been widely used in a range of psychiatric disorders, providing new insights into neural brain circuits and neuroreceptor functions in vivo. These imaging techniques allow researchers to study not only the configuration of brain structures but also aspects of normal and anomalous human behavior more accurately.

Differential patterns of neuronal activation in the brainstem and hypothalamus following peripheral injection of GLP-1, oxyntomodulin and lithium chloride in mice detected by manganese-enhanced magnetic resonance imaging (MEMRI)

We have used manganese-enhanced magnetic resonance imaging (MEMRI) to show distinct patterns of neuronal activation within the hypothalamus and brainstem of fasted mice in response to peripheral injection of the anorexigenic agents glucagon-like peptide-1 (GLP-1), oxyntomodulin (OXM) and lithium chloride. Administration of both GLP-1 and OXM resulted in a significant increase in signal intensity (SI) in the area postrema of fasted mice, reflecting an increase in neuronal activity within the brainstem. In the hypothalamus, GLP-1 administration induced a significant reduction in SI in the paraventricular nucleus and an increase in the ventromedial hypothalamic nucleus whereas OXM reduced SI in the arcuate and supraoptic nuclei of the hypothalamus. These data indicate that whilst these related peptides both induce a similar effect on neuronal activity in the brainstem they generate distinct patterns of activation within the hypothalamus. Furthermore, the hypothalamic pattern of signal intensity generated by GLP-1 closely matches that generated by peripheral injection of LiCl, suggesting the anorexigenic effects of GLP-1 may be in part transmitted via nausea circuits. This work provides a framework by which the temporal effects of appetite modulating agents can be recorded simultaneously within hypothalamic and brainstem feeding centres.

Appetitive traits and child obesity: measurement, origins and implications for intervention

Childhood obesity has multiple causes, most of them capable of explaining only one part of the problem. The population-wide impact of sedentary lifestyles and availability of energy-dense food is undeniable, but substantial individual differences in body weight persist, suggesting that individuals respond differently to the 'obesogenic' environment. One plausible mechanism for this variation is the early expression of appetitive traits, including low responsiveness to internal satiety signals, high responsiveness to external food cues, high subjective reward experienced when eating liked foods and preferences for energy-dense foods. Case-control studies support the existence of abnormalities in these traits among obese children compared with normal-weight children, and correlations between psychometric measures of child appetite and child weight suggest that appetitive trait profiles may not only promote obesity but also protect against it. The origins of appetitive traits are as yet uncharted, but will include both genetic and environmental influences. Parental feeding style may affect the development of appetite but the exact nature of the relevant behaviours is unclear and many studies are cross-sectional or begin late in childhood, obscuring causal relationships. Future research should explore determinants and biological mechanisms by using prospective designs beginning early in life, measuring relevant biomarkers such as gut hormones and incorporating neuroimaging and genotyping technologies. Potential clinical applications include the identification of 'at risk' children early in life and interventions to modify appetitive traits or ameliorate their impact on intake and weight.

Low dopamine striatal D2 receptors are associated with prefrontal metabolism in obese subjects: possible contributing factors

Dopamine's role in inhibitory control is well recognized and its disruption may contribute to behavioral disorders of discontrol such as obesity. However, the mechanism by which impaired dopamine neurotransmission interferes with inhibitory control is poorly understood. We had previously documented a reduction in dopamine D2 receptors in morbidly obese subjects. To assess if the reductions in dopamine D2 receptors were associated with activity in prefrontal brain regions implicated in inhibitory control we assessed the relationship between dopamine D2 receptor availability in striatum with brain glucose metabolism (marker of brain function) in ten morbidly obese subjects (BMI>40 kg/m(2)) and compared it to that in twelve non-obese controls. PET was used with [(11)C]raclopride to assess D2 receptors and with [(18)F]FDG to assess regional brain glucose metabolism. In obese subjects striatal D2 receptor availability was lower than controls and was positively correlated with metabolism in dorsolateral prefrontal, medial orbitofrontal, anterior cingulate gyrus and somatosensory cortices. In controls correlations with prefrontal metabolism were not significant but comparisons with those in obese subjects were not significant, which does not permit to ascribe the associations as unique to obesity. The associations between striatal D2 receptors and prefrontal metabolism in obese subjects suggest that decreases in striatal D2 receptors could contribute to overeating via their modulation of striatal prefrontal pathways, which participate in inhibitory control and salience attribution. The association between striatal D2 receptors and metabolism in somatosensory cortices (regions that process palatability) could underlie one of the mechanisms through which dopamine regulates the reinforcing properties of food.

Brain activity in hunger and satiety: an exploratory visually stimulated FMRI study

OBJECTIVE

To explore neuroanatomical sites of eating behavior, we have developed a simple functional magnetic resonance imaging (fMRI) paradigm to image hunger vs. satiety using visual stimulation.

METHODS AND PROCEDURES

Twelve healthy, lean, nonsmoking male subjects participated in this study. Pairs of food-neutral and food-related pictures were presented in a block design, after a 14-h fast and 1 h after ad libitum ingestion of a mixed meal. Statistically, a general linear model for serially autocorrelated observations with a P level<0.001 was used.

RESULTS

During the hunger condition, significantly enhanced brain activity was found in the left striate and extrastriate cortex, the inferior parietal lobe, and the orbitofrontal cortices. Stimulation with food images was associated with increased activity in both insulae, the left striate and extrastriate cortex, and the anterior midprefrontal cortex. Nonfood images were associated with enhanced activity in the right parietal lobe and the left and right middle temporal gyrus. A significant interaction in activation pattern between the states of hunger and satiety and stimulation with food and nonfood images was found for the left anterior cingulate cortex, the superior occipital sulcus, and in the vicinity of the right amygdala.

DISCUSSION

These preliminary data from a homogenous healthy male cohort suggest that central nervous system (CNS) activation is not only altered with hunger and satiety but that food and nonfood images have also specific effects on regional brain activity if exposure takes place in different states of satiety. Wider use of our or a similar approach would help to establish a uniform paradigm to map hunger and satiety to be used for further experiments.

Diffusion-weighted imaging features of brain in obesity

PURPOSE

Obesity is characterized by an altered distribution of body fluid. However, distribution of fluid (extracellular/intracellular) in brain tissues has not been studied in obese subjects yet. The purpose of this study was to detect possible brain diffusion changes especially in satiety and hunger related centers in obese subjects by diffusion weighted imaging (DWI).

METHODS

Conventional MRI and DWI of the brain was obtained from 81 obese patients (obese=68, morbid obese=13) and 29 age-matched, nonobese. The apparent diffusion coefficient (ADC) values were calculated in hypothalamus; amygdala; hippocampal gyrus; thalamus; insula; cingulate gyrus; orbitofrontal, dorsomedial and dorsolateral frontal, middle temporal and occipital cortex; cerebellum; midbrain and corpus striatum.

RESULTS

The ADC values of hypothalamus, hippocampal gyrus, amygdala, insula, cerebellum and midbrain were significantly increased in patients (n:81) when compared to nonobese subjects. The ADC values of thalamus, hippocampal gyrus, amygdala, orbitofrontal, occipital, dorsolateral and middle temporal cortex, insula and midbrain were significantly increased in morbid obese when compared to nonobese subjects. The ADC values of orbitofrontal and occipital cortex were significantly higher in morbid obese than the values in the obese. The body mass index positively correlated with ADC values of amygdala, insula, orbitofrontal and middle temporal cortex.

CONCLUSION

We observed increased ADC values of distinct locations related to satiety and hunger that suggest altered fluid distribution and/or vasogenic edema in obese subjects. Awareness of this abnormalities in brain tissue composition/function in obesity may contribute to better understanding of the underlying mechanisms.

The temporal sequence of gut peptide CNS interactions tracked in vivo by magnetic resonance imaging

Hormonal satiety signals secreted by the gut play a pivotal role in the physiological control of appetite. However, therapeutic exploitation of the gut-brain axis requires greater insight into the interaction of gut hormones with CNS circuits of appetite control. Using the manganese ion (Mn2+) as an activity-dependent magnetic resonance imaging (MRI) contrast agent, we showed an increase in signal intensity (SI) in key appetite-regulatory regions of the hypothalamus, including the arcuate, paraventricular, and ventromedial nuclei, after peripheral injection of the orexigenic peptide ghrelin. Conversely, administration of the anorexigenic hormone peptide YY(3-36) caused a reduction in SI. In both cases, the changes in SI recorded in the hypothalamic arcuate nucleus preceded the effect of these peptides on food intake. Intravenous Mn2+ itself did not significantly alter ghrelin-mediated expression of the immediate early gene product c-Fos, nor did it cause abnormalities of behavior or metabolic parameters. We conclude that manganese-enhanced MRI constitutes a powerful tool for the future investigation of the effects of drugs, hormones, and environmental influences on neuronal activity.

Less activation in the left dorsolateral prefrontal cortex in the reanalysis of the response to a meal in obese than in lean women and its association with successful weight loss

BACKGROUND

We previously found that obese men have less activation in the left dorsolateral prefrontal cortex (LDLPFC) in response to a meal than do lean men, which indicates an association between this altered neuronal response and the pathophysiology of obesity.

OBJECTIVES

The objectives of the study were to extend this finding in obese women and to investigate activity in this region in women with a history of severe obesity who have successfully lost weight (ie, formerly obese women, sometimes called postobese women).

DESIGN

We reanalyzed previously collected data to compare postmeal (after receiving a liquid meal) with premeal (after a 36-h fast) regional cerebral blood flow, a marker of neuronal activity, by using (15)O-water positron emission tomography in 10 lean [26 +/- 6% body fat (BF)], 9 obese (39 +/- 3%BF) and 8 formerly obese (28 +/- 4%BF) right-handed women. Data were analyzed by using a 2-level, random-effect analysis of variance.

RESULTS

The regional cerebral blood flow in the LDLPFC differed in response to the meal across the 3 groups (P < 0.001, uncorrected for multiple comparisons). Post hoc group comparisons showed that obese women had significantly less activation in this area than did lean and formerly obese women. No significant difference between formerly obese and lean women was found.

CONCLUSIONS

These results extend our previous findings, indicating that obese women have less activation in the LDLPFC in response to a meal than do lean or formerly obese women. Neuronal activity in this region did not differ significantly between the latter 2 groups. Longitudinal studies are needed to determine whether these differences in neuronal activity change with or predict weight change.

Enhanced activation of reward mediating prefrontal regions in response to food stimuli in Prader-Willi syndrome

BACKGROUND

Individuals with Prader-Willi syndrome (PWS) exhibit severe disturbances in appetite regulation, including delayed meal termination, early return of hunger after a meal, seeking and hoarding food and eating of non-food substances. Brain pathways involved in the control of appetite in humans are thought to include the hypothalamus, frontal cortex (including the orbitofrontal, ventromedial prefrontal, dorsolateral prefrontal and anterior cingulate areas), insula, and limbic and paralimbic areas. We hypothesised that the abnormal appetite in PWS results from aberrant reward processing of food stimuli in these neural pathways.

METHODS

We compared functional MRI blood oxygen level dependent (BOLD) responses while viewing pictures of food in eight adults with PWS and eight normal weight adults after ingestion of an oral glucose load.

RESULTS

Subjects with PWS demonstrated significantly greater BOLD activation in the ventromedial prefrontal cortex than controls when viewing food pictures. No significant differences were found in serum insulin, glucose or triglyceride levels between the groups at the time of the scan.

CONCLUSIONS

Individuals with PWS had an increased BOLD response in the ventromedial prefrontal cortex compared with normal weight controls when viewing pictures of food after an oral glucose load. These findings suggest that an increased reward value for food may underlie the excessive hunger in PWS, and support the significance of the frontal cortex in modulating the response to food in humans. Our findings in the extreme appetite phenotype of PWS support the importance of the neural pathways that guide reward related behaviour in modulating the response to food in humans.

VBM signatures of abnormal eating behaviours in frontotemporal lobar degeneration

The brain bases of specific human behaviours in health and disease are not well established. In this voxel-based morphometric (VBM) study we demonstrate neuroanatomical signatures of different abnormalities of eating behaviour (pathological sweet tooth and increased food consumption, or hyperphagia) in individuals with frontotemporal lobar degeneration (FTLD). Sixteen male patients with FTLD were assessed using the Manchester and Oxford Universities Scale for the Psychopathological Assessment of Dementia and classified according to the presence or absence of abnormal eating behaviours. Volumetric brain magnetic resonance imaging was performed in all patients and in a group of nine healthy age-matched male controls and grey matter changes were assessed using an optimised VBM protocol. Compared with healthy controls, the FTLD group had a typical pattern of extensive bilateral grey matter loss predominantly involving the frontal and temporal lobes. Within the FTLD group, grey matter changes associated with different abnormal behaviours were assessed independently using a covariate-only model. The development of pathological sweet tooth was associated with grey matter loss in a distributed brain network including bilateral posterolateral orbitofrontal cortex (Brodmann areas 12/47) and right anterior insula. Hyperphagia was associated with more focal grey matter loss in anterolateral OFC bilaterally (Brodmann area 11). In accord with emerging evidence in humans and other species, our findings implicate distinct components of a multi-component brain network in the control of specific aspects of eating behaviour.

Postprandial glucagon-like peptide-1 (GLP-1) response is positively associated with changes in neuronal activity of brain areas implicated in satiety and food intake regulation in humans

Postprandial glucagon-like peptide-1 (GLP-1) secretion can act as a meal termination signal in animals and humans. We tested the hypothesis that the postprandial changes in plasma GLP-1 concentrations are associated with changes in the human brain activity in response to satiety by performing a post-hoc analysis of a cross-sectional study of neuroanatomical correlates of hunger and satiation using (15)O-water positron-emission tomography (PET). Forty-two subjects (22M/20F, age 31+/-8 years) spanning a wide range of adiposity (body fat: 7-44%) were included in this analysis. Outcome measures included changes in PET-measured regional cerebral blood flow (rCBF) and plasma concentrations of GLP-1, glucose, insulin, and free-fatty acids (FFA), elicited by the administration of a satiating amount of a liquid formula meal. The peak postprandial increases in plasma GLP-1 concentrations were correlated with increases in rCBF in the left dorsolateral prefrontal cortex (including the left middle and inferior frontal gyri), previously implicated in PET studies of human satiation, and the hypothalamus, previously implicated in the regulation of food intake in animal and human studies, both before and after adjustment for sex, age, body fat, and changes in plasma glucose, insulin, and serum FFA concentrations. The postprandial GLP-1 response is associated with activation of areas of the human brain previously implicated in satiation and food intake regulation.

Less activation of the left dorsolateral prefrontal cortex in response to a meal: a feature of obesity

BACKGROUND

In an exploratory positron emission tomography study of postprandial regional cerebral blood flow, which is a marker of neuronal activity, obese men differed from lean men in several brain regions, including the prefrontal cortex. The subjects received a meal proportional to their body size; therefore, the meal volume was different for each person.

OBJECTIVE

We investigated whether differences in the brain responses of obese and lean men to a meal represent satiety or feelings of gastric distension.

DESIGN

We studied 9 lean (x +/- SD body fat: 15 +/- 5%; age: 33 +/- 10 y) and 9 obese (body fat: 31 +/- 4%; age: 32 +/- 10 y) men given a fixed amount (400 mL) of a liquid meal. We compared their results with those in 11 lean (body fat: 16 +/- 5%; age: 35 +/- 8 y) and 11 obese (body fat: 33 +/- 5%; age: 28 +/- 5 y) previously studied men given a meal proportional to their body size. We performed analyses by using a two-level, random-effects approach in the STATISTICAL PARAMETRIC MAPPING software package and a significance level of P < or = 0.001, uncorrected for multiple comparisons.

RESULTS

Compared with lean men, obese men had consistently less postprandial activation in the left dorsolateral prefrontal cortex, irrespective of meal size.

CONCLUSION

Because the dorsolateral prefrontal cortex has been implicated in the inhibition of inappropriate behavior, satiety, and meal termination, differential responses of neuronal activity to food intake in this area may contribute to a propensity for obesity or to the difficulty in losing weight experienced by obese men.

Differential hypothalamic neuronal activation following peripheral injection of GLP-1 and oxyntomodulin in mice detected by manganese-enhanced magnetic resonance imaging

The anorexigenic gut hormones oxyntomodulin (OXM) and glucagon-like peptide-1 (GLP-1) are thought to physiologically regulate appetite and food intake. Using manganese-enhanced magnetic resonance imaging, we have shown distinct patterns of neuronal activation in the hypothalamus in response to intraperitoneal injections into fasted mice of 900 and 5400 nmol/kg OXM or 900 nmol/kg GLP-1. Administration of OXM at either dose resulted in a reduced rate of signal enhancement, reflecting a reduction in neuronal activity, in the arcuate, paraventricular, and supraoptic nuclei of the hypothalamus. Conversely, GLP-1 caused a reduction in signal enhancement in the paraventricular nucleus only and an increase in the ventromedial hypothalamic nucleus. Our data show that these two apparently similar peptides generate distinct patterns of activation within the hypothalamus, suggesting that GLP-1 and OXM may act via different hypothalamic pathways.

Changes in extracellular hypothalamic glucose in relation to feeding

The aim of the present in vivo microdialysis study was to investigate the relation between feeding and changes in glucose concentrations in the rat ventromedial hypothalamus (VMH). Absolute ambient glucose concentrations in VMH were 1.43 mm in non-deprived rats as compared to 0.94 mm after 24-h food deprivation. To examine whether feeding influences hypothalamic glucose, changes of glucose concentration over time were determined relative to a baseline. Experiments were conducted in relation to both, nutritional state (food-deprived rats vs. non-deprived rats) and feeding conditions throughout the experiment (freely feeding rats vs. rats without access to food). The results of this microdialysis study show clearly that glucose concentration in the VMH of rats increases significantly in relation to food intake. The data demonstrate that a 24-h food deprivation before the experiment further augments this increase (up to 350% from baseline) as compared to non-deprived conditions (up to 60% from baseline). However, the magnitude of food related increase in VMH glucose does not correlate with the individual amount of food eaten. In conclusion, the present study shows for the first time that VMH glucose concentrations increase with food intake in the early dark phase, indicating that such changes do not only occur after pharmacological treatment, but also under physiological feeding conditions. The results further indicate that the feeding related increase in VMH glucose depends on the nutritional state of the organism.

Obesity and its therapy: from genes to community action

Like many diseases, the causes of obesity are complex, and their investigation requires novel approaches. Given the many contributors to our weight status, as well as the dynamic nature, genomic tools must be applied in an ecological model. Evaluating disparate factors can be difficult, such as feeding behavior, nutritional genomics, and gene-environment interaction. Many of these behaviors are being evaluated in animal models and hold great promise for targeted interventions in the future.

Successful dieters have increased neural activity in cortical areas involved in the control of behavior

OBJECTIVE

To investigate whether dietary restraint, a landmark of successful dieting, is associated with specific patterns of brain responses to the sensory experience of food and meal consumption.

DESIGN AND SUBJECTS

Cross-sectional study of the brain's response to the sensory experience of food and meal consumption in nine successful dieters (age: 38+/-7 years, body fat (%): 28+/-3) and 20 non-dieters (age: 31+/-9 years, body fat (%): 33+/-9), all women.

MEASUREMENTS

Changes in brain activity in response to the sensory experience of food and meal consumption were assessed by using positron emission tomography and (15)O water as a radiotracer. Body fatness was assessed by dual X-ray absorptiometry. Subjective ratings of hunger and fullness were measured by visual analogue scale. Dietary restraint, disinhibition and hunger were assessed by the Three Factor Eating Questionnaire.

RESULTS

Successful dieters had a significantly higher level of dietary restraint compared to non-dieters. In response to meal consumption, successful dieters had a greater activation in the dorsal prefrontal cortex (DPFC), dorsal striatum and anterior cerebellar lobe as compared to non-dieters. In response to the same stimulation, the orbitofrontal cortex (OFC) was significantly more activated in non-dieters as compared to successful dieters. Dietary restraint was positively correlated with the response in the DPFC and negatively with the response in the OFC. The responses in the DPFC and OFC were negatively intercorrelated.

CONCLUSION

Cortical areas involved in controlling inappropriate behavioral responses, such as the DPFC, are particularly activated in successful dieters in response to meal consumption. The association between the degree of dietary restraint and the coordinated neural changes in the DPFC and OFC raises the possibility that cognitive control of food intake is achieved by modulating neural circuits controlling food reward.

The metabolic syndrome, depression, and cardiovascular disease: interrelated conditions that share pathophysiologic mechanisms

This article introduces the metabolic syndrome as a clinical phenotype with consequences for diagnosis and treatment that go beyond the different clinical specialties involved. A life-course approach is suggested as a means of understanding the complex interrelations between the metabolic syndrome, depression, and cardiovascular disease. Pathophysiologic mechanisms that these conditions share are discussed in detail. These considerations provide arguments for a more integrative approach to patients in general that surpass the current disease-centered services such as endocrinology, psychiatry, and cardiology.

Neural mechanisms underlying hyperphagia in Prader-Willi syndrome

OBJECTIVE

Prader-Willi syndrome (PWS) is a genetic disorder associated with developmental delay, obesity, and obsessive behavior related to food consumption. The most striking symptom of PWS is hyperphagia; as such, PWS may provide important insights into factors leading to overeating and obesity in the general population. We used functional magnetic resonance imaging to study the neural mechanisms underlying responses to visual food stimuli, before and after eating, in individuals with PWS and a healthy weight control (HWC) group.

RESEARCH METHODS AND PROCEDURES

Participants were scanned once before (pre-meal) and once after (post-meal) eating a standardized meal. Pictures of food, animals, and blurred control images were presented in a block design format during acquisition of functional magnetic resonance imaging data.

RESULTS

Statistical contrasts in the HWC group showed greater activation to food pictures in the pre-meal condition compared with the post-meal condition in the amygdala, orbitofrontal cortex, medial prefrontal cortex (medial PFC), and frontal operculum. In comparison, the PWS group exhibited greater activation to food pictures in the post-meal condition compared with the pre-meal condition in the orbitofrontal cortex, medial PFC, insula, hippocampus, and parahippocampal gyrus. Between-group contrasts in the pre- and post-meal conditions confirmed group differences, with the PWS group showing greater activation than the HWC group after the meal in food motivation networks.

DISCUSSION

Results point to distinct neural mechanisms associated with hyperphagia in PWS. After eating a meal, the PWS group showed hyperfunction in limbic and paralimbic regions that drive eating behavior (e.g., the amygdala) and in regions that suppress food intake (e.g., the medial PFC).

The hypothalamus, hormones, and hunger: alterations in human obesity and illness

Obesity is a major global epidemic, with over 300 million obese people worldwide, and nearly 1 billion overweight adults. Being overweight carries significant health risks, reduced quality of life, and impaired socioeconomic success, with profound consequences for health expenditure. The most successful treatment for obesity is gastric bypass surgery, which acts in part by reducing appetite through alterations in gut hormones. Circulating gut hormones, secreted or suppressed after eating food, act in the brain, particularly the hypothalamus, to alter hunger and fullness. Stomach-derived ghrelin increases food intake even in those with anorexia from chronic illness, while pancreatic polypeptide (PP), intestinal peptide YY 3-36 (PYY), oxyntomodulin, and other hormones reduce food intake and appetite. While obese subjects have appropriate reductions in orexigenic ghrelin, other gut-hormone disturbances may contribute to obesity such as reduced anorexigenic PYY and PP. Prader-Willi syndrome (PWS) arises from the loss of paternally inherited genes on chromosome 15q11-13, leading to life-threatening insatiable hunger and obesity from early childhood, through developmental brain, particularly hypothalamic defects. The study of genetically homogenous causes of abnormal-feeding behavior helps our understanding of appetite regulation. PWS subjects have inappropriately elevated plasma ghrelin for their obesity, at least partly explained by preserved insulin sensitivity. It remains unproven if their hyperghrelinemia or other gut-hormone abnormalities contribute to the hyperphagia in PWS, in addition to brain defects. Postmortem human hypothalamic studies and generation of animal models of PWS can also provide insight into the pathophysiology of abnormal-feeding behavior. Changes in orexigenic NPY and AGRP hypothalamic neurons, or anorexigenic oxytocin neurons have been found in illness and PWS. Functional neuroimaging studies, using PET and fMRI, will also allow us to tease apart the hormonal and brain pathways responsible for controlling human appetite, and their defects in obesity.

Neural representations of hunger and satiety in Prader–Willi syndrome

OBJECTIVE

To investigate the neural basis of the abnormal eating behaviour in Prader-Willi syndrome (PWS), using brain imaging. We predicted that the satiety response in those with PWS would be delayed and insensitive to food intake.

DESIGN AND PARTICIPANTS

The design of this study was based on a previous investigation of the neural activation associated with conditions of fasting and food intake in a nonobese, non-PWS group. The findings were used to generate specific hypotheses regarding brain regions of interest for the current study, in which 13 adults with PWS took part (mean +/- s.d. age = 29 +/- 6; BMI = 31.5 +/- 5.1; IQ = 71 +/- 8, six were female).

MEASUREMENTS

Regional cerebral blood flow was measured using positron emission tomography in three sessions: one following an overnight fast and two following disguised energy controlled meals of similar volume and appearance--one of 1674 kJ (400 kcal) and another of 5021 kJ (1200 kcal). Subjective ratings of hunger, fullness and desire to eat, and blood plasma levels of glucose, insulin, leptin, ghrelin and PYY were measured before and after each imaging session.

RESULTS

The neural representation of hunger, after an overnight fast, was similar to that found in nonobese individuals in the control study. In contrast, after food intake, the patterns of neural activation previously associated with satiety were not found, even after the higher-energy load. Lateral and medial orbitofrontal cortical activation was associated with consumption of the 400- and 1200-kcal meals, respectively. The medial orbitofrontal activation, however, was only found in those who had shown a large percentage change in fullness ratings following the higher-energy meal.

CONCLUSION

We conclude that there is a dysfunction in the satiety system in those with PWS. These findings suggest that brain regions associated with satiety are insensitive even to high-energy food intake in those with the syndrome. This may be the neural basis of the hyperphagia seen in PWS.

Neural mechanisms underlying food motivation in children and adolescents

Dramatic increases in childhood obesity necessitate a more complete understanding of neural mechanisms of hunger and satiation in pediatric populations. In this study, normal weight children and adolescents underwent functional magnetic resonance imaging (fMRI) scanning before and after eating a meal. Participants showed increased activation to visual food stimuli in the amygdala, medial frontal/orbitofrontal cortex, and insula in the pre-meal condition; no regions of interest responded in the post-meal condition. These results closely parallel previous findings in adults. In addition, we found evidence for habituation to food stimuli in the amygdala within the pre-meal session. These findings provide evidence that normal patterns of neural activity related to food motivation begin in childhood. Results have implications for obese children and adults, who may have abnormal hunger and satiation mechanisms.

In pursuit of neural risk factors for weight gain in humans

Obesity is a multifactorial disease associated with an increased risk of type 2 diabetes, coronary artery disease, cancer, and consequently, with a reduced length of life. Metabolic phenotypes of reduced energy expenditure have been associated with weight gain, but their contribution has been estimated to be relatively small. On the other hand, excessive food intake is likely to be the major determinant of positive energy balances and it is underlied by both non-conscious (homeostatic) and conscious (perceptual, emotional, and cognitive) phenomena processed in the brain. Functional neuroimaging is a promising tool to investigate these neural substrates in humans, because it provides a measurement of state-dependent brain regional activity, bridging the gap between neural events and behavioral responses. Using this technology, a few studies have provided the first evidence of functional differences between obese and lean individuals in the brain's response to energy intake and investigated the presence of neural risk factors of weight gain.

Endocannabinoids and food consumption: comparisons with benzodiazepine and opioid palatability-dependent appetite

The endocannabinoid system consists of several endogenous lipids, including anandamide and 2-arachidonoyl-glycerol (2-AG), and constitute a retrograde signalling system, which modulates neurotransmitter release and synaptic plasticity. Specific brain-type cannabinoid receptors (CB(1)) are widely distributed in the central nervous system, and are localized presynaptically. Mounting evidence, reviewed here, indicates that cannabinoids can act to increase food consumption, and cannabinoid CB(1) receptor antagonists/inverse agonists reduce food intake and suppress operant responding for food rewards. Hence, endocannabinoids provide the first example of a retrograde signalling system, which is strongly implicated in the control of food intake. Benzodiazepine and opioid palatability-dependent appetite are well-established processes supported by several sources of convergent evidence; they provide pharmacological benchmarks against which to evaluate the endocannabinoids. To date, evidence that endocannabinoids specifically modulate palatability as an affective evaluative process is insufficient and not compelling. Endocannabinoids may have important clinical utility in the treatment of human obesity and forms of eating disorders.

Sensory experience of food and obesity: a positron emission tomography study of the brain regions affected by tasting a liquid meal after a prolonged fast

The sensory experience of food is a primary reinforcer of eating and overeating plays a major role in the development of human obesity. However, whether the sensory experience of a forthcoming meal and the associated physiological phenomena (cephalic phase response, expectation of reward), which prepare the organism for the ingestion of food play a role in the regulation of energy intake and contribute to the development of obesity remains largely unresolved. We used positron emission tomography (PET) and 15O-water to measure changes in regional cerebral blood flow (rCBF) and to assess the brain's response to the oral administration of 2 ml of a liquid meal (Ensure Plus, 1.5 kcal/ml) after a 36-h fast and shortly before consuming the same meal. Twenty-one obese (BMI > 35 kg/m2, 10M/11F, age 28 +/- 6 years, body fat 40 +/- 6%) and 20 lean individuals (BMI < 25 kg/m2, 10M/10F, age 33 +/- 9 years, body fat 21 +/- 7%) were studied. Compared to lean individuals, obese individuals had higher fasting plasma glucose (83.3 +/- 6.2 vs. 75.5 +/- 9.6 mg/dl; P = 0.0003) and insulin concentrations (6.1 +/- 3.5 vs. 2.5 +/- 1.7 microU/ml; P < 0.0001) and were characterized by a higher score of dietary disinhibition (i.e., the susceptibility of eating behavior to emotional factors and sensory cues, 5.7 +/- 3.6 vs. 3.5 +/- 2.7; P = 0.01) assessed by the Three Factor Eating Questionnaire. In response to the sensory experience of food, differences in rCBF were observed in several regions of the brain, including greater increases in the middle-dorsal insula and midbrain, and greater decreases in the posterior cingulate, temporal, and orbitofrontal cortices in obese compared to lean individuals (P < 0.05, after small volume correction). In a multiple regression model, percentage of body fat (P = 0.04), glycemia (P = 0.01), and disinhibition (P = 0.07) were independent correlates of the neural response to the sensory experience of the meal in the middle-dorsal insular cortex (R2 = 0.45). We conclude that obesity is associated with an abnormal brain response to the sensory aspects of a liquid meal after a prolonged fast especially in areas of the primary gustatory cortex. This is only partially explained by the elevated glycemia and high level of disinhibition which characterize individuals with increased adiposity. These results provide a new perspective on the understanding of the neuroanatomical correlates of abnormal eating behavior and their relationship with obesity in humans.

Neural contributions to the motivational control of appetite in humans

The motivation to eat in humans is a complex process influenced by intrinsic mechanisms relating to the hunger and satiety cascade, and extrinsic mechanisms based on the appetitive incentive value of individual foods, which can themselves induce desire. This study was designed to investigate the neural basis of these two factors contributing to the control of motivation to eat within the same experimental design using positron emission tomography. Using a novel counterbalanced approach, participants were scanned in two separate sessions, once after fasting and once after food intake, in which they imagined themselves in a restaurant and considered a number of items on a menu, and were asked to choose their most preferred. All items were tailored to each individual and varied in their incentive value. No actual foods were presented. In response to a hungry state, increased activation was shown in the hypothalamus, amygdala and insula cortex as predicted, as well as the medulla, striatum and anterior cingulate cortex. Satiety, in contrast, was associated with increased activation in the lateral orbitofrontal and temporal cortex. Only activity in the vicinity of the amygdala and orbitofrontal cortex was observed in response to the processing of extrinsic appetitive incentive information. These results suggest that the contributions of intrinsic homeostatic influences, and extrinsic incentive factors to the motivation to eat, are somewhat dissociable neurally, with areas of convergence in the amygdala and orbitofrontal cortex. The findings of this study have implications for research into the underlying mechanisms of eating disorders.