How exposure to chronic stress contributes to the development of type 2 diabetes: A complexity science approach.

Chronic stress contributes to the onset of type 2 diabetes (T2D), yet the underlying etiological mechanisms are not fully understood. Responses to stress are influenced by earlier experiences, sex, emotions and cognition, and involve a complex network of neurotransmitters and hormones, that affect multiple biological systems. In addition, the systems activated by stress can be altered by behavioral, metabolic and environmental factors. The impact of stress on metabolic health can thus be considered an emergent process, involving different types of interactions between multiple variables, that are driven by non-linear dynamics at different spatiotemporal scales. To obtain a more comprehensive picture of the links between chronic stress and T2D, we followed a complexity science approach to build a causal loop diagram (CLD) connecting the various mediators and processes involved in stress responses relevant for T2D pathogenesis. This CLD could help develop novel computational models and formulate new hypotheses regarding disease etiology.

Stress, rhythm, choice and the munchies - tribute to Mary F. Dallman.

In December 2021, we lost a pioneer in the field of stress research who inspired generations of scientists. Mary Dallman was an expert on the hypothalamic-pituitary-adrenal (HPA) axis, its interactions with a wide variety of other physiological systems and the impact of chronic changes of HPA function on energy metabolism and adiposity. She was not only an excellent scientist, she was a great role model and mentor for young scientists, especially women. She encouraged and supported many of her trainees even long after they left the lab. Her outside-the-box thinking, the fun and crazy discussions we had in the lab proved to be a beautiful basis for my own future research.

Disruption of lateral hypothalamic calorie detection by a free choice high fat diet.

During the last few decades, the consumption of low-calorie sweeteners, as a substitute for caloric sweeteners, has sharply increased. Although research shows that caloric versus low-calorie sweeteners can have differential effects on the brain, it is unknown which neuronal populations are responsible for detecting the difference between the two types of sweeteners. Using in vivo two-photon calcium imaging, we investigated how drinking sucrose or sucralose (a low-calorie sweetener) affects the activity of glutamatergic neurons in the lateral hypothalamus. Furthermore, we explored the consequences of consuming a free-choice high fat diet on the calorie detection abilities of these glutamatergic neurons. We found that glutamatergic neurons indeed can discriminate sucrose from water and sucralose, and that consumption of a free-choice high fat diet shifts the glutamatergic neuronal response from sucrose-specific to sucralose-specific, thereby disrupting calorie detection. These results highlight the disruptive effects of a diet high in saturated fat on calorie detection in the lateral hypothalamus.

The Clash of Two Epidemics: the Relationship Between Opioids and Glucose Metabolism.

PURPOSE OF REVIEW: We are currently in the midst of a global opioid epidemic. Opioids affect many physiological processes, but one side effect that is not often taken into consideration is the opioid-induced alteration in blood glucose levels. RECENT FINDINGS: This review shows that the vast majority of studies report that opioid stimulation increases blood glucose levels. In addition, plasma levels of the endogenous opioid ß-endorphin rise in response to low blood glucose. In contrast, in hyperglycaemic baseline conditions such as in patients with type 2 diabetes mellitus (T2DM), opioid stimulation lowers blood glucose levels. Furthermore, obesity itself alters sensitivity to opioids, changes opioid receptor expression and increases plasma ß-endorphin levels. Thus, opioid stimulation can have various side effects on glycaemia that should be taken into consideration upon prescribing opioid-based medication, and more research is needed to unravel the interaction between obesity, glycaemia and opioid use.

Effects of Neuropeptide Y administration into the lateral hypothalamus on intake of free-choice high-fat high-sucrose diet components of the male Wistar rat.

Introduction: Neuropeptide Y (NPY) signaling in the brain plays an important role in energy regulation, and is altered during diet-induced obesity. Yet, NPY function during the consumption of specific diet components remains to be fully determined. We have previously demonstrated that consumption of a saturated fat component (free-choice high-fat; fcHF), a sucrose solution (high-sugar; fcHS), or both (fcHFHS) combined with a standard diet (chow and water) has diverse effects on Npy expression in the arcuate nucleus and the sensitivity to intraventricular NPY administration. Arcuate NPY neurons project to the lateral hypothalamus (LHA), and NPY administration in the LHA potently promotes chow intake in rats on a standard diet. However, it is currently unclear if short-term consumption of a palatable free-choice diet alters NPY function in the LHA. Therefore, we assessed the effects of intra-LHA NPY administration on intake in rats following one-week consumption of a fcHF, fcHS, or fcHFHS diet.Methods: Male Wistar rats consumed a fcHF, fcHS, fcHFHS, or control (CHOW) diet for one week before NPY (0.3 µg / 0.3 µL) or phosphate-buffered saline (0.3 µL) was administered into the LHA. Intake was measured 2h later. fcHFHS-fed rats were divided into high-fat (fcHFHS-hf) and low-fat (fcHFHS-lf) groups based on differences in basal fat intake.Results: Intra-LHA NPY administration increased chow intake in fcHFHS- (irrespective of basal fat intake), fcHF- and CHOW-fed rats. Intra-LHA NPY infusion increased fat intake in fcHF-, fcHFHS-hf, but not fcHFHS-lf, rats. Intra-LHA NPY infusion did not increase caloric intake in fcHS-fed rats.Discussion: Our data demonstrate that the effects of intra-LHA NPY on caloric intake differ depending on the consumption of a fat or sugar component, or both, in a free-choice diet. Our data also indicate that baseline preference for the fat diet component modulates the effects of intra-LHA NPY in fcHFHS-fed rats.

Loss of Microglial Insulin Receptor Leads to Sex-Dependent Metabolic Disorders in Obese Mice.

Obesity and type 2 diabetes mellitus (T2DM) are highly prevalent disorders, associated with insulin resistance and chronic inflammation. The brain is key for energy homeostasis and contains many insulin receptors. Microglia, the resident brain immune cells, are known to express insulin receptors (InsR) and to be activated by a hypercaloric environment. The aim of this study was to evaluate whether microglial insulin signaling is involved in the control of systemic energy homeostasis and whether this function is sex-dependent. We generated a microglia-specific knockout of the InsR gene in male and female mice and exposed them to control or obesogenic dietary conditions. Following 10 weeks of diet exposure, we evaluated insulin tolerance, energy metabolism, microglial morphology and phagocytic function, and neuronal populations. Lack of microglial InsR resulted in increased plasma insulin levels and insulin resistance in obese female mice. In the brain, loss of microglial InsR led to a decrease in microglial primary projections in both male and female mice, irrespective of the diet. In addition, in obese male mice lacking microglial InsR the number of proopiomelanocortin neurons was decreased, compared to control diet, while no differences were observed in female mice. Our results demonstrate a sex-dependent effect of microglial InsR-signaling in physiology and obesity, and stress the importance of a heterogeneous approach in the study of diseases such as obesity and T2DM.

Localization of fibroblast growth factor 23 protein in the rat hypothalamus.

Fibroblast growth factor 23 (FGF23) is an endocrine growth factor and known to play a pivotal role in phosphate homeostasis. Interestingly, several studies point towards a function of FGF23 in the hypothalamus. FGF23 classically activates the FGF receptor 1 in the presence of the co-receptor αKlotho, of both gene expression in the brain was previously established. However, studies on gene and protein expression of FGF23 in the brain are scarce and have been inconsistent. Therefore, our aim was to localise FGF23 gene and protein expression in the rat brain with focus on the hypothalamus. Also, we investigated the protein expression of αKlotho. Adult rat brains were used to localise and visualise FGF23 and αKlotho protein in the hypothalamus by immunofluorescence labelling. Furthermore, western blots were used for assessing hypothalamic FGF23 protein expression. FGF23 gene expression was investigated by qPCR in punches of the arcuate nucleus, lateral hypothalamus, paraventricular nucleus, choroid plexus, ventrolateral thalamic nucleus and the ventromedial hypothalamus. Immunoreactivity for FGF23 and αKlotho protein was found in the hypothalamus, third ventricle lining and the choroid plexus. Western blot analysis of the hypothalamus confirmed the presence of FGF23. Gene expression of FGF23 was not detected, suggesting that the observed FGF23 protein is not brain-derived. Several FGF receptors are known to be present in the brain. Therefore, we conclude that the machinery for FGF23 signal transduction is present in several brain areas, indeed suggesting a role for FGF23 in the brain.

Lipid Droplets Accumulate in the Hypothalamus of Mice and Humans with and without Metabolic Diseases.

BACKGROUND: In peripheral tissues, the lipid droplet (LD) organelle links lipid metabolism, inflammation, and insulin resistance. Little is known about the brain LDs. OBJECTIVES: We hypothesized that hypothalamic LDs would be altered in metabolic diseases. METHODS: We used immunofluorescence labeling of the specific LD protein, PLIN2, as the approach to visualize and quantify LDs. RESULTS: LDs were abundant in the hypothalamic third ventricle wall layer with similar heterogeneous distributions between control mice and humans. The LD content was enhanced by high-fat diet (HFD) in both wild-type and in low-density lipoprotein receptor deficient (Ldlr -/- HFD) mice. Strikingly, we observed a lower LD amount in type 2 diabetes mellitus (T2DM) patients when compared with non-T2DM patients. CONCLUSIONS: LDs accumulate in the normal hypothalamus, with similar distributions in human and mouse. Moreover, metabolic diseases differently modify LD content in mouse and human. Our results suggest that hypothalamic LD accumulation is an important target to the study of metabolism.

Dopamine D1 receptor signalling in the lateral shell of the nucleus accumbens controls dietary fat intake in male rats.

Central dopamine signaling regulates reward-related aspects of feeding behavior, and during diet-induced obesity dopamine receptor signaling is altered. Yet, the influence of dopamine signaling on the consumption of specific dietary components remains to be elucidated. We have previously shown that 6-hydroxydopamine-mediated lesions of dopamine neuron terminals in the lateral shell of the nucleus accumbens promotes fat intake in rats fed a multi-component free-choice high-fat high-sugar (fcHFHS) diet. It is however not yet determined which dopamine receptors are responsible for this shift towards fat preference. In this study, we assess the effects of D1-or D2 receptor acute inhibition in the lateral shell of the nucleus accumbens on fcHFHS diet consumption. We report that infusion of the D1 receptor antagonist SCH2 3390, but not the D2 receptor antagonist raclopride, promotes dietary fat consumption in male Sprague Dawley rats on a fcHFHS diet during 2 h after infusion. Furthermore, anatomical analysis of infusion sites revealed that the rostral region, but not the caudal region, of the lateral shell of the nucleus accumbens is sensitive to the D1 receptor inhibition effects on fat consumption. Our data highlight a role for D1 receptors in the rostral region of the lateral shell of the nucleus accumbens to control dietary fat consumption.

The Role of the Gut Microbiota in the Gut-Brain Axis in Obesity: Mechanisms and Future Implications.

Interaction between the gut and the brain is essential for energy homeostasis. In obesity, this homeostasis is disrupted, leading to a positive energy balance and weight gain. Obesity is a global epidemic that affects individual health and strains the socioeconomic system. Microbial dysbiosis has long been reported in obesity and obesity-related disorders. More recent literature has focused on the interaction of the gut microbiota and its metabolites on human brain and behavior. Developing strategies that target the gut microbiota could be a future approach for the treatment of obesity. Here, we review the microbiota-gut-brain axis and possible therapeutic options.

Is leptin resistance the cause or the consequence of diet-induced obesity?

BACKGROUND/OBJECTIVES: Obesity is strongly associated with leptin resistance. It is unclear whether leptin resistance results from the (over)consumption of energy-dense diets or if reduced leptin sensitivity is also a pre-existing factor in rodent models of diet-induced obesity (DIO). We here tested whether leptin sensitivity on a chow diet predicts subsequent weight gain and leptin sensitivity on a free choice high-fat high-sucrose (fcHFHS) diet. METHODS: Based upon individual leptin sensitivity on chow diet, rats were grouped in leptin sensitive (LS, n = 22) and leptin resistant (LR, n = 19) rats (P = 0.000), and the development of DIO on a fcHFHS diet was compared. The time-course of leptin sensitivity was measured over weeks in individual rats. RESULTS: Both on a chow and a fcHFHS diet, high variability in leptin sensitivity was observed between rats, but not over time per individual rat. Exposure to the fcHFHS diet revealed that LR rats were more prone to develop DIO (P = 0.013), which was independent of caloric intake (p ≥ 0.320) and the development of diet-induced leptin resistance (P = 0.769). Reduced leptin sensitivity in LR compared with LS rats before fcHFHS diet exposure, was associated with reduced leptin-induced phosphorylated signal transducer and activator of transcription 3 (pSTAT3) levels in the dorsomedial and ventromedial hypothalamus (P ≤ 0.049), but not the arcuate nucleus (P = 0.558). CONCLUSIONS: A pre-existing reduction in leptin sensitivity determines the susceptibility to develop excessive DIO after fcHFHS diet exposure. Rats with a pre-existing reduction in leptin sensitivity develop excessive DIO without eating more calories or altering their leptin sensitivity.

Timing of caloric intake during weight loss differentially affects striatal dopamine transporter and thalamic serotonin transporter binding.

Recent studies have shown that meal timing throughout the day contributes to maintaining or regaining weight after hypocaloric diets. Although brain serotonin and dopamine are well known to be involved in regulating feeding, it is unknown whether meal timing during energy restriction affects these neurotransmitter systems. We studied the effect of a 4 wk hypocaloric diet with either 50% of daily calories consumed at breakfast (BF group) or at dinner (D group) on hypothalamic and thalamic serotonin transporter (SERT) binding and on striatal dopamine transporter (DAT) binding. The BF and D groups lost a similar amount of weight. Striatal DAT and thalamic SERT binding increased in the BF group, while decreasing in the D group after the diet (ΔDAT 0.37 ± 0.63 vs. -0.53 ± 0.77, respectively; P = 0.005; ΔSERT 0.12 ± 0.25 vs. -0.13 ± 0.26 respectively, P = 0.032). Additional voxel-based analysis showed an increase in DAT binding in the ventral striatum in the BF group and a decrease in the dorsal striatum in the D group. During weight loss, striatal DAT and thalamic SERT binding increased weight independently when 50% of daily calories were consumed at breakfast, whereas it decreased when caloric intake was highest at dinner. These findings may contribute to the earlier reported favorable effect of meal timing on weight maintenance after hypocaloric diets.-Versteeg, R. I., Schrantee, A., Adriaanse, S. M., Unmehopa, U. A., Booij, J., Reneman, L., Fliers, E., la Fleur, S. E., Serlie, M. J. Timing of caloric intake during weight loss differentially affects striatal dopamine transporter and thalamic serotonin transporter binding.

The effects of overnight nutrient intake on hypothalamic inflammation in a free-choice diet-induced obesity rat model.

Consumption of fat and sugar induces hyperphagia and increases the prevalence of obesity and diabetes type 2. Low-grade inflammation in the hypothalamus, a key brain area involved in the regulation of energy homeostasis is shown to blunt signals of satiety after long term high fat diet. The fact that this mechanism can be activated after a few days of hyperphagia before apparent obesity is present led to our hypothesis that hypothalamic inflammation is induced with fat and sugar consumption. Here, we used a free-choice high-fat high-sugar (fcHFHS) diet-induced obesity model and tested the effects of differential overnight nutrient intake during the final experimental night on markers of hypothalamic inflammation. Male Wistar rats were fed a control diet or fcHFHS diet for one week, and assigned to three different feeding conditions during the final experimental night: 1) fcHFHS-fed, 2) fed a controlled amount of chow diet, or 3) fasted. RT-qPCR and Western blot were utilized to measure hypothalamic gene and protein expression, of cytokines and intermediates of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway. Lastly, we investigated the effects of acute fat intake on markers of hypothalamic inflammation in fat-naïve rats. fcHFHS-fed rats consumed more calories, increased adipose tissue, and showed elevated expression of hypothalamic inflammation markers (increased phosphorylation of NF-κB protein, Nfkbia and Il6 gene expression) compared to chow-fed rats. These effects were evident in rats consuming relative high amounts of fat. Removal of the fat and sugar, or fasting, during the final experimental night ameliorated hypothalamic inflammation. Finally, a positive correlation was observed between overnight acute fat consumption and hypothalamic NF-κB phosphorylation in fat-naïve rats. Our data indicate that one week of fcHFHS diet, and especially the fat component, promotes hypothalamic inflammation, and removal of the fat and sugar component reverses these detrimental effects.

Insulin deficiency results in reversible protein kinase A activation and tau phosphorylation.

Alzheimer's disease (AD) is a highly prevalent multifactorial disease for which Diabetes Mellitus (DM) is a risk factor. Abnormal phosphorylation and aggregation of tau is a key hallmark of AD. In animal models, DM induces or exacerbates the phosphorylation of tau, suggesting that DM may influence the risk at AD by directly facilitating tau pathology. Previously we reported that tau phosphorylation induced in response to metabolic stress is reversible. Since identification and understanding of early players in tau pathology is pivotal for therapeutic intervention, we here investigated the mechanism underlying tau phosphorylation in the diabetic brain and its potential for reversibility. To model DM we used streptozotocin-treatment to induce insulin deficiency in rats. Insulin depletion leads to increased tau phosphorylation in the brain and we investigated the activation status of known tau kinases and phosphatases in this model. We identified protein kinase A (PKA) as a tau kinase activated by DM in the brain. The potential relevance of this signaling pathway to AD pathogenesis is indicated by the increased level of active PKA in temporal cortex of early stage AD patients. Our data indicate that activation of PKA and tau phosphorylation are associated with insulin deficiency per se, rather than the downstream energy deprivation. In vitro studies confirm that insulin deficiency results in PKA activation and tau phosphorylation. Strikingly, both active PKA and induced tau phosphorylation are reversed upon insulin treatment in the steptozotocin animal model. Our data identify insulin deficiency as a direct trigger that induces the activity of the tau kinase PKA and results in tau phosphorylation. The reversibility upon insulin treatment underscores the potential of insulin as an early disease-modifying intervention in AD and other tauopathies.

The importance of hormonal circadian rhythms in daily feeding patterns: An illustration with simulated pigs.

The interaction between hormonal circadian rhythms and feeding behaviour is not well understood. This study aimed to deepen our understanding of mechanisms underlying circadian feeding behaviour in animals, using pigs, Sus scrofa, as a case study. Pigs show an alternans feeding pattern, that is, a small peak of feed intake at the beginning of the day and a larger peak at the end of the day. We simulated the feeding behaviour of pigs over a 24h period. The simulation model contained mechanisms that regulate feeding behaviour of animals, including: processing of feed in the gastrointestinal tract, fluctuation in energy balance, circadian rhythms of melatonin and cortisol and motivational decision-making. From the interactions between these various processes, feeding patterns (e.g. feed intake, meal frequency, feeding rate) emerge. These feeding patterns, as well as patterns for the underlying mechanisms (e.g. energy expenditure), fitted empirical data well, indicating that our model contains relevant mechanisms. The circadian rhythms of cortisol and melatonin explained the alternans pattern of feeding in pigs. Additionally, the timing and amplitude of cortisol peaks affected the diurnal and nocturnal peaks in feed intake. Furthermore, our results suggest that circadian rhythms of other hormones, such as leptin and ghrelin, are less important in circadian regulation of feeding behaviour than previously thought. These results are relevant to animal species with a metabolic and endocrine system similar to that of pigs, such as humans. Moreover, the modelling approach to understand feeding behaviour can be applied to other animal species.

Serotonin Transporter Binding in the Diencephalon Is Reduced in Insulin-Resistant Obese Humans.

BACKGROUND: Altered brain dopaminergic and serotonergic pathways have been shown in obese rodents and humans, but it is unknown whether this is related to obesity per se or to the metabolic derangements associated with obesity. METHODS: We performed a case-control study in insulin-sensitive obese (ISO) and insulin-resistant obese (IRO) subjects (n = 12) and age-matched lean controls (n = 8) and measured serotonin transporter (SERT) binding in the whole diencephalon and specifically in the hypothalamus, as well as dopamine transporter (DAT) binding in the striatum using 123I- FP-CIT single-photon emission computed tomography. We assessed insulin sensitivity using the homeostatic model assessment of insulin resistance. RESULTS: BMI did not differ between the IRO and ISO subjects. SERT binding in the diencephalon was significantly lower in IRO than in ISO subjects, but was not different between lean and obese subjects. SERT binding in the hypothalamus tended to be reduced in obese versus lean subjects, but was not different between IRO and ISO subjects. Striatal DAT binding was similar between lean and obese subjects as well as between ISO and IRO subjects. CONCLUSIONS: We conclude that SERT binding in the diencephalon is reduced in insulin-resistant subjects independently of body weight, while hypothalamic SERT binding tends to be lower in obesity, with no difference between insulin-resistant and insulin-sensitive subjects. This suggests that the metabolic perturbations associated with obesity independently affect SERT binding within the diencephalon.

Effects of a free-choice high-fat high-sugar diet on brain PER2 and BMAL1 protein expression in mice.

The suprachiasmatic nucleus (SCN) times the daily rhythms of behavioral processes including feeding. Beyond the SCN, the hypothalamic arcuate nucleus (ARC), involved in feeding regulation and metabolism, and the epithalamic lateral habenula (LHb), implicated in reward processing, show circadian rhythmic activity. These brain oscillators are functionally coupled to coordinate the daily rhythm of food intake. In rats, a free choice high-fat high-sugar (fcHFHS) diet leads to a rapid increase of calorie intake and body weight gain. Interestingly, under a fcHFHS condition, rats ingest a similar amount of sugar during day time (rest phase) as during night time (active phase), but keep the rhythmic intake of regular chow-food. The out of phase between feeding patterns of regular (chow) and highly rewarding food (sugar) may involve alterations of brain circadian oscillators regulating feeding. Here, we report that the fcHFHS diet is a successful model to induce calorie intake, body weight gain and fat tissue accumulation in mice, extending its effectiveness as previously reported in rats. Moreover, we observed that whereas in the SCN the day-night difference in the PER2 clock protein expression was similar between chow-fed and fcHFHS-fed animals, in the LHb, this day-night difference was altered in fcHFHS-exposed animals compared to control chow mice. These findings confirm previous observations in rats showing disrupted daily patterns of feeding behavior under a fcHFHS diet exposure, and extend our insights on the effects of the diet on circadian gene expression in brain clocks.

Impact of nutrients on circadian rhythmicity.

The suprachiasmatic nucleus (SCN) in the mammalian hypothalamus functions as an endogenous pacemaker that generates and maintains circadian rhythms throughout the body. Next to this central clock, peripheral oscillators exist in almost all mammalian tissues. Whereas the SCN is mainly entrained to the environment by light, peripheral clocks are entrained by various factors, of which feeding/fasting is the most important. Desynchronization between the central and peripheral clocks by, for instance, altered timing of food intake can lead to uncoupling of peripheral clocks from the central pacemaker and is, in humans, related to the development of metabolic disorders, including obesity and Type 2 diabetes. Diets high in fat or sugar have been shown to alter circadian clock function. This review discusses the recent findings concerning the influence of nutrients, in particular fatty acids and glucose, on behavioral and molecular circadian rhythms and will summarize critical studies describing putative mechanisms by which these nutrients are able to alter normal circadian rhythmicity, in the SCN, in non-SCN brain areas, as well as in peripheral organs. As the effects of fat and sugar on the clock could be through alterations in energy status, the role of specific nutrient sensors will be outlined, as well as the molecular studies linking these components to metabolism. Understanding the impact of specific macronutrients on the circadian clock will allow for guidance toward the composition and timing of meals optimal for physiological health, as well as putative therapeutic targets to regulate the molecular clock.

Hypercaloric diets with increased meal frequency, but not meal size, increase intrahepatic triglycerides: a randomized controlled trial.

UNLABELLED: American children consume up to 27% of calories from high-fat and high-sugar snacks. Both sugar and fat consumption have been implicated as a cause of hepatic steatosis and obesity but the effect of meal pattern is largely understudied. We hypothesized that a high meal frequency, compared to consuming large meals, is detrimental in the accumulation of intrahepatic and abdominal fat. To test this hypothesis, we randomized 36 lean, healthy men to a 40% hypercaloric diet for 6 weeks or a eucaloric control diet and measured intrahepatic triglyceride content (IHTG) using proton magnetic resonance spectroscopy ((1) H-MRS), abdominal fat using magnetic resonance imaging (MRI), and insulin sensitivity using a hyperinsulinemic euglycemic clamp with a glucose isotope tracer before and after the diet intervention. The caloric surplus consisted of fat and sugar (high-fat-high-sugar; HFHS) or sugar only (high-sugar; HS) and was consumed together with, or between, the three main meals, thereby increasing meal size or meal frequency. All hypercaloric diets similarly increased body mass index (BMI). Increasing meal frequency significantly increased IHTG (HFHS mean relative increase of 45%; P = 0.016 and HS mean relative increase of 110%; P = 0.047), whereas increasing meal size did not (2-way analysis of variance [ANOVA] size versus frequency P = 0.03). Abdominal fat increased in the HFHS-frequency group (+63.3 ± 42.8 mL; P = 0.004) and tended to increase in the HS-frequency group (+46.5 ± 50.7 mL; P = 0.08). Hepatic insulin sensitivity tended to decrease in the HFHS-frequency group while peripheral insulin sensitivity was not affected. CONCLUSION: A hypercaloric diet with high meal frequency increased IHTG and abdominal fat independent of caloric content and body weight gain, whereas increasing meal size did not. This study suggests that snacking, a common feature in the Western diet, independently contributes to hepatic steatosis and obesity. ( TRIAL REGISTRATION: www.clinicaltrials.gov; nr.NCT01297738.)

Circadian influences on feeding behavior.

Feeding, like many other biological functions, displays a daily rhythm. This daily rhythmicity is controlled by the circadian timing system of which the central master clock is located in the hypothalamic suprachiasmatic nucleus (SCN). Other brain areas and tissues throughout the body also display rhythmic functions and contain the molecular clock mechanism known as peripheral oscillators. To generate the daily feeding rhythm, the SCN signals to different hypothalamic areas with the lateral hypothalamus, paraventricular nucleus and arcuate nucleus being the most prominent. With respect to the rewarding aspects of feeding behavior, the dopaminergic system is also under circadian influence. However the SCN projects only indirectly to the different reward regions, such as the ventral tegmental area where dopamine neurons are located. In addition, high palatable, high caloric diets have the potential to disturb the normal daily rhythms of physiology and have been shown to alter for example meal patterns. Around a meal several hormones and peptides are released that are also under circadian influence. For example, the release of postprandial insulin and glucagon-like peptide following a meal depend on the time of the day. Finally, we review the effect of deletion of different clock genes on feeding behavior. The most prominent effect on feeding behavior has been observed in Clock mutants, whereas deletion of Bmal1 and Per1/2 only disrupts the day-night rhythm, but not overall intake. Data presented here focus on the rodent literature as only limited data are available on the mechanisms underlying daily rhythms in human eating behavior.