Inverse relationship between brown fat thermogenesis and meal size: the thermostatic control of food intake revisited

Not feeling the heat? Effects of dietary protein on satiation and satiety in mice are not due to its impact on body temperature

Dietary protein modulates food intake (FI) via unclear mechanism(s). One possibility is that higher protein leads to greater post-ingestive heat production (Specific dynamic action: SDA) leading to earlier meal termination (increased satiation), and inhibition of further intake (increased satiety). The influence of dietary protein on feeding behaviour in C57BL/6J mice was tested using an automated FI monitoring system (BioDAQ), simultaneous to body temperature (Tb). Total FI, inter meal intervals (IMI, satiety) and meal size (MS, satiation) were related to changes in Tb after consuming low (5%, LP), moderate (15%, MP) and high (30%, HP) protein diets. Diets were tested over three conditions: 1) room temperature (RT, 21 ± 1 °C), 2) room temperature and running wheels (RTRW) and 3) low temperature (10 °C) and running wheels (LTRW). The differences between diets and conditions were also compared using mixed models. Mice housed at RT fed HP diet, reduced total FI compared with LP and MP due to earlier meal termination (satiation effect). FI was lowered in RTRW conditions with no differences between diets. FI significantly increased under LTRW conditions for all diets, with protein content leading to earlier meal termination (satiation) but not the intervals between feeding bouts (satiety). Tb fell immediately after feeding in all conditions. Despite a reduction in total FI in mice fed HP, mediated via increased satiation, this effect was not linked to increased Tb during meals. We conclude effects of dietary protein on intake are not mediated via SDA and Tb.

Leptin mediates postprandial increases in body temperature through hypothalamus-adrenal medulla-adipose tissue crosstalk

Meal ingestion increases body temperature in multiple species, an effect that is blunted by obesity. However, the mechanisms responsible for these phenomena remain incompletely understood. Here we show that refeeding increases plasma leptin concentrations approximately 8-fold in 48-hour-fasted lean rats, and this normalization of plasma leptin concentrations stimulates adrenomedullary catecholamine secretion. Increased adrenal medulla-derived plasma catecholamines were necessary and sufficient to increase body temperature postprandially, a process that required both fatty acids generated from adipose tissue lipolysis and β-adrenergic activation of brown adipose tissue (BAT). Diet-induced obese rats, which remained relatively hyperleptinemic while fasting, did not exhibit fasting-induced reductions in temperature. To examine the impact of feeding-induced increases in body temperature on energy balance, we compared rats fed chronically by either 2 carbohydrate-rich boluses daily or a continuous isocaloric intragastric infusion. Bolus feeding increased body temperature and reduced weight gain compared with continuous feeding, an effect abrogated by treatment with atenolol. In summary, these data demonstrate that leptin stimulates a hypothalamus-adrenal medulla-BAT axis, which is necessary and sufficient to induce lipolysis and, as a result, increase body temperature after refeeding.

The gut hormone secretin triggers a gut-brown fat-brain axis in the control of food intake

NEW FINDINGS

What is the topic of this review? Brown fat's role in meal-associated thermogenesis and the related consequences for energy balance regulation with a focus on the gut hormone secretin, which has been identified as the endocrine molecular mediator of meal-associated brown fat thermogenesis. What advances does it highlight? The finding of the secretin-induced gut-brown fat-brain axis creates new opportunities to manipulate brown fat and thereby energy balance in a natural way while living in a thermoneutral environment. The role of brown fat as a mere catabolic heater organ needs to be revised and more attention should be directed towards the regulatory role of brown fat beyond energy expenditure.

ABSTRACT

Brown fat research concentrates on the energy expenditure function of this heating organ, whereas previous evidence for a role of brown fat in regulating energy intake has been mostly neglected. Ingestion of a single mixed meal activates human brown fat thermogenesis to the same degree as cold. In mice, activation of brown fat thermogenesis with a β₃ -adrenergic receptor agonist inhibits food intake. Pharmacological β-blockade, however, inhibits neither meal-associated thermogenesis nor food intake. We recently identified the gut hormone secretin as a non-adrenergic activator of brown fat. In vivo, secretin treatment acutely increases energy expenditure and inhibits food intake in wild-type, but not in uncoupling protein 1 (UCP1)-knockout (KO) mice, which lack thermogenic brown fat function. Concurrently, secretin alters gene expression of melanocortinergic peptides of hypothalamic neurons in wild-type mice, but not UCP1-KO. Blocking endogenous secretin with a neutralizing antibody attenuates brown fat thermogenesis during refeeding, increases food intake of mice, and alters ad libitum feeding behaviour. Taken together, these findings demonstrate that secretin triggers an endocrine gut-brown adipose tissue-brain axis in the control of satiation. We hypothesize that meal-associated activation of brown adipose tissue thermogenesis induced by secretin results in a rise in brain temperature and increased melanocortinergic signalling. Taken together, brown fat is not a mere heating organ dissipating excess calories but also involved in gut-brain communication in the control of food intake.

Inter-organ communication: a gatekeeper for metabolic health

Multidirectional interactions between metabolic organs in the periphery and the central nervous system have evolved concomitantly with multicellular organisms to maintain whole-body energy homeostasis and ensure the organism's adaptation to external cues. These interactions are altered in pathological conditions such as obesity and type 2 diabetes. Bioactive peptides and proteins, such as hormones and cytokines, produced by both peripheral organs and the central nervous system, are key messengers in this inter-organ communication. Despite the early discovery of the first hormones more than 100 years ago, recent studies taking advantage of novel technologies have shed light on the multiple ways used by cells in the body to communicate and maintain energy balance. This review briefly summarizes well-established concepts and focuses on recent advances describing how specific proteins and peptides mediate the crosstalk between gut, brain, and other peripheral metabolic organs in order to maintain energy homeostasis. Additionally, this review outlines how the improved knowledge about these inter-organ networks is helping us to redefine therapeutic strategies in an effort to promote healthy living and fight metabolic disorders and other diseases.

Intact innervation is essential for diet-induced recruitment of brown adipose tissue

The possibility that recruitment and activation of brown adipose tissue (BAT) thermogenesis could be beneficial for curtailing obesity development in humans prompts a need for a better understanding of the control of these processes [that are often referred to collectively as diet-induced thermogenesis (DIT)]. Dietary conditions are associated with large changes in blood-borne factors that could be responsible for BAT recruitment, but BAT is also innervated by the sympathetic nervous system. To examine the significance of the innervation for DIT recruitment, we surgically denervated the largest BAT depot, i.e., the interscapular BAT depot in mice and exposed the mice at thermoneutrality to a high-fat diet versus a chow diet. Denervation led to an alteration in feeding pattern but did not lead to enhanced obesity, but obesity was achieved with a lower food intake, as denervation increased metabolic efficiency. Conclusively, denervation totally abolished the diet-induced increase in total UCP1 protein levels observed in the intact mice, whereas basal UCP1 expression was not dependent on innervation. The denervation of interscapular BAT did not discernably hyper-recruit other BAT depots, and no UCP1 protein could be detected in the principally browning-competent inguinal white adipose tissue depot under any of the examined conditions. We conclude that intact innervation is essential for diet-induced thermogenesis and that circulating factors cannot by themselves initiate recruitment of brown adipose tissue under obesogenic conditions. Therefore, the processes that link food intake and energy storage to activation of the nervous system are those of significance for the further understanding of diet-induced thermogenesis.

Brown adipocyte glucose metabolism: a heated subject

The energy expending and glucose sink properties of brown adipose tissue (BAT) make it an attractive target for new obesity and diabetes treatments. Despite decades of research, only recently have mechanistic studies started to provide a more complete and consistent picture of how activated brown adipocytes handle glucose. Here, we discuss the importance of intracellular glycolysis, lactate production, lipogenesis, lipolysis, and beta-oxidation for BAT thermogenesis in response to natural (temperature) and artificial (pharmacological and optogenetic) forms of sympathetic nervous system stimulation. It is now clear that together, these metabolic processes in series and in parallel flexibly power ATP-dependent and independent futile cycles in brown adipocytes to impact on whole-body thermal, energy, and glucose balance.

Postprandial Oxidative Metabolism of Human Brown Fat Indicates Thermogenesis

Human studies suggest that a meal elevates glucose uptake in brown adipose tissue (BAT). However, in postprandial state the thermogenic activity and the metabolism of non-esterified fatty acids (NEFAs) in BAT remain unclear. Using indirect calorimetry combined with positron emission tomography and computed tomography (PET/CT), we showed that whole-body and BAT thermogenesis (oxygen consumption) increases after the ingestion of a mixed carbohydrate-rich meal, to the same extent as in cold stress. Postprandial NEFA uptake into BAT is minimal, possibly due to elevated plasma insulin inhibiting lipolysis. However, the variation in postprandial NEFA uptake is linked to BAT thermogenesis. We identified several genes participating in lipid metabolism to be expressed at higher levels in BAT compared with white fat in postprandial state, and to be positively correlated with BAT UCP1 expression. These findings suggest that substrates preferred by BAT in postprandial state are glucose or LPL-released NEFAs due to insulin stimulation.

The Environmental Pollutants Perfluorooctane Sulfonate and Perfluorooctanoic Acid Upregulate Uncoupling Protein 1 (UCP1) in Brown-Fat Mitochondria Through a UCP1-Dependent Reduction in Food Intake

The environmental pollutants perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) cause a dramatic reduction in the size of the major adipose tissue depots and a general body weight decrease when they are added to the food of mice. We demonstrate here that this is mainly due to a reduction in food intake; this reduction was not due to food aversion. Remarkably and unexpectedly, a large part of the effect of PFOA/PFOS on food intake was dependent on the presence of the uncoupling protein 1 (UCP1) in the mice. Correspondingly, PFOA/PFOS treatment induced recruitment of brown adipose tissue mitochondria: increased oxidative capacity and increased UCP1-mediated oxygen consumption (thermogenesis). In mice pair-fed to the food intake during PFOA/PFOS treatment in wildtype mice, brown-fat mitochondrial recruitment was also induced. We conclude that we have uncovered the existence of a regulatory component of food intake that is dependent upon brown adipose tissue thermogenic activity. The possible environmental consequences of this novel PFOA/PFOS effect (a possible decreased fitness) are noted, as well as the perspectives of this finding on the general understanding of control of food intake control and its possible extension to combatting obesity.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

The sustained development of preneoplastic lesions depends on high protein intake

The effects of sequential alterations in the feeding of two levels of dietary protein (5% and 20% casein) on the postinitiation development of aflatoxin B1- (AFB1) induced gamma-glutamyl transpeptidase-positive (GGT+) preneoplastic foci were examined. Weanling male Fischer 344 rats fed AIN-76A diet (20% protein) were administered 10 intragastric doses of AFB1 (1 dose/day during the 14-day dosing period excluding weekends) at 250 micrograms/kg body wt (initiation). After AFB1 tissue clearance, rats were randomly assigned to dietary treatment groups. During the next 12 weeks (promotion), they developed AFB1-induced GGT+ preneoplastic lesions. The 12-week promotion period was subdivided into four three-week periods, during which rats were fed isocaloric diets containing 20% casein during all four periods (20:20:20:20), 5% casein during all four periods (5:5:5:5), or sequentially altered casein levels (20:5:20:5 and 5:20:5:20). Rats were killed at 3,6,9, and 12 weeks to examine the dependence of GGT+ foci development on protein intake. Animals fed 5% casein diets developed significantly fewer (p < 0.01) GGT+ foci than animals fed 20% casein diets despite greater total caloric intake. Similarly, in the intervention groups, preneoplastic development was enhanced when the 20% casein diet was fed and inhibited when the 5% casein diet was fed. These results indicate that the sustained development of AFB1-induced preneoplastic foci depends on a high protein intake. Alternatively, these results suggest that low protein intake inhibits lesion development.

Hepatic thermogenesis relation to food intake in the conscious rat

It is well known that the body temperature increases during food intake. However, probably because of high thermal conduction within the body, it is rather difficult to determine the main organ that is the source of participates heat production in association with food intake. Our study revealed that increased temperature in the liver during food intake was more predominant than that in the other parts of the abdomen. To further confirm this, we attempted to measure the temperature of both the liver and blood in the portal vein, simultaneously. The temperature of the liver was always higher than that of the portal blood. This implies that hepatic thermogenesis is continuous. The difference between these two temperatures may indicate whether or not hepatic thermogenesis contributes to the increase in body temperature during food consumption. It becomes apparent that the participation of thermogenesis of the liver, along with that of the brown adipose tissue in body temperature increase, is influenced by the composition of food, such as high or low protein in the diet.

Effects of food restriction and adrenalectomy in rats with VMH or PVH lesions

The effects of adrenalectomy in rats with ventromedial or paraventricular hypothalamic lesions have been studied in two experiments. Rats with ventromedial hypothalamic lesions or lesions in the paraventricular nucleus were allowed to gain weight for fourteen days at which time they were adrenalectomized. Before adrenalectomy, animals with VMH lesions ate more, gained significantly more weight than animals with lesions in the paraventricular nucleus, and both were significantly heavier and consumed more food than sham-operated controls. Following adrenalectomy, food intake decreased and both groups of lesioned animals lost weight. The animals with VMH lesions stabilized at weights above the control animals. Implantation of corticosterone enhanced weight gain and food intake in animals with lesions in either the paraventricular nucleus or the ventromedial hypothalamus. In the second experiment, one subgroup of rats with VMH lesions was adrenalectomized, and allowed to eat ad lib. Two other groups of sham-operated rats with VMH lesions served as controls. One group ate ad lib and one group was pair fed to the food intake of the adrenalectomized VMH-lesioned rats. Weight gain in the adrenalectomized VMH-lesioned rats and the pair-fed VMH-lesioned controls was similar and less than the VMH-lesioned rats eating ad lib. GDP binding to interscapular brown adipose tissue was related to the degree of weight gain, not to the presence of the VMH lesion. These data show that corticosterone is essential for the expression of obesity in both PVH- and VMH-lesioned rats. They also argue that the reduction in the activity of the sympathetic nervous system of VMH-lesioned rats as estimated by the GDP binding to mitochondria from brown adipose tissue is associated with hyperphagia.

Meal associated changes in brown fat thermogenesis and glycogen

Data indicate a close association between a decrease in feeding-induced brown adipose tissue (BAT) thermogenesis and an increase in food consumption. The present study examines the hypothesis that feeding-induced BAT thermogenesis, or feeding-induced changes in BAT glycogen, a mobile form of energy store and a correlate of BAT thermogenesis, may modulate feeding behavior. We report that propranolol, which completely abolished meal-induced BAT thermogenesis, did not evoke intake of a larger meal. Though BAT glycogen concentration is a sensitive measure of the state of feeding, on a meal to meal basis it does not correlate with hunger and satiety. Hence the hypothesis is not supported by the current data. We also report that meal-induced BAT hypertrophy and glycogen deposition can be dissociated from meal-induced BAT thermogenesis.

Comparison of brown adipose tissue thermogenesis induced by congeners and isomers of phenylpropanolamine

The present experiment compared the effects of intraperitoneal injection (.0223 mMol/kg) of several phenethylamine congeners and isomers including amphetamine (AMP), ephedrine (EPH), methoxyphenamine (MET), norpseudoephedrine (NOR), pseudoephedrine (PS) and phenylpropanolamine (PPA) on in vivo interscapular brown adipose tissue temperature in adult male rats. Comparisons of isomer potency revealed that the 1-isomer was more thermogenic than the d-isomer for EPH and PPA but not for AMP, NOR and PS. Congener potency order was: AMP greater than PPA greater than EPH greater than NOR = MET greater than PS. The implications of these data for the weight-reducing activity of these compounds is discussed.

Effects of guanethidine sympathectomy on feeding, drinking, weight gain and amphetamine anorexia in the rat

Adult female rats that underwent sympathectomy induced by guanethidine treatment (10, 20 or 40 mg/kg) exhibited markedly increased water intake, but did not display significant alterations of either food intake, body weight, or the Lee Index of obesity. Guanethidine treatment did not attenuate amphetamine anorexia as evidenced by comparable dose-dependent reductions in food intake to d-amphetamine sulfate (0.25, 0.50, 1.0, and 2.0 mg/kg) in sympathectomized and control rats. These data are not consistent with the hypothesis that amphetamine anorexia is partially mediated via enhanced BAT thermogenesis.

Guanethidine sympathectomy does not prevent meal-induced increases in the weight or oxygen consumption of brown fat

The interscapular brown adipose tissue (BAT) of adult rats that were neonatally sympathectomized with guanethidine (GUA) consumed less oxygen but weighed the same as BAT from intact controls. In response to a 2-hr mixed-constituent meal, BAT from sympathectomized and control rats showed similar increases in oxygen uptake and weight. These data suggest that some functions of BAT can be maintained even without sympathetic stimulation.

Hyperinsulinemia and obesity in the dorsolateral tegmental rat

Adult female rats (N = 26) were prepared with either sham lesions or electrolytic lesions of the dorsolateral tegmentum (DLT). Body weight gain, adiposity, 72-hour food intake and day/night food intake, serum glucose and serum insulin levels were measured 15 and 17 days post-operatively. Dorsolateral tegmental lesions produced moderate weight gains and enhanced adiposity as assessed by the Lee Index. Although dorsolateral tegmental rats were hyperphagic only at night, hyperinsulinemia was observed during the day and the night. Hyperinsulinemia may contribute to the obesifying action of tegmental lesions in rats fed high-fat diets.

Compositional and metabolic changes in brown adipose tissue following a single test meal

In rats maintained on a scheduled feeding plan, the hypertrophy of brown adipose tissue (BAT) observed after a low-protein/high-carbohydrate single test meal was accompanied by significant changes in the percentage of all major constituents of the tissue. There was a fall in the percentage of water (P less than 0.01), a rise in the percentage of fat (P less than 0.05), and a rise in the percentage of glycogen (P less than 0.001). The largest absolute changes following a meal were in the fat content, which almost doubled, and in the glycogen content, which exhibited about a four-fold increase. Deposition of fat in the BAT following the test meal was accompanied by a three-fold increase in the rate of fatty acid synthesis (P less than 0.05). The in vitro respiration rate of BAT was usually significantly increased in the meal-fed rats, but the effect of replacing the protein content of the test meal with starch was not clear. A lower protein, higher starch diet (9% of calories from protein, 72% from starch) resulted in a trend for a larger thermic effect than a higher protein, lower starch diet (27% of calories from protein, 54% from starch).