Yann Ravussin

Responses of gut microbiota to diet composition and weight loss in lean and obese mice.

Maintenance of a reduced body weight is accompanied by a decrease in energy expenditure beyond that accounted for by reduced body mass and composition, as well as by an increased drive to eat. These effects appear to be due—in part—to reductions in circulating leptin concentrations due to loss of body fat. Gut microbiota have been implicated in the regulation of body weight. The effects of weight loss on qualitative aspects of gut microbiota have been studied in humans and mice, but these studies have been confounded by concurrent changes in diet composition, which influence microbial community composition. We studied the impact of 20% weight loss on the microbiota of diet‐induced obese (DIO: 60% calories fat) mice on a high‐fat diet (HFD). Weight‐reduced DIO (DIO‐WR) mice had the same body weight and composition as control (CON) ad‐libitum (AL) fed mice being fed a control diet (10% calories fat), allowing a direct comparison of diet and weight‐perturbation effects. Microbial community composition was assessed by pyrosequencing 16S rRNA genes derived from the ceca of sacrificed animals. There was a strong effect of diet composition on the diversity and composition of the microbiota. The relative abundance of specific members of the microbiota was correlated with circulating leptin concentrations and gene expression levels of inflammation markers in subcutaneous white adipose tissue in all mice. Together, these results suggest that both host adiposity and diet composition impact microbiota composition, possibly through leptin‐mediated regulation of mucus production and/or inflammatory processes that alter the gut habitat.

Effects of Chronic Weight Perturbation on Energy Homeostasis and Brain Structure in Mice

Maintenance of reduced body weight in lean and obese human subjects results in the persistent decrease in energy expenditure below what can be accounted for by changes in body mass and composition. Genetic and developmental factors may determine a central nervous system (CNS)-mediated minimum threshold of somatic energy stores below which behavioral and metabolic compensations for weight loss are invoked. A critical question is whether this threshold can be altered by environmental influences and by what mechanisms such alterations might be achieved. We examined the bioenergetic, behavioral, and CNS structural responses to weight reduction of diet-induced obese (DIO) and never-obese (CON) C57BL/6J male mice. We found that weight-reduced (WR) DIO-WR and CON-WR animals showed reductions in energy expenditure, adjusted for body mass and composition, comparable (-10-15%) to those seen in human subjects. The proportion of excitatory synapses on arcuate nucleus proopiomelanocortin neurons was decreased by ∼50% in both DIO-WR and CON-WR mice. These data suggest that prolonged maintenance of an elevated body weight (fat) alters energy homeostatic systems to defend a higher level of body fat. The synaptic changes could provide a neural substrate for the disproportionate decline in energy expenditure in weight-reduced individuals. This response to chronic weight elevation may also occur in humans. The mouse model described here could help to identify the molecular/cellular mechanisms underlying both the defense mechanisms against sustained weight loss and the upward resetting of those mechanisms following sustained weight gain.

A Missing Link in Body Weight Homeostasis: The Catabolic Signal of the Overfed State

Mammals regulate fat mass so that increases or reductions in adipose tissue mass activate responses that favor return to ones previous weight. A reduction in fat mass activates a system that increases food intake and reduces energy expenditure; conversely, overfeeding and rapid adipose tissue expansion reduces food intake and increases energy expenditure. With the identification of leptin nearly two decades ago, the central circuit that defends against reductions in body fat was revealed. However, the systems that defend against rapid expansion of fat mass remain largely unknown. Here we review the physiology of the overfed state and evidence for a distinct regulatory system, which unlike the leptin-mediated system, we propose primarily measures a functional aspect of adipose tissue and not total mass per se.

Estimating energy expenditure in mice using an energy balance technique

Objective:To compare, in mice, the accuracy of estimates of energy expenditure (EE) using an energy balance technique (TEEbal: food energy intake and body composition change) vs indirect calorimetry (TEEIC).Subjects:In 32 male C57BL/6J mice, EE was estimated using an energy balance (caloric intake minus change in body energy stores) method over a 37-day period. EE was also measured in the same animals by indirect calorimetry. These measures were compared.Results:The two methods were highly correlated (r2=0.87: TEEbal=1.07*TEEIC–0.22, P<0.0001). By Bland–Altman analysis, TEEbal estimates were slightly higher (4.6±1.5%; P<0.05) than TEEIC estimates (Bias=0.55 kcal per 24 h).Conclusion:TEEbal can be performed in ‘home cages’ and provides an accurate integrated long-term measurement of EE while minimizing potentially confounding stress that may accompany the use of indirect calorimetry systems. The technique can also be used to assess long-term energy intake.

Effects of chronic leptin infusion on subsequent body weight and composition in mice: Can body weight set point be reset?

Circulating leptin concentrations correlate with fat mass and signal the status of somatic energy stores to the brain. Previous studies suggest that diet-induced elevations of body weight increase body weight “set-point”. To assess whether chronic hyperleptinemia is responsible for this shift in defended body weight, we elevated circulating leptin concentrations in lean mice to those comparable to diet-induced obese mice for eighteen weeks. We hypothesized that following cessation of leptin infusion, a higher body weight would be defended. Compared to saline-infused controls, leptin-infused mice had elevated circulating leptin concentrations, gained less weight, yet had similar metabolic rates. Following cessation of leptin administration, leptin-infused mice gained some weight yet plateaued at 5–10% below controls. These results suggest that, unlike mice rendered hyperleptinemic by diet-induced weight gain, leptin-infused mice do not subsequently “defend” a higher body weight, suggesting that hyperleptinemia per se does not mimic the CNS consequences of chronic weight gain.

Effects of a novel MC4R agonist on maintenance of reduced body weight in diet‐induced obese mice

The physiology of the weight‐reduced (WR) state suggests that pharmacologic agents affecting energy homeostasis may have greater efficacy in WR individuals. Our aim was to establish a protocol that allows for evaluation of efficacy of weight maintenance agents and to assess the effectiveness of AZD2820, a novel melanocortin 4 receptor (MC4R) agonist in such a paradigm.

Weight Perturbation Alters Leptin Signal Transduction in a Region-Specific Manner throughout the Brain

Diet-induced obesity (DIO) resulting from consumption of a high fat diet (HFD) attenuates normal neuronal responses to leptin and may contribute to the metabolic defense of an acquired higher body weight in humans; the molecular bases for the persistence of this defense are unknown. We measured the responses of 23 brain regions to exogenous leptin in 4 different groups of weight- and/or diet-perturbed mice. Responses to leptin were assessed by quantifying pSTAT3 levels in brain nuclei 30 minutes following 3 mg/kg intraperitoneal leptin. HFD attenuated leptin sensing throughout the brain, but weight loss did not restore central leptin signaling to control levels in several brain regions important in energy homeostasis, including the arcuate and dorsomedial hypothalamic nuclei. Effects of diet on leptin signaling varied by brain region, with results dependent on the method of weight loss (restriction of calories of HFD, ad lib intake of standard mouse chow). High fat diet attenuates leptin signaling throughout the brain, but some brain regions maintain their ability to sense leptin. Weight loss restores leptin sensing to some degree in most (but not all) brain regions, while other brain regions display hypersensitivity to leptin following weight loss. Normal leptin sensing was restored in several brain regions, with the pattern of restoration dependent on the method of weight loss.

Energy homeostasis in leptin deficient Lepob/ob mice

Maintenance of reduced body weight is associated both with reduced energy expenditure per unit metabolic mass and increased hunger in mice and humans. Lowered circulating leptin concentration, due to decreased fat mass, provides a primary signal for this response. However, leptin deficient (Lepob/ob) mice (and leptin receptor deficient Zucker rats) reduce energy expenditure following weight reduction by a necessarily non-leptin dependent mechanisms. To identify these mechanisms, Lepob/ob mice were fed ad libitum (AL group; n = 21) or restricted to 3 kilocalories of chow per day (CR group, n = 21). After losing 20% of initial weight (in approximately 2 weeks), the CR mice were stabilized at 80% of initial body weight for two weeks by titrated refeeding, and then released from food restriction. CR mice conserved energy (-17% below predicted based on body mass and composition during the day; -52% at night); and, when released to ad libitum feeding, CR mice regained fat and lean mass (to AL levels) within 5 weeks. CR mice did so while their ad libitum caloric intake was equal to that of the AL animals. While calorically restricted, the CR mice had a significantly lower respiratory exchange ratio (RER = 0.89) compared to AL (0.94); after release to ad libitum feeding, RER was significantly higher (1.03) than in the AL group (0.93), consistent with their anabolic state. These results confirm that, in congenitally leptin deficient animals, leptin is not required for compensatory reduction in energy expenditure accompanying weight loss, but suggest that the hyperphagia of the weight-reduced state is leptin-dependent.

Evidence for a Non-leptin System that Defends against Weight Gain in Overfeeding

Weight is defended so that increases or decreases in body mass elicit responses that favor restoration of ones previous weight. While much is known about the signals that respond to weight loss and the central role that leptin plays, the lack of experimental systems studying the overfed state has meant little is known about pathways defending against weight gain. We developed a system to study this physiology and found that overfed mice defend against increased weight gain with graded anorexia but, unlike weight loss, this response is independent of circulating leptin concentration. In overfed mice that are unresponsive to orexigenic stimuli, adipose tissue is transcriptionally and immunologically distinct from fat of ad libitum-fed obese animals. These findings provide evidence that overfeeding-induced obesity alters adipose tissue and central responses in ways that are distinct from ad libitum obesity and activates a non-leptin system to defend against weight gain.