Alterations in energy expenditure in Roux-en-Y gastric bypass rats persist at thermoneutrality

BACKGROUND: The compensatory decrease in energy expenditure (EE) in response to body weight loss is attenuated by Roux-en-Y gastric bypass (RYGB) surgery in rats. The thermoneutral zone (TNZ) is at higher temperatures in rodents than in humans. Consequently, rodents may be under moderate cold stress if EE is measured at room temperature, leading to increased EE due to adaptive thermogenesis. We speculated that the reported alterations in EE of RYGB rats at room temperature are caused by higher adaptive thermogenesis and are therefore not present at thermoneutrality. METHODS: Male Wistar rats were randomized for RYGB or sham surgery. Some sham rats were body weight matched (BWM) to the RYGB rats by food restriction, the others received ad libitum access to food (AL). EE, body temperature, physical activity and food intake were measured at ambient temperatures between 22 and 32 degrees C to determine the TNZ. Adaptive thermogenesis requires beta 3adrenergic receptor-mediated uncoupling protein-1 (UCP-1) expression in brown adipose tissue (BAT). The in vivo thermogenic capacity of BAT was determined by administering the beta 3-adrenergic agonist CL316,243, and UCP-1 protein expression was measured at room temperature. RESULTS: The TNZ was between 28 and 30 degrees C for AL and RYGB and between 30 and 32 degrees C for BWM rats, respectively. In contrast to AL and BWM rats, EE was not significantly higher at room temperature than at thermoneutrality in RYGB rats, reflecting a lack of adaptive thermogenesis. Consistently, both the thermogenic capacity of BAT and UCP-1 expression were decreased in RYGB compared with AL rats at room temperature. CONCLUSIONS: Our data confirm that the decrease in EE after body weight loss is attenuated by RYGB surgery and show that this effect persists at thermoneutrality. Contrary to our hypothesis, we found that adaptive thermogenesis at room temperature is reduced in RYGB rats.