Dietary Fat and Carbohydrate Content Modulate Energy Expenditure via Gut Bacteria

Dietary composition impacts the composition of the gut microbiota, and consuming high caloric diets (either high fat, HFD, or high carbohydrate, HCD) result in increased weight gain. Previous studies have estimated that bacteria contribute 1–3% (~1–3 kg) of an adult human's body mass and that bacteria typically exhibit an ~7 kcal/kg/hr metabolic rate, which implies a total contribution of 150–450 kcal/d to the host's apparent energy expenditure. As gut bacteria live within a largely anaerobic environment, we hypothesize that the biomass and composition of the gut microbiota contribute to whole-animal energy homeostasis through anaerobic resting metabolic rate (RMR). Subsequently, we hypothesize that that HFD and HCD contribute to excess weight gain through the modulation of the biomass and composition of the gut microbiota and thus anaerobic RMR. Wildtype C57BL/6J mice were grown to 12 weeks of age on standard chow (Teklad 7913 or 2920x) before remaining on chow or switching to a HFD (45% kcal from fat) or HCD (70% kcal from sucrose) for one week. Mice were then anesthetized with ketamine/xylazine and examined for total (aerobic + anaerobic) RMR by direct calorimetry before and after acute cecectomy. Direct calorimetry methods were required, as conventional methods such as respirometry are blind to anaerobic processes.