Previous studies have estimated that gut bacteria -which is largely anaerobic- contribute 1-3% (~1-3 kg) of an adult human’s body mass and that bacteria typically exhibit ~7 kcal/kg/hr metabolic rate, which can equals to ~150-450 kcal/d to the host’s apparent energy expenditure.We hypothesized that the biomass and composition of the bacterial community would both contribute to the metabolic rate of the gut microbiota, and that the energy flux through this biomass would be anaerobic in nature.


Total resting metabolic rate (tRMR) was assessed using direct calorimetry; aerobic (aRMR) was determined using respirometry, and anaerobic RMR (anRMR) was determined as the difference in results between the two methods.Wildtype C57BL/6J mice were anesthetized with ketamine/xylazine to measure baseline tRMR, and tRMR after surgical removal of the cecum (as a way to reducebiomass of gut microbiota)


Cecectomy resulted in8% reduction in tRMR within minutes of surgery (sham n=12 pre 0.116±0.004; cecectomy n=13 pre 0.115±0.004; sham post 0.113±0.004; cecectomy post 0.104±0.005 p<0.05 vs pre, in kcal/hr). Next, a cohort of C57BL/6J mice brought to obesity by either high-fat diet (HFD) or high-carb diet (HCD) underwent Roux-en-Y gastric bypass (RYGB) or sham surgery.After one week, tRMR was significantly increased by 10% in the RYGB group maintained on the HFD (sham n=8, 0.151±0.003 vs RYGB n=10, 0.184±0.002 kcal/hr, p<0.05) while aRMR remained unchanged, indicating that the increase in tRMR was completely due to an increase in anRMR. On the other hand, no significant change was observed in tRMR between RYGB and sham-operated mice that were maintained on HCD (sham n=8, 0.150±0.003 vs RYGB n=8, 0.149±0.003 kcal/hr, p=NS).


Collectively these data highlight a major contribution of gut microbiota to total energy expenditure in mice, and support the general concept that the energy flux through this bacterial biomass is anaerobic and subject to composition of the diet consumed.