Intestinal Microbial Contribution to Metabolic Leucine Input in Adult Men12

New estimates of the indispensable amino acid requirements of adult humans are much higher than previously thought and questions the adequacy of cereal-based diets of low protein quality. However, dietary amino acid requirements may be supplemented by contributions from the intestinal microbiota. This study measured the contribution of intestinal microbes to leucine input in healthy adult men. Fourteen adult men were studied during each of 2 11-d periods (before and after intestinal antimicrobial treatment), in which leucine was supplied at 1.25 times the estimated average requirement (EAR) (d 1–7) and at 2.5 times the EAR (d 8–11) providing an L-amino acid diet. We estimated fasting- and fed-state leucine oxidation on d 7 and d 11 using a13C-leucine tracer infusion. The microbial contribution to body leucine input was calculated from the relationship of leucine oxidation to leucine intake and the reduction in leucine oxidation after antimicrobial treatment. Antimicrobial treatment did not affect the slope of the relationship of leucine oxidation to leucine intake. Mean and fed-state leucine oxidation declined by ∼13 and 20%, respectively (both P < 0.05) after antimicrobial treatment with the 1.25 EAR diet, but not with the 2.5 EAR diet. The contribution of the intestinal microbiota to body leucine input was estimated to be between 19 and 22% at the 1.25 EAR diet. The contribution of the intestinal microbiota to body amino acid homeostasis may be significant at maintenance intakes, but its long-term nutritional importance remains to be determined.