The gut is the major site for nutrient digestion and absorption. Dietary nutrients go through gut level metabolism before their absorption. The gut harbors dense and diverse microbes, with the microbial cell number ten folds of the host cells. Diets provide nutrients for both gut microbes and the animal host. At the gut level, both the gut epithelial cells and microbes need nutrients for their growth and metabolism. These gut level interactions affect nutrient metabolic fates, either to epithelial cells, or microbial cells or absorbed for the host, and establish the gut level of nutrient metabolism, which can be defined as the gut nutrition. For amino acids (AAs), one third of essential AAs were reported to be catabolized in the gut. Thus, gut level interactions considerably affect AAs metabolism and their availability for the animal host. This paper reviews recent research progress on the mediation of gut microbes, especially the small intestinal microbes, in AA metabolism. Intestinal microbes derived from the pig were greatly involved in AAs metabolism in AA specific and gut segment and niche specific patterns. Around 16% of lysine was used for microbial cells while 57% of Leucine and 49% Phenylalanine were used for microbial cells, respectively. Gut lumen microbes derived from pig small intestine seemed to prefer AA degradation while gut mucosal microbes showed more AA synthesis. While porcine enterocytes mainly metabolize BCAAs, intestinal microbes showed great capacity of metabolizing Lys, Arg, Thr, Glu, but with limited ability to metabolize BCAA. With in-feed antibiotics in pigs, gut level of AAs significantly reduced while circular level of AAs sharply increased, coupled with upregulated expression of gut AA transporters and receptors of the porcine small intestine. Thus, gut microbial load influences greatly the AA pool for the host. Dietary protein sources and AA forms affect gut microbiota. Gut microbes show preference for their utilization of different AAs. Lactobacillus is one of the most dominant genera in the small intestine of pigs, with many species as beneficial to gut health of the pig. With a probiotic Lactobacillus strain as example, it was found to grow preferentially with peptide-rich rather than amino acid-rich substrates, as reflected by enhanced growth and peptide-bound AA utilization, and peptide transporter upregulations. Further, AA forms drive distinct metabolic patterns of the Lactobacillus strain, with more production of lactate in peptide-rich substrates, beneficial to the small intestine health, while more production of acetate in AA-rich substrates. These results suggest that an increased capability of utilizing peptide-bound AAs (PBAAs) contributes to the dominance of Lactobacillus, indicating AA utilization as a deterministic factor affecting intestinal microbial distribution. These findings provide new insights into the microbe-gut nutrition interplay, influencing the nutrient availability to the animal host.

Zhu, W. 2025. Gut level interactions affecting AA metabolism.2025 American Society of Animal Science Midwest Section meeting, Abstract 254.