available to the microbiota in the gut in mammals. In addition to serving as the materials for
protein synthesis, AAs can act as precursors for numerous metabolic end products in
reactions involving the intestinal mucosa and microbiota. After penetrating the epithelial
barrier, microbial metabolites can enter and accumulate in the host circulatory system,
where they are sensed by immune cells and then elicit a wide range of biological functions …