The gut microbiota is not itself an important source of amino acids during periods of adequate protein intake. Some commensal members produce biologically active components from amino acids. Amino acid supplementation in a mouse model of ulcerative colitis has been shown to promote overall growth of commensal microbiota. The effect was considered to be mediated via the stimulatory effect on mucin production by amino acid supplementation. Here, MUC5AC is linked to ulcerative colitis.