PYY and type 2 diabetes mellitus: Nutritional fiber intervention of T2D patients has enriched a functional group of 15 strains of acetic acid and butyric acid-producing bacteria such as Faecalibacterium prausnitzii, Eubacterium rectale, and Ruminococcus sp., and inhibited those strains capable of producing indole and hydrogen sulfide, leading to an increased butyric acid level which promoted glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) secretion to elevate blood insulin level.