Peptide YY (PYY), a hormone produced by gut endocrine cells that was already known to control appetite, also plays an important role in maintaining the balance of fungi in the digestive system of mammals, according to new research from the University of Chicago. In a study published August 3, 2023 in Science, researchers found that specialized immune cells in the small intestine called Paneth cells express a form of PYY that prevents the fungus Candida albicans from turning into its more virulent form. PYY was already known to be produced by endocrine cells in the gut as a hormone that signals satiety, or when an animal has had enough to eat. The new research shows that it also functions as an antimicrobial peptide that selectively allows commensal yeast forms of C. albicans to flourish while keeping its more dangerous forms in check.“ So little is known about what regulates these fungi in our in our microbiome. We know that they're there, but we have no idea what keeps them in a state that provides health benefit to us,” said Eugene B. Chang, MD, Martin Boyer Professor of Medicine at UChicago and senior author of the study. “We now think that this peptide we discovered is actually important for maintaining fungal commensalism in the gut.”