Microbiota-derived isovalerate ameliorates sex-specific gut barrier dysfunction in malnutrition

Malnutrition increases intestinal permeability and the risk of sepsis, yet mechanisms underlying malnutrition-induced gut barrier dysfunction are poorly defined. Here, we aimed to determine how the gut microbiome and microbiota-derived metabolites influence intestinal barrier function in the malnourished host. We induced malnutrition in specific pathogen-free (SPF) and germ-free (GF) mice using a low-protein, low-fat diet. Colonic permeability was quantified in Ussing chambers and invasive bacteria were cultured from liver and spleen. Targeted metabolomics identified microbial metabolites depleted in malnutrition. Candidate metabolites were screened in human-derived colonoid monolayers and administered to malnourished mice to determine whether gut barrier dysfunction can be rescued. Malnutrition thinned the colonic mucus layer, increased gut barrier permeability, and facilitated bacterial translocation in male, but not female, SPF mice. Malnourished GF mice exhibited normal barrier function. In the malnourished intestine, a subset of microbial short-chain fatty acids, the branched-chain fatty acids (BCFAs), was depleted in SPF mice of both sexes. Treating human-derived colonoid monolayers with BCFAs, especially isovalerate, increased transepithelial electrical resistance and altered the expression of genes associated with epithelial junction complexes. In malnourished male SPF mice, either enemas with isovalerate or gavages with its branched-chain amino acid fermentation substrate, leucine, restored the localization of the integral membrane protein claudin-8 within the colonic crypt and reduced barrier permeability. Together, these findings identify BCFAs, including isovalerate, as microbiota-derived regulators of intestinal junction complexes and barrier integrity. We propose the branched-chain amino acid leucine as a microbiota-directed precision nutrition therapy that could target intestinal barrier dysfunction in malnutrition.