Microbiota-Derived Isovalerate Ameliorates Sex-Specific Gut Barrier Dysfunction in Malnutrition

Malnutrition is a major global health challenge that increases intestinal permeability and susceptibility to sepsis, yet the mechanisms driving barrier dysfunction remain poorly defined. We aimed to identify how the gut microbiome and microbiota-derived metabolites regulate intestinal barrier integrity during malnutrition. We used a low-protein, low-fat diet (LPLFD) to induce malnutrition in specific pathogen-free (SPF) and germ-free (GF) mice. Colonic permeability and mucus thickness were quantified. Metabolomics identified microbiota-derived metabolites altered by malnutrition. Human colonoids were used to test mechanistic effects of candidate metabolites. Barrier restoration was evaluated following colonic administration of isovalerate or oral supplementation with its precursor amino acid, leucine. LPLFD-induced malnutrition increased colonic permeability and reduced mucus thickness in male, but not female, SPF mice. These defects were absent in malnourished GF mice, indicating a microbiota-dependent and sexually dimorphic mechanism of barrier disruption. Metabolomic analysis revealed reduced colonic levels of branched-chain fatty acids (BCFAs) in malnourished mice. Supplementation of human colonoids with the BCFA isovalerate improved barrier function and altered expression of genes associated with epithelial junctional complexes. Restoring isovalerate levels, either directly via colonic administration or indirectly through oral leucine supplementation, partially rescued barrier defects in malnourished male mice. These findings identify BCFAs, particularly isovalerate, as essential microbiota-derived regulators of intestinal barrier integrity during malnutrition. This work reveals a sex-specific, microbiota-dependent pathway of barrier dysfunction and highlights microbial metabolites as promising therapeutic targets for mitigating sepsis risk in undernourished populations.