What role does Vitamin D 25-OH play in gut health?

Vitamin D (25-OH) is the circulating storage form, converted in the liver to active 1,25-dihydroxyvitamin D, which modulates immune function. In the gut, it supports calcium absorption and influences intestinal barrier integrity and the microbiome. Observational studies link low 25-OH levels to inflammatory bowel disease (IBD), but causality remains unclear. Randomized controlled trials of supplementation in IBD show mixed results. Evidence for a direct role of 25-OH in gut health is largely mechanistic and associative. Deficiency may impair gut immune regulation, but measuring 25-OH alone cannot explain individual gut issues. Consumer tests like MyBody-X measure 25-OH, but interpretation for gut health lacks robust validation. Caveat: Do not supplement based solely on test results without medical guidance. Evidence grade: mechanistic/unclear.