Vitamin D and the gut microbiome: a systematic review of in vivo studies.

Variation in the human microbiome has been linked with a variety of physiological functions, including immune regulation and metabolism and biosynthesis of vitamins, hormones, and neurotransmitters. Evidence for extraskeletal effects of vitamin D has been accruing and it has been suggested that the effect of vitamin D on health is partially mediated through the microbiome. We aimed to critically evaluate the evidence linking vitamin D and the gastrointestinal microbiome.