Short communication: Metabolic synthesis of vitamin D2 by the gut microbiome

The origin of vitamin D₂ in herbivorous animals was investigated in vivo in sheep and in bovine as well as mouse gastrointestinal tracts. A high concentration of 25-hydroxyvitamin D₂ in blood plasma of sheep both in summer and winter appeared to be incompatible with the undetectable level of vitamin D₂ in the pasture on which the sheep were grazing. Studies with bovine rumen contents from a cow grazing the same pasture as the sheep, demonstrated an increased concentration of vitamin D₂ on anaerobic incubation in a ‘Rusitec’ artificial rumen, which was further enhanced when cellulose powder was added as a fermentation substrate. The colon contents of mice that were fed from weaning on a vitamin D-free diet were found to contain vitamin D₂. The results of these comparative studies in 3 animal species indicated that vitamin D₂ was being generated by microbial anaerobic metabolism in the gastrointestinal tract.