Short communication: Can Vitamin D be supplied from the large intestine?

The discovery that vitamin D₂ is being generated by anaerobic microbial metabolism in the alimentary tract, raises the question whether such a source of vitamin D could contribute to vitamin D supply for the animal hosting this microbial production system. In ruminants, this microbial generation in the forestomach allows vitamin D₂ to be readily absorbed when it reaches the small intestine, contributing to vitamin D₂ and 25-hydroxyvitamin D₂ [25(OH)D₂] found in their tissues. In monogastric animals like humans, the microbial generation of vitamin D₂ is occurring in the large intestine. There is evidence that vitamin D hydroxy metabolites, delivered to the lumen of the colon can be absorbed. However, the parent vitamin D is more lipophilic than its metabolites, and like lipophilic vitamin K₂ being produced by bacteria in the hindgut, may be poorly absorbed by the colon mucosa. It is now apparent that colon mucosal cells have the proteins megalin and cubilin in their basal membrane. These glycoproteins perform endocytosis of circulating proteins including vitamin D binding protein [DBP]. Inside the cell, DBP binds to cytoplasmic actin and thus provides an array of high affinity binding sites for vitamin D and its functional metabolites. Any traces of vitamin D₂ that may diffuse into the colon mucosal cells from the lumen would thus be retained and accumulate on the DBP-actin. It would then be a substrate for functional hydroxylase metabolism for local endocrine action in these cells, and subsequent delivery of 25(OH)D₂ by diffusion to apo-DBP in the circulation.