Microbial biosynthesis of vitamin D ₂

Abstract In the early history of vitamin D research most of the studies on its chemistry and function were performed with vitamin D 2 which was readily obtained by UV irradiation of ergosterol from yeast. Yet, in the physiological economy of vitamin D for most vertebrates, including humans, fish and especially for birds, vitamin D 3 produced in skin by solar irradiation of 7-dehydrocholesterol, is the natural form of vitamin D. Vitamin D 2 , as a dietary supplement, while of comparable potency to vitamin D 3 in most mammals, has been found in nature only when ergosterol in fungi is inadvertently exposed to solar UV radiation. Nevertheless, some herbivorous animals, horses and elephants, seem to maintain adequate vitamin D status with vitamin D 2 rather than vitamin D 3 . The source of that vitamin D 2 has been assumed to be the traces derived from ergosterol in endophytic fungi exposed to the sun on grass being consumed. However, outdoor grazing sheep in winter maintained adequate vitamin D status with vitamin D 2 , yet no vitamin D 2 could be detected on the grass they were consuming. Bovine rumen contents, fermenting in an artificial rumen, had an increase in vitamin D 2 concentration, particularly when cellulose fibre was added as a fermentation substrate. Furthermore, mice being raised from weaning on a vitamin D-free diet had vitamin D 2 in their colon contents. This review examines anaerobic microbial production of vitamin D 2 in the alimentary tract, in the dark, and describes a natural function for vitamin D 2 in microorganisms and potentially for gut health.