Microbial biosynthesis of vitamin D₂

In the early history of vitamin D research most of the studies on its chemistry and function were performed with vitamin D₂ 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 D₃ produced in skin by solar irradiation of 7-dehydrocholesterol, is the natural form of vitamin D. Vitamin D₂, as a dietary supplement, while of comparable potency to vitamin D₃ 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 D₂ rather than vitamin D₃. The source of that vitamin D₂ 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 D₂, yet no vitamin D₂ could be detected on the grass they were consuming. Bovine rumen contents, fermenting in an artificial rumen, had an increase in vitamin D₂ 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 D₂ in their colon contents. This review examines anaerobic microbial production of vitamin D₂ in the alimentary tract, in the dark, and describes a natural function for vitamin D₂ in microorganisms and potentially for gut health.