The first reference to the physiological effect of sunlight on vitamin D was illustrated by the Greek historian Herodotus. The most well-known source of vitamin D is via synthesis in the skin induced by sun exposure. The potential myriad effects of this vitamin in human health and disease have led to an escalating interest in vitamin D inadequacy and the best methods to normalize suboptimal levels. These potential effects include inhibition of cell proliferation, promotion of cell differentiation, and apoptosis which may in turn have roles in cancer, immunity, and many organ systems, ,. The paracrine and autocrine effects of vitamin D depend on genetic transcription, unique to the type of cell expressing nuclear vitamin D receptors. Furthermore, vitamin D is an enabling agent that, when present in optimal concentrations, has no perceptible effect on calcium absorption in its own right however, it permits or facilitates flexible physiologic response to varying calcium need. Vitamin D and calcium are often used in the same sentence because they work closely together, vitamin D’s primary role is to control the levels of calcium found in the bloodstream by constantly allowing calcium and phosphate absorption from the intestine or taking calcium from bones.

The endocrine effects of vitamin D are mainly involved in serum calcium homeostasis. Vitamin D is actually a fat-soluble prohormone steroid that has endocrine, paracrine and autocrine functions. The paradox with ‘vitamin D’ is that diet per se is usually poor in vitamin D except for cod or other fish oils or food fortified with this vitamin. It is somewhat ironic that vitamin D, through a historical accident, became classified as a ‘vitamin’, owing to the fact that vitamin is conventionally defined as ‘essential item needed in the diet’.

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