What is Vitamin D?

Vitamin D is a group of fat-soluble prohormones, the two major forms of which are vitamin D2 (or ergocalciferol) and vitamin D3 (or cholecalciferol). Vitamin D obtained from sun exposure, food, and supplements, is biologically inert and must undergo two hydroxylation reactions to be activated in the body. Calcitriol (1,25-Dihydroxycholecalciferol) is the active form of vitamin D found in the body. The term vitamin D also refers to these metabolites and other analogues of these substances.

Calcitriol plays an important role in the maintenance of several organ systems. However, its major role is to increase the flow of calcium into the bloodstream, by promoting absorption of calcium and phosphorus from food in the intestines, and reabsorption of calcium in the kidneys; enabling normal mineralization of bone and preventing hypocalcemic tetany. It is also necessary for bone growth and bone remodeling by osteoblasts and osteoclasts.

Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Deficiency can arise from inadequate intake coupled with inadequate sunlight exposure; disorders that limit its absorption; conditions that impair conversion of vitamin D into active metabolites, such as liver or kidney disorders; or, rarely, by a number of hereditary disorders. Vitamin D deficiency results in impaired bone mineralization and leads to bone softening diseases, rickets in children and osteomalacia in adults, and possibly contributes to osteoporosis.

Vitamin D plays a number of other roles in human health including inhibition of calcitonin release from the thyroid gland. Calcitonin acts directly on osteoclasts, resulting in inhibition of bone resorption and cartilage degradation. Vitamin D can also inhibit parathyroid hormone secretion from the parathyroid gland, modulate neuromuscular and immune function and reduce inflammation.

Various forms of vitamin D

We know about 5 forms of vitamin D, of which vitamins D2 and D3 are the major forms as far as humans are concerned. They are known collectively as calciferol.

* Vitamin D1, molecular compound of ergocalciferol with lumisterol.

* Vitamin D2, ergocalciferol (made from ergosterol).

It is produced by invertebrates (animals without a spine, vertebral column), fungus and plants in response to sunlight (UV irradiation). Humans and other vertebrates do not produce vitamin D2. We don't know much about what vitamin D2 does in invertebrates. We know that ergosterol is a good absorber of ultraviolet radiation which can damage DNA, RNA and protein; consequently many scientists believe it may serve as a sunscreen that protects organisms from sunlight damage.

* Vitamin D3, cholecalciferol (made from 7-dehydrocholesterol).

Vitamin D3 is made in the skin when 7-dehydrocholesterol reacts with ultraviolet light at 270-300 nm wavelengths - peak vitamin D3 production occurs between 295-297 nm. It is only when the UV index is greater than 3 that these UVB wavelengths are present.

A UV index of more than 3 occurs every day in the tropics, every day during some of spring, all of summer, and parts of autumn in temperate areas, and hardly ever at all in the arctic circles. Temperate regions are all regions outside the tropics and arctic circles. The number of days of the year when the UV index is greater than 3 become fewer the further you move away from the tropics.

A human requires ten to fifteen minutes of sun exposure at least twice a week on the face, arms, hands, or back without sunscreen with a greater than 3 UV index for adequate amounts of vitamin D3. Longer exposure results in the extra vitamin supply being degraded as fast as it is generated.

* Vitamin D4, 22-dihydroergocalciferol.

* Vitamin D5, sitocalciferol (made from 7-dehydrositosterol).




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