That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it (as vitamin D3) when sun exposure is adequate (hence its nickname, the "sunshine vitamin").
Although vitamin D is commonly called a vitamin, it is not in the sense an essential dietary vitamin as it can be synthesized in adequate amounts by all mammals from sunlight. Vitamin D fits within the definition of vitamin as it is "an organic compound required as a vital nutrient in tiny amounts by an organism." An organic chemical compound (or related set of compounds) is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. As with other compounds called vitamins, it was discovered in an effort to find the dietary substance that was lacking in a disease, namely, rickets, the childhood form of osteomalacia. Additionally, like other compounds called vitamins, in the developed world vitamin D is added to staple foods, such as milk, to avoid disease due to deficiency.
Measures of serum levels (from a vitamin D3 blood test) reflect endogenous synthesis from exposure to sunlight as well as intake from the diet, and it is believed that synthesis may contribute generally to the maintenance of adequate serum concentrations. The evidence indicates that the synthesis of vitamin D from sun exposure works in a feedback loop that prevents toxicity but, because of uncertainty about the cancer risk from sunlight, no recommendations are issued by the Institute of Medicine, USA, for the amount of sun exposure required to meet vitamin D requirements. Accordingly, the Dietary Reference Intakes for vitamin D assume that no synthesis occurs and that all of a person's vitamin D is from their diet, although that will rarely occur in practice.
Human Body need sunlight?
"...like a woman's stillborn baby, who never saw the sun.(Psalms 58:8)
Vitamin D is converted to calcidiol in the liver. Part of the calcidiol is converted by the kidneys to calcitriol, the biologically-active form of vitamin D. This circulates as a hormone in the blood, regulating the concentration of calcium and phosphate in the bloodstream and promoting the healthy growth and remodeling of bone. Calcidiol is also converted to calcitriol outside of the kidneys for other purposes, such as the proliferation, differentiation and apoptosis of cells; calcitriol also affects neuromuscular function and inflammation.
Vitamin D has a key role in bone health. Supplements of vitamin D have not been found to improve other health
outcomes.The best evidence of benefit is for bone health.