Vitamin D
About Vitamin D
Vitamin D is a fat soluble secosteroid that the skin can synthesize on exposure to ultraviolet B light, which is why it is often described as a hormone as well as a vitamin. It occurs as vitamin D2, ergocalciferol, made by fungi and yeasts, and vitamin D3, cholecalciferol, formed in skin and found in animal foods, with D3 generally more effective at raising blood levels. Vitamin D is activated by two hydroxylations, first in the liver to 25 hydroxyvitamin D, the main circulating form, and then in the kidney to the active hormone 1,25 dihydroxyvitamin D, which increases intestinal absorption of calcium and phosphate. Official recommended intakes vary by age. Deficiency causes rickets in children, with soft bones and bowed legs, and osteomalacia in adults, with bone pain and muscle weakness. Low status has been linked in observational studies to infections, autoimmune disease and some cancers, though trials have given mixed results. Food sources include fatty fish, cod liver oil, egg yolks, fortified foods and mushrooms exposed to ultraviolet light.
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