It is activated by two protein enzyme hydroxylation steps, the first in the liver and the second in the kidneys. Vitamin D from the diet, or from skin synthesis, is biologically inactive.

Dietary recommendations typically assume that all of a person's vitamin D is taken by mouth, because sun exposure in the population is variable and recommendations about the amount of sun exposure that is safe are uncertain in view of the skin cancer risk. Mushrooms exposed to ultraviolet light contribute useful amounts of vitamin D 2. and other countries, cow's milk and plant-derived milk substitutes are fortified with vitamin D, as are many breakfast cereals. Only a few foods, such as the flesh of fatty fish, naturally contain significant amounts of vitamin D. Cholecalciferol and ergocalciferol can be ingested from the diet and supplements. The major natural source of the vitamin is synthesis of cholecalciferol in the lower layers of epidermis of the skin, through a photo-chemical reaction of UVB light, from the sun exposure (specifically UVB radiation) or UVB lamps. In humans, the most important compounds in this group are vitamin D 3 ( cholecalciferol) and vitamin D 2 ( ergocalciferol). Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. Rickets, osteoporosis, vitamin D deficiency

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