Sources
In foods vitamin D exists as cholecalciferol (D3) derived from dehydrocholesterol, or as ergocalciferol (D2)formed by UV irradiation of the plant sterol, egosterol. Relatively few foods provide the vitamin. Humans as well as the cow’s milk contain negligible amount of vitamin D. fish, eggs, animal liver and cheese are the food sources of vitamin D.
Synthesis in skin
Cholesterol in the skin is converted to 7-dehydricholesterol (7-DHC), which is formed in the highest concentration in the stratum spinosum and next in stratum basale, with little in the dermis. Absorption of a quantum of light in the wavelength range 250-310 nm results in cleavage of the 9,10 bond in the sterol B ring to yield pre-cholecalciferol which undergoes thermal isomerization to yield cholecalciferol and vitamin D.
In liver
D3 from the diet and skin are transported in the blood stream bound to a special alpha-globulin, the D3-binding protein (DBP), to the liver where it is hydroxylated at carbon 25, to yield 25-hydroxy D3.
Requirement of vitamin d
There is uncertainty about the magnitude of the requirement the vitamin at any age, due to the variability between individuals in the duration of exposure to sun light. Read the article on “Recommended daily allowances of some vitamins”.
The fate of dietary vitamin D apparently differs from the fate of D3 formed in skin.
Vitamin d deficiency
Deficiency of vitamin D can result from a number of factors including: inadequate intake coupled with inadequate sunlight (UVB) exposure, disorders that limit its absorption from the gastrointestinal tract, conditions that impair conversion of vitamin D into active metabolites, such as liver or kidney disorders and body characteristics such as skin color and body fat. Rarely deficiency can result from a number of hereditary disorders.Deficiency results in impaired bone mineralization, and leads to bone softening diseases including:
Rickets,
Osteomalacia
Osteoporosis
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