Vitamin D requirements and reference intakes
It is difficult to determine requirements for dietary vitamin D, since the major source is synthesis in the skin. Before the development of methods for mea-surement of calcidiol the diagnosis of subclinical rickets was by detection of elevated alkaline phospha-tase in plasma; nowadays, the main criterion of ade-quacy is the plasma concentration of calcidiol.
In older people with little sunlight exposure, a dietary intake of 10 μg of vitamin D/day results in a plasma calcidiol concentration of 20 nmol/l, the lower end of the reference range for younger adults at the end of winter. Therefore, the reference intake for older people is 10 μg/day, whereas average intakes of vitamin D from unfortified foods are less than 4 μg/day.
There is increasing evidence that high vitamin D status is associated with a lower incidence of various cancers, diabetes, and the metabolic syndrome, sug-gesting that desirable intakes are higher than current reference intakes. Widespread fortification of foods would improve vitamin D status, but might also put a significant proportion of the population at risk of hypervitaminosis and hypercalcemia. Increased sun-light exposure will improve vitamin D status without the risks of toxicity, but excessive sunlight exposure is a cause of skin cancer. The main problem in trying to balance improved vitamin D status through increased sunlight exposure, and increased risk of skin cancer, is that there is very little information on the amount of sunlight exposure required for the synthesis of a given amount of vitamin D.
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