Vitamin D requirements and reference intakes

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 little evidence to establish what are appro-priate plasma concentrations of calcidiol; certainly the lower end of the reference range for young adults at the end of winter in a temperate climate is a mini-malist goal, and is not much higher than the level at which biochemical signs of deficiency occur. However, unfortified foods will not meet even this goal.

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.