Contributions of sunlight and diet to vitamin D status

Vitamin D is made in the skin using ultraviolet radiation of specific low wavelength, 290-315 nm (UVB). For many parts of the world there is a period when there is insufficient intensity of UVB to make vitamin D, which is reflected by a clear seasonal variation in vitamin D status. Sun avoidance practices, melanin in pigmented skin, and sun protection creams (sunscreen), if used properly, can dramatically reduce vitamin D synthesis. Few foods naturally contain vitamin D, although some countries fortify foods with vitamin D. Regulatory mechanisms in the skin mean there is no danger of vitamin D toxicity through sunlight synthesis. Although oral vitamin D is potentially toxic with high-dose supplements, there is a wide safety margin. Long-term safety data covering a range of potential adverse outcomes are limited.