Figuring out all the factors that can affect your vitamin D level is complicated. Your body makes vitamin D when sunlight hits the skin. You can also get the vitamin from food (but few foods are a natural source of vitamin D) or by taking a supplement. The process by which the body makes vitamin D is complex. It starts when the skin absorbs rays in the invisible ultraviolet B (UVB) part of the light spectrum. The liver and kidneys also make a form of the vitamin that the body can use. A number of factors affect a person's vitamin D levels. Here are six important ones.
Where you live. The further away you live from the equator, the less vitamin D-producing UVB light reaches the Earth's surface in winter. Short days and clothes that cover the legs and arms also limit exposure to UVB.
Air quality. Airborne carbon particles from burning fossil fuels, wood and other materials scatter and absorb UVB rays, reducing vitamin D production. In contrast, ozone absorbs UVB radiation, allowing holes in the ozone layer caused by pollution to improve vitamin D levels.
Use of sunscreen. Sunscreen prevents sunburn by blocking UVB light. Theoretically, this means that using sunscreens lowers vitamin D levels. But in practice, very few people put on enough sunscreen to block all UVB light, or use sunscreen infrequently, so the effects of sunscreen on vitamin D may not be that important.
Skin color. Melanin is the substance in the skin that makes it dark. It "competes" for UVB with the substance in the skin that stimulates the body's vitamin D production. As a result, dark-skinned people usually need more UVB exposure than fair-skinned people to generate the same amount of vitamin D.
Weight. Body fat absorbs vitamin D, so it has been suggested to provide a vitamin D fund for rainy days:a source of the vitamin when intake is low or production is reduced. But studies have also shown that being obese is correlated with low vitamin D levels and that being overweight can affect the bioavailability of vitamin D.
Age. Compared to younger people, older people have lower levels of the substance in the skin that converts UVB light into the vitamin D precursor. There is also experimental evidence that the elderly are less efficient vitamin D producers than younger people.
