Vitamin D plays a fundamental role in calcium absorption and bone health. The body cannot absorb calcium or make new bone without it. Vitamin D facilitates calcium absorption in the intestine by a complex mechanism that scientists have only recently begun to understand.

Imagine vitamin D as a key that unlocks a door to your body, allowing calcium to leave the intestine and enter the bloodstream. Vitamin D also helps reclaim calcium in the kidneys, where calcium is filtered and prepared for excretion in the urine.

The body gets vitamin D in two ways. It is naturally synthesized in the skin upon exposure to ultraviolet radiation from sunlight. That’s why it’s called the “sunshine vitamin.” How much vitamin D you produce depends upon how long you are out in the sun; in general, at least 15 minutes of exposure per day is required to create the 400 IU required for normal body function. Many factors, however, can modulate the effects of sun exposure, including the time of year and time of day when you are outside and the clothes you wear. Many people now put on sunscreen before going outside, to avoid harmful overexposure to ultraviolet rays. But sunscreen also blocks vitamin-D absorption. So if you wear sunscreen all of the time you are outdoors or do not have regular exposure to the sun, you must depend upon the other source for vitamin D to meet your body’s need: diet.

Though most people are able to obtain enough vitamin D naturally, many studies show that vitamin-D production decreases in the elderly, in people who are housebound, and during winter. Elderly people in convalescent homes, who are unable to get outside, are at high risk for vitamin-D deficiency. Some people may require vitamin-D supplements to achieve the RDA of 400 IU. Most multivitamins do contain 400 IU of vitamin D.

Vitamin D is found in eggs, butter, liver, and fatty fish (such as mackerel). Milk is usually fortified with vitamin D, so all dairy products are a rich source of this vitamin. One cup of fortified milk provides about 100 IU of vitamin D. Check the nutrition label on the milk you buy to make sure it is supplemented with vitamin D.

Vitamin D, whether you get it from the sun, from the food you eat, and/or from supplements, circulates in the blood, where it is activated to a form that is responsible for modulating calcium absorption from the intestine, as well as other functions. Vitamin-D activation has two steps. First the liver changes vitamin D into an intermediary factor, called calcidiol. Then from calcidiol the kidneys produce the active vitamin-D metabolite, calcitriol.