Vitamin D is a nutrient found in some foods that is needed for health and to maintain strong bones. It does so by helping the body absorb calcium from food and supplements. Vitamin D is important to the body in many other ways as well. Vitamin D plays a role in cell growth, immunity and reducing inflammation. Vitamin D is found in cells throughout the body.
Very few foods naturally have vitamin D. Fortified foods provide most of the vitamin D in American diets. Fatty fish such as salmon, tuna, and mackerel are among the best sources of vitamin D. Almost all of the U.S. milk supply is fortified with 400 IU of vitamin D per quart. But foods made from milk, like cheese and ice cream, are usually not fortified. Vitamin D is added to many breakfast cereals and to some brands of orange juice, yogurt and margarine; check the labels
The body makes vitamin D when skin is directly exposed to the sun, and most people meet at least some of their vitamin D needs this way. Skin exposed to sunshine indoors, through a window, will not produce vitamin D. Cloudy days, shade, and having dark-colored skin also cut down on the amount of vitamin D the skin makes. People who avoid the sun or who cover their bodies with sunscreen or clothing should include good sources of vitamin D in their diets or take a supplement.
Older adults are at increased risk of developing vitamin D insufficiency for many reasons. First, as they age their skin cannot synthesize vitamin D as efficiently. Second, they are more likely to spend time indoors especially if they have physical limitations or live in an assisted living community or a nursing home. Finally, older adults may not have adequate intakes of vitamin D from food. In addition, people with dark skin, because their skin has less ability to produce vitamin D from the sun, are more likely to be deficient. Obese people, because their body fat binds to some vitamin D and prevents it from getting into the blood are also at risk of deficiency.
As people get older they develop, or are at risk of developing osteoporosis. This is true for men as well as women. Together with calcium, vitamin D helps protect older adults from osteoporosis. Supplements of both vitamin D3 (800 IU/day) and calcium (500–1,200 mg/day) have been shown to reduce the risk of bone loss and fractures in elderly people aged 62–85 years. Men and women should talk with their health care providers about their needs for vitamin D and calcium supplements as part of an overall health plan.
Source: Office of Dietary Supplements NIH