Vitamin D is a hormone that is manufactured in the body when skin is exposed to sunlight. It can also be obtained from a few dietary sources, mainly fortified foods such as milk. Many people are deficient in vitamin D, especially those who are over 60, live at northern latitudes, have darker skin, or are rarely outdoors. In the northern U.S. and in Canada, blood levels drop markedly in the winter. Many young people also have low blood levels of vitamin D, according to some recent studies. Obesity is associated with reduced blood levels.

Claims, purported benefits: There has been a lot of recent research on vitamin D. Some has focused on its known benefits, notably its crucial role in working with calcium to keep bones strong. But many studies have looked at its potential to reduce the risk of everything from some common cancers and multiple sclerosis to diabetes, hypertension, and age-related muscle weakness. A recent meta-analysis found that people taking vitamin D supplements had a 7% reduction in total mortality rates.

Bottom line: The research is promising. The potential health benefits have been linked to high blood levels of vitamin D—levels higher than most Americans and Canadians have. Since few foods supply vitamin D, and depending on sun exposure to produce enough of it in the skin is unpredictable and can cause skin cancer, supplements are the best option for most people.

Many experts now advise people to get tested for vitamin D, and to take enough supplements to get blood levels of 25-hydroxyvitamin D to at least 30 to 40 ng/ml. Talk to your doctor about testing, especially if you are over 60 or have low bone density. In any case, consider taking 800 to 1,000 IU of supplemental vitamin D a day (multivitamins usually supply only 400 IU). For most people, that should raise blood levels to the desirable range or at least close to it. Some people with low blood levels of vitamin D may need even higher intakes.