Supplement Your Knowledge of Vitamin D

May 19, 2008 -- The big news hit last week: Women who are deficient in vitamin D when they get breast cancer have a much higher chance of dying from their cancer.

That's surprising news. But just as surprising is the fact that many women -- and many men -- are deficient in vitamin D or have insufficient blood levels of this important vitamin.

How many? Data suggest many of us don't get the vitamin D we need. For example, a 2007 study of childbearing women in the Northern U.S. found insufficient vitamin D levels in 54% of black women and in 42% of white women.

WebMD's FAQ addresses these questions:

• Why do I need vitamin D?
• How can I get enough vitamin D?
• Which foods contain vitamin D?
• How much vitamin D do I need?
• Can I get too much vitamin D?
• What kind of vitamin D is best?
• Does vitamin D interact with other medications?

Why do I need vitamin D?

Your body must have vitamin D to absorb calcium and promote bone growth. Too little vitamin D results in soft bones in children (rickets) and fragile, misshapen bones in adults (osteomalacia). You also need vitamin D for other important body functions.

Vitamin D deficiency has now been linked to breast cancer, colon cancer, prostate cancer, heart disease, depression, weight gain, and other maladies. These studies show that people with higher levels of vitamin D have a lower risk of disease, although they do not definitively prove that lack of vitamin D causes disease -- or that vitamin D supplements would lower risk.

The Vitamin D Council -- a scientist-led group promoting vitamin D deficiency awareness -- suggests vitamin D treatment might be found helpful in treating or preventing autism, autoimmune disease, cancer, chronic pain, depression, diabetes, heart disease, hyperparathyroidism, high blood pressure, flu, neuromuscular diseases, and osteoporosis. However, there have been no definitive clinical trials.

The best known benefit of vitamin D is its role in helping calcium build strong bones. But that's far from the whole story. Vitamin D helps regulate the immune system and the neuromuscular system. Vitamin D also plays major roles in the life cycle of human cells.

Vitamin D is so important that your body makes it by itself -- but only after skin exposure to sufficient sunlight. This is a problem for people in northern climates. In the U.S., only people who live south of a line drawn from Los Angeles to Columbia, S.C., get enough sunlight for vitamin D production throughout the year.

Dark skin absorbs less sunlight, so people with dark skin do not get as much vitamin D from sun exposure as do light-skinned people. This is a particular problem for African-Americans in the northern U.S.