Q: I'm 70 years old. Should I be taking a vitamin D supplement?

A: Vitamin D is essential to maintain bone health. A below-normal blood level increases your risk of osteoporosis, falls and fractures. As many as 1 out of 3 people in the United States have low blood levels of vitamin D.

There are three main forms of vitamin D. The active form found in our bodies is 1,25-dihydroxyvitamin D. The body makes the active form from vitamin D2 and vitamin D3.

The skin makes vitamin D3 when it is exposed to ultraviolet light (sunlight). It is also found in foods and supplements. Vitamin D2 is found only in food and supplements. However, few natural foods are rich in vitamin D. That's why milk, orange juice and other products have vitamin D added.

Today, people get less exposure to sunlight, especially during the winter. And we absorb vitamin D less efficiently with age.

Besides bone health, studies have shown that people with low levels of vitamin D may have increased risks of cancer, high blood pressure, heart disease, stroke and other blood vessel diseases.

But it's way too soon to call it a wonder vitamin. No studies have proven that the low level of vitamin D causes any of these problems. Nor have they shown that taking vitamin D pills prevents them from occurring.

The Recommended Dietary Allowance (RDA) for vitamin D is 600 international units (IU) a day for everyone ages 1 to 70 and 800 IU a day for those 71 and older.

Personally, I recommend a daily pill containing 1,000 IU of vitamin D3. Some doctors suggest that this dose is too low. The Institute of Medicine says that up to 2,000 IU of vitamin D per day is safe.

To maintain bone health and prevent fractures, you also need to get enough calcium. How much you need is debated because too much calcium taken as pills may be harmful. Recent studies suggest that high-dose calcium supplements might increase the risks of heart disease and certain types of cancer. Unlike calcium in your diet, calcium in pills also can cause kidney stones.

The current advice is 1,000 to 1,200 milligrams total calcium per day for adults. The recommended upper limit is 2,000 milligrams per day. Most people can easily get enough calcium through their diet.

(Howard LeWine, M.D., is a practicing internist at Brigham and Women's Hospital, Boston, Mass.,and Chief Medical Editor of Internet Publishing at Harvard Health Publications, Harvard Medical School.)

(For additional consumer health information, please visit http://www.health.harvard.edu.)