Which is more important for maintaining strong bones--calcium or vitamin D?

Most people say calcium. But the right answer is vitamin D, according to growing evidence, including a new study in the Journal of the American Medical Association. In fact, the new study of 2310 adults finds that as long as you get sufficient vitamin D, getting more than 800 milligrams of calcium a day is totally unnecessary for bone health.

The study by researchers at the University of Iceland, concludes that getting enough vitamin D is more important than getting extra high doses of calcium, such as 1200 mg daily, often recommended by doctors and other health professionals. It's a myth that calcium is a magic protector of bones and defense against osteoporosis and fractures, according to many experts.

The truth is, says the new study, as long as you take vitamin D supplements and/or get sufficient sunshine, a daily intake of 800 mg of calcium is quite enough for bone health. Some experts even point out that overdosing on calcium in futile efforts to fend off weak bones, may be detrimental to the heart and possibly promote prostate cancer.  Harvard's Walter Willett favors only 500-800 mg calcium a day, pointing out that populations with the lowest intake of calcium have the lowest rates of osteoporosis.    

How much vitamin D is enough? It's unclear, because people who live in sunny climates manufacture more vitamin D. Those who get less sun, such as in the winter in northern climates, are most apt to be deficient. The new study suggests that 500 IU of vitamin D daily may be enough throughout the year, but that during winter, some may need 700 IU of D daily. 

People worried about insufficient vitamin D can eat more fish and consume vitamin-D fortified foods, including milk. But supplements of vitamin D are the most reliable insurance, especially if you are not regularly exposed to sunlight, say the researchers.  (Steingrimsdottir, L. JAMA,  2005:294:2336-2341)

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