In draft recommendations that appeared on its website, the U.S. Preventive Services Task Force (USPSTF), which makes recommendations on a range of prevention issues, said studies of the supplements suggest they do little to prevent fractures at doses lower than 400 IU of vitamin D and 1,000 milligrams (mg) of calcium daily.

But it is clear that they come with a slightly increased risk of side effects, such as kidney stones.

"It's been known for some time that that is too low of a dose," said Silvina Levis, of the Osteoporosis Center at the University of Miami Miller School of Medicine in Florida.

The Institute of Medicine, an advisory panel to the U.S. government, recommends that men and women get at least 600 IU of vitamin D and at least 1,000 mg of calcium every day. The exact recommendations vary depending on age and sex.

Vitamin D and calcium supplements are often recommended for women to prevent fractures, according to the USPSTF. The supplements are widely available and are usually inexpensive.

The recommendations come as part of a review based on 19 randomized controlled trials, and found supplementing a person's diet with 400 IU of vitamin D and 1,000 mg calcium may offer some protection against broken bones in the elderly.

But the increased risk of kidney stones means there is "no net benefit," the panel found.

"We're fortunate that we have large studies that tell us with a moderate degree of certainty that, in these dosages, this supplementation is not effective in postmenopausal women with the goal of preventing fractures," said Kirsten Bibbins-Domingo, a member of the panel and a professor at the University of California, San Francisco.

For higher doses of the supplements, the evidence is still too limited to make recommendations either way.

Bibbins-Domingo added that it's important for people to know that the new recommendation only applies to postmenopausal women and those taking supplements.

"We know vitamin D is very important for the body and it's important for everyone to eat a healthy diet that includes vitamin D and calcium," she said. SOURCE: bit.ly/K7kSAx

(Reporting from New York by Andrew Seaman at Reuters Health; editing by Elaine Lies and Bob Tourtellotte)

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