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From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.

MELISSA BLOCK, HOST:

And I'm Melissa Block. Vitamin D is being promoted these days for all sorts of things, but an expert panel is raising questions about two big reasons people take the so-called sunshine vitamin.

NPR's Rob Stein has the story.

ROB STEIN, BYLINE: Evidence has been mounting in recent years that Vitamin D might have lots of health benefits. Here's Bruce Hollis of the Medical University of South Carolina.

BRUCE HOLLIS: It has potential benefits in cancer prevention, cancer treatment, cardiovascular prevention, infection prevention, prevention of autoimmune disease.

STEIN: So a prominent panel of experts decided to take a look at two of the most common reasons people take Vitamin D and come up with a set of recommendations. Kirsten Bibbins-Domingo is a member of the U.S. Preventive Services Taskforce.

KIRSTEN BIBBINS-DOMINGO: This is the first recommendation that we have, looking specifically at the prevention of fractures and prevention of cancer.

STEIN: The taskforce analyzed data from the best 16 studies of Vitamin D and calcium to prevent bone fractures. Specifically, they looked at taking 400 international units of Vitamin D combined with 1,000 milligrams of calcium every day and she says what the taskforce found for older women is...

BIBBINS-DOMINGO: It really is not effective for preventing fractures. So for post-menopausal women who are seeking to do that, this type of supplementation really isn't effective for that purpose.

STEIN: Now, Bibbins-Domingo says the calcium in Vitamin D can be helpful. It seems to help prevent falls among elderly people, but it doesn't seem to protect post-menopausal women from getting bone fractures and the jury's still out for younger women.

The panel also looked at cancer. The taskforce could only find three studies it considered really good enough.

BIBBINS-DOMINGO: We have insufficient evidence. We can't really make a statement on that.

STEIN: The taskforce can't rule out that higher doses may work, but Bibbins-Domingo says, in the meantime, there are some real well known risks of taking supplements.

BIBBINS-DOMINGO: There is a small but increased risk of kidney stones by taking the Vitamin D and calcium supplementation.

STEIN: Kidney stones may seem minor compared to cancer, but when you're giving healthy people anything for a long time, she says, you have to be careful.

BIBBINS-DOMINGO: In prevention, we are taking, generally, people who are otherwise healthy, who don't have symptoms, who are taking, in this case, a supplement potentially for a long period of time in order to achieve some health goal in the future. And I think for those purposes we have to set the bar high and really understand whether those purposes can be achieved.

STEIN: But Vitamin D proponents like Bruce Hollis dismiss the taskforce's conclusions. For one thing, he says, the doses studied were far too low to really do much.

HOLLIS: Well, the amounts of Vitamin D they're recommending are extraordinarily low. They're basically the levels an infant should get. To me, it's appalling. I think the report is a sham.

STEIN: And Hollis and others say there are many other studies that indicate higher doses have lots of benefits and are safe.

HOLLIS: We've monitored thousands of patients taking these levels of Vitamin D that everybody's worried about that there'll be toxicity, yet the human body can make that much in the sun.

STEIN: Bibbins-Domingo can't rule out that other doses might turn out to be useful.

BIBBINS-DOMINGO: It may be that higher doses are beneficial. But that would be an area that we would certainly want to see the studies to really show that the benefits are there and that they outweigh any potential risk of higher doses.

STEIN: So it's clear the debate over the risks and benefits of Vitamin D supplements isn't going to end anytime soon. The taskforce plans to take public comment for about a month before issuing final recommendations.

Rob Stein, NPR News.

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