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LINDA WERTHEIMER, host:

This is MORNING EDITION. I'm Linda Wertheimer.

STEVE INSKEEP, host:

And I'm Steve Inskeep.

This morning in Your Health, new advice on vitamin D. Nutrition researchers are pushing for a big increase in the daily recommended dose of what's sometime called the sunshine vitamin. Dozens of recent studies suggest that deficiencies of vitamin D make people more vulnerable to everything from fractures to certain cancers to diabetes.

NPR's Allison Aubrey reports.

ALLISON AUBREY reporting:

The doctor who coined the term aerobics decades ago and pioneered exercise as preventive medicine has a new recommendation, take more vitamin D.

Dr. KEN COOPER (Cooper Institute): We used to think that we get all the vitamin D that we need daily by exposure to sun.

AUBREY: Physician Ken Cooper now runs the nonprofit Cooper Institute in Dallas. Part of what they do is study vitamins and market doses and combinations backed by science. Cooper explains UV rays from the sun enable our bodies to synthesize vitamin D. The problem is study shows two-thirds of Americans don't get enough.

In the winter, the rays aren't strong enough and in the summer many of us are either all lathered up in sunscreen or covered up in work clothes. Researcher Beth Dawson-Hughes, who runs an osteoporosis center at Tuft's University, says she tries to soak in the sun as she makes rounds on campus.

Ms. BETH DAWSON-HUGHES (Researcher, Osteoporosis Center, Tufts University): I walk to and from the clinic with hands and arms exposed, but probably that didn't take me 10 minutes. So I would say I have not gotten my daily dose today.

AUBREY: For this reason and because it's tough to get adequate amounts through diet, Dawson-Hughes takes a vitamin D supplement, as do all of her patients. She said the old news, going back to studies down in the 1980's is that a combination of vitamin D and calcium seems to significantly cut the risk of bone fractures.

Ms. DAWSON-HUGHES: We now see that vitamin D also affects muscle performance and risk of falling.

AUBREY: The key it seems lies in taking enough vitamin D. To get the benefit, people participating in studies have been given at least 800 international units of D per day. That's double the amount currently recommended by the government-sponsored Food and Nutrition Board. This group sets the standards that many healthcare providers and vitamin makers follow.

Mr. WALTER WILLETT (Director, Nutrition Researcher, Harvard School of Public Health): I think at this point the case for raising the recommended levels of vitamin D intake is very strong.

AUBREY: Walter Willett directs nutrition research at the Harvard School of Public Health. He says the benefits of D seem to extend far beyond bone health.

Mr. WILLETT: There are many lines of evidence that people need more vitamin D.

AUBREY: Take for instance cancer prevention. A few decades ago, researchers found that colon cancer rates were higher in northern parts of the country where sunlight exposure is lowest. From this lead, Willett's group at Harvard designed a study. It took blood samples from 30,000 healthy women to find out exactly how much D they had in their bodies, then followed the women for years to see which ones developed colon cancer.

Mr. WILLETT: We found that women who had the lowest blood levels of vitamin D had about double the risk of cancer, compared to women who had the highest blood levels of vitamin D.

AUBREY: The study does not prove that vitamin D protects against cancer, but two other lines of evidence help build the case. Lab researchers doing test tube and animal studies have found that vitamin D reduces the rate of cell multiplication. And most recently, scientists have found that a genetic variation in the vitamin D receptor, which transmits signals from vitamin D to cells, is associated with risk of breast cancer.

As researchers try to replicate these findings, there is also new work into understanding how vitamin D may affect the risk of multiple sclerosis, asthma and diabetes. Again, Beth Dawson-Hughes of Tufts.

Ms. DAWSON-HUGHES: We and many other researchers are seeing that individuals who have higher vitamin D levels have a lower risk of developing Type II diabetes.

AUBREY: The best evidence comes from a randomized controlled trial which found people taking a vitamin D-calcium supplement had a better glucose or sugar tolerance than those taking a placebo. The big picture here is that researchers have many reasons to be excited by the accumulating evidence. And even though many associations remain unproven, one key issue has been worked out. Scientists have lots of new data to show high doses of vitamin D, up to 4,000 international units per day, are not toxic to the body. So since it can't hurt, and it probably does help, researchers are betting they can convince the Food and Nutrition Board to increase the daily recommended level.

The first meeting was called a few weeks ago, and the number many are aiming for is 1,000 International Units of vitamin D per day. This is the dose that the Cooper Institute's scientific advisory board recently adopted for their products.

Allison Aubrey, NPR News, Washington.

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