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Most people in the U.S. or Canada between the ages of 1 and 70 only need 600 international units of vitamin D a day, a new report finds. That's higher than the target on food labels, but much lower than the 2,000 IUs some scientists recommend.
NBC News and news services
updated 11/30/2010 7:05:50 PM ET 2010-12-01T00:05:50

For the past few years vitamin D has been the "it" vitamin, with studies wildly trumpeting the supplement's role in strengthening bones, reducing the risk of some cancers, heart disease, along with fighting autoimmune diseases and diabetes. But long-awaited new dietary guidelines say there's no proof that megadoses of the "sunshine vitamin" prevent cancer, diabetes or other conditions.

While some people will need a bit more vitamin D than they're already getting, some studies suggest that too much could actually cause some kinds of cancer, according to the panel of experts at the prestigious Institute of Medicine, the health arm of the National Academy of Sciences.

"More is not necessarily better," cautioned Dr. Joann Manson of Harvard Medical School, who co-authored the Institute of Medicine's report being released Tuesday.

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While the panel found powerful evidence that vitamin D — together with calcium — can protect bones, the new report could dampen the national sunshine vitamin fad.

Most people in the U.S. and Canada — from age 1 to age 70 — need to consume no more than 600 international units of vitamin D a day to maintain health, the report found. People in their 70s and older need as much as 800 IUs. The report set those levels as the "recommended dietary allowance" for vitamin D.

That's only a bit higher than the target of 400 IUs set by today's government-mandated food labels, and higher than 1997 recommendations by the Institute of Medicine that ranged from 200 to 600 IUs, depending on age.

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But it's far below the 2,000 IUs a day that some scientists recommend, pointing to studies that suggest people with low levels of vitamin D are at increased risk of certain cancers or heart disease.

"This is a stunning disappointment," said Dr. Cedric Garland of the University of California, San Diego, who wasn't part of the institute's study and says the risk of colon cancer in particular could be slashed if people consumed enough vitamin D.

"Have they gone far enough [in raising recommended levels]? In my opinion probably not, but it's a step in the right direction," added prominent vitamin D advocate Dr. Michael Holick of Boston University Medical Center, who said the new guidelines draw needed attention to the vitamin D debate and encourage more food marketers to fortify their products with it.

Vitamin D and calcium go hand in hand, and you need a lifetime of both to build and maintain strong bones. But the two-year study by the Institute of Medicine's panel of experts concluded research into vitamin D's possible roles in other diseases is conflicting. Some studies show no effect, or even signs of harm.

How much should you take?

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A National Cancer Institute study last summer was the latest to report no cancer protection from vitamin D and the possibility of an increased risk of pancreatic cancer in people with the very highest D levels. Super-high doses — above 10,000 IUs a day — are known to cause kidney damage, and Tuesday's report sets 4,000 IUs as an upper daily limit — but not the amount people should strive for.

Vitamin D is the latest supplement to have been heavily touted based on animal studies and observations in humans and failed to live up to expectations during rigorous testing. For example, vitamins C and E, beta carotene and lycopene were believed to prevent cancer or heart disease, but didn't pan out, and sometimes caused harm, when tested.

Stay tuned: To help settle the issue, Manson is heading a government-funded study that's recruiting 20,000 healthy older Americans to test whether taking 2,000 IUs of vitamin D really will lower their risk for heart disease, a stroke or certain cancers.

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In the meantime, it's hard to consume 600 IUs of vitamin D from food alone. A cup of D-fortified milk or orange juice has about 100 IUs. The best sources may be fatty fish — some servings of salmon can provide about a day's supply. Other good sources are D-fortified cereals.

But here's the report's big surprise: While some people truly are seriously deficient in vitamin D, the average American in fact already has enough circulating in his or her blood — because we also make vitamin D from sun exposure, and because many people already take multivitamins or other D-containing dietary supplements.

Wait a minute: Headlines in recent years have insisted the opposite, that a majority of people don't get enough vitamin D, especially during the winter. What explains the contradiction?

A relatively inexpensive blood test measures something called 25-hydroxy vitamin D, the active form in the body. Most testing laboratories are using a too-high cutoff for those blood levels, said report co-author Dr. Clifford Rosen of the Maine Medical Center. The report says at least 20 nanograms is adequate for bone health, while many labs instead list people as low if their blood levels are below 30 ng. Serious vitamin D deficiencies are diagnosed when levels dip well below 20, something that hasn't changed.

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Rosen called the state of vitamin D testing "the wild, wild West," and said he hoped that "with this report, we can at least temper people's enthusiasm for just taking tons of supplements."

As for calcium, the report recommended already accepted levels to go along with your daily D — about 1,000 milligrams of calcium a day for most adults, 700 to 1,000 mg for young children, and 1,300 mg for teenagers and menopausal women. Too much can cause kidney stones; the report said that risk increases once people pass 2,000 mg a day.

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It's true that most studies link poor health to vitamin D levels that are below 20 ng, said preventive cardiologist Dr. Erin Michos, a Johns Hopkins University School of Medicine professor who wasn't part of the study.

But, "I'm not sure I'm going to dramatically change my practice," said Michos, who pushes her patients to boost their levels until they're between 30 and 50 ng.

And even if future trials show that vitamin D does help protect against other diseases, the amounts required for bone health should be perfectly sufficient, panel members told NBC News.

The Associated Press and NBC's Robert Bazell contributed to this report

Video: New guidelines for Vitamin D, Calcium

  1. Closed captioning of: New guidelines for Vitamin D, Calcium

    >>> we turn now to today's news about health. surprising, especially for women, who have had it hammered into their heads that they need more calcium to keep their bones healthy. a new report from the institute of medicine says that that's not always true. and it says -- much of what especially women have been told about vitamin d is a myth as well. our chief science correspondent robert bazell has more on the new findings.

    >> reporter: when it comes to bone health, there is no debate. calcium and vitamin d are critical. but today's report holds surprises. in a world where there's calcium in dairy products and it is added to everything from tums to orange juice , many people including older women, don't need calcium supplements .

    >> many individuals will be able to obtain these recommended dietary allowances from diet.

    >> reporter: the other big news? vitamin d , we get it from exposure to the sun, certain food and from supplements. in recent years, research has suggested vitamin d might lower the risk for all sorts of conditions. including various cancers, heart disease , diabetes and auto immune disorders. because we tend to spend less time in the sun given the worries about skin cancer , many people have been taking large doses of vitamin d supplements.

    >> we were asked to review the available evidence about the effects of calcium and vitamin d .

    >> reporter: but the panel found the scientific evidence for the benefits of vitamin d beyond bone health, is very weak.

    >> the data as a whole were often inconclusive and sometimes contradictory.

    >> as with calcium, the panel found most people get enough vitamin d from the sun and their diet and don't need supplements. a simple blood kes can reveal if someone is vitamin d deficient. a prominent bone specialist worries that those who need supplements will decide not to take them because of this report.

    >> we've never told people to take extra. we've told people to take enough. and if the diet is not providing enough, you need some from supplements to get up to what we consider to be enough.

    >> reporter: some people like carroll baggerly continue to doubt the experts. she insists megadoses of vitamin d help her in many ways.

    >> my own personal well being it's critical.

    >> reporter: and others are confuse.

    >> one person comes up with something and then in a few more months someone comes up with something else.

    >> i make my own decisions and take what i feel is appropriate.

    >> reporter: the panel warns that too much calcium or vitamin d can cause problems including kidney stones . so how much of both should you take? the report has lots of specific numbers and you can read it on our website, nightly dot msnbc.com.

    >> i agree with that last woman. robert bazell thanks as always.

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