WASHINGTON - Got milk? You may need a couple cups more than today's food labels say to get enough vitamin D for strong bones. But don't go overboard: Long-awaited new dietary guidelines say there's no proof that megadoses prevent cancer or other ailments - sure to frustrate backers of the so-called sunshine vitamin.

The decision by the prestigious Institute of Medicine, the health arm of the National Academy of Sciences, could put some brakes on the nation's vitamin D craze, warning that super-high levels could be risky.

"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.

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 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.

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 my opinion probably not, but it's a step in the right direction," added prominent vitamin D researcher Dr. Michael Holick of Boston University Medical Center, who said the new levels draw needed attention to the vitamin D debate and encourage more food fortification.

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's D possible roles in other diseases is conflicting. Some studies show no effect, or even signs of harm.

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.

And Manson pointed to history's cautionary tales: A list of other supplements - vitamins C and E and beta carotene - plus menopause hormone pills that once were believed to prevent cancer or heart disease didn't pan out, and sometimes caused harm, when put to rigorous testing.

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.

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?

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.

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.

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.

Lauran Neergaard covers health and medical issues for The Associated Press in Washington.