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.

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 today.

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 its “recommended dietary allowance” levels for vitamin D a bit higher than the target of 400 IUs set by today’s government-mandated food labels.

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.

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.

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.

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.