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 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.

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.

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.