(CBS/AP) Call it a dose of reality for doctors who have been pushing high doses of vitamin D.
A new report from the prestigious Institute of Medicine recommends higher doses of vitamin D, but says there's no proof that megadoses of the vitamin prevent cancer or heart disease. In fact, the report says, high levels of the "sunshine vitamin" could be hazardous.
"More is not necessarily better," said Dr. Joann Manson of Harvard Medical School, who co-authored the report.
Most people in the U.S. and Canada - from age 1 to age 70 - need to consume no more than 600 international units (IU) of vitamin D a day, according to the report. People in their 70s and older need as much as 800 IU. The report set those levels as the "recommended dietary allowance" for vitamin D.
That's a bit higher than the 400 IU target set by today's U.S. government-mandated food labels, and higher than the Institute's 1997 recommendations, which ranged from 200 to 600 IU, depending on age. But it's well below the 2,000 IU a day that some scientists have been recommending, pointing to studies that suggest people with low levels of vitamin D are at 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. He said the risk of colon cancer in particular could be slashed if people consumed enough vitamin D.
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's expert panel concluded that 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 highest D levels. Doses above 10,000 IU a day are known to cause kidney damage, and today's report sets 4,000 IU 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.
It's hard to consume 600 IU of vitamin D from food alone. A cup of D-fortified milk or orange juice has about 100 IU. 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 deficient in vitamin D, the average person already has enough circulating in his or her blood. That's because we also make vitamin D from sun exposure, and because many people already take multivitamins or other D-containing dietary supplements.