MONDAY, Jan. 7 (HealthDay News) -- Low blood levels of vitamin D -- sourced through sunlight, some foods and supplements -- are associated with an increased risk of cardiovascular problems such as heart attack and stroke, U.S. researchers report.

Their five years of research with participants in the Framingham Heart Study included 1,739 people, average age 59, living in that Massachusetts city.

The research team found that those with the lowest levels of vitamin D had a 62 percent greater risk of a cardiovascular event than those with the next highest levels, according to a report published in the Jan. 7 issue of Circulation.

It's still too early to recommend routine vitamin D supplementation, however, said study author Dr. Thomas J. Wang, an assistant professor of medicine at Harvard Medical School. He believes that there is still not enough evidence to put vitamin D deficiency on the same level as high cholesterol and other known risk factors.

"It is probably premature to consider vitamin D in the same light as those risk factors because this is one of the first clinical studies," Wang said. "We don't know whether increasing vitamin D levels with some kind of supplement decreases risk. That would require some kind of trial."

Vitamin D is known to be essential for strong bones, since it facilitates the body's uptake of calcium. One major source of the vitamin is sunlight -- an hour or so of sunlight on the skin each week allows the skin to produce blood levels of about 30 nanograms of vitamin D per liter of blood, more than enough to prevent a deficiency such as rickets, for example. Blacks need longer exposures than whites, however, since their skin pigment reduces formation of the vitamin.

Food sources of vitamin D include milk (which is fortified) and oily fishes such as salmon. Current recommendations from the U.S. Institute of Medicine call for a daily intake of vitamin D ranging from 200 International Units (IU) for young people, to 400 IU for the middle-aged, to 600 IU for older people.

But getting that amount from food and sunlight may not be easy, said Robert U. Simpson, a professor of pharmacology at the University of Michigan, whose group was the first to identify vitamin D receptors in heart cells. For example, a glass of fortified milk contains only about 100 IU of vitamin D.

Simpson believes the recommended levels are actually still too low. "People should get at least 1,000 IU, perhaps 2,000," he said. "I take 1,000 IU a day. I recommend that intake right now as part of a multivitamin supplement."

Wang is much more cautious.

"I would still emphasize that studies like ours are not enough to alter practice," he said. Wang said he does not take any vitamin supplement, but he concedes that the recommended 600 IU daily intake for older people is "very, very difficult to achieve," especially in the winter in northern parts of the country.

"Virtually all elderly people have to take supplements," Wang said.

The cardiovascular risk associated with low vitamin D levels was especially great for people who also had high blood pressure. Their incidence of cardiovascular events was double that of people with higher blood levels of vitamin D.

There have been similar studies of other vitamins indicating that they might help prevent heart disease, Wang noted. But efforts at supplementation with those vitamins failed to show a benefit in large-scale, controlled trials, Wang noted.

"The question becomes how much data such as ours is needed to justify a controlled trial," he said. "The answer is, more than we have now."

More information

There's a fact sheet on vitamin D at the U.S. Office of Dietary Supplements.

SOURCES: Thomas J. Wang, M.D., assistant professor, medicine, Harvard Medical School, Boston; Robert U. Simpson, Ph.D., professor, pharmacology, University of Michigan, Ann Arbor; Jan. 8, 2008, Circulation

Copyright © 2008 ScoutNews, LLC. All rights reserved.