WEDNESDAY, Jan. 6, 2010 (HealthDay News) -- New research indicates that the darker skin of blacks may increase their risk of heart disease and stroke because it reduces production of vitamin D, which is made during exposure to sunlight.
Several studies have associated low levels of vitamin D with an increased risk of cardiovascular disease and "the biggest source of vitamin D levels is sunlight," said Dr. Kevin Fiscella, a professor of family medicine and community and preventive medicine at the University of Rochester, and co-author of a paper in the January/February issue of the Annals of Family Medicine. "People with dark skin who live at higher latitudes, where the intensity of sunlight is less, may be at greater risk."
But the issue abounds with unanswered questions, starting with whether there is a real cause-and-effect relationship of vitamin D levels and cardiovascular risk, and ending with whether supplements that increase blood levels of the vitamin lower that risk, Fiscella said.
"We don't truly know the answer," Fiscella said. "That is the really pivotal question, what happens to cardiovascular risk if you correct blood levels of vitamin D. We do know that small supplements for middle-aged people don't seem to have any effect."
In the study, Fiscella and Dr. Peter Franks of the University of California, Davis, looked at data on more than 15,000 U.S. adults in a national nutritional study. They found that overall, the 25 percent of adults with the lowest levels of vitamin D had a 40 percent higher risk of cardiovascular death. When they singled out blacks, the report found a 38 percent higher incidence of such deaths than among whites. Most of that difference was related to lower levels of vitamin D.
"The first issue is clarifying whether vitamin D is truly an independent risk factor for cardiovascular disease," Fiscella said. There are reasons to believe that it is, since too-low levels of the vitamin are associated with development of high blood pressure, kidney disease and diabetes, he said, but the case is not proven.
A second issue concerns the proper level of intake of the vitamin. "A consensus is evolving that the current levels recommended are too low, and those with darker skin need higher levels," Fiscella said.
The current recommendation is a daily intake of 400 International Units (IUs) for most adults, and 600 IU for those over 70. Fiscella declined to make a recommendation.
There was no such hesitation on the part of Dr. James O'Keefe Jr., director of preventive cardiology at the Mid America Heart Institute in Kansas City, who has done his own studies of vitamin D and the heart.
"I recommend for most people 2,000 IU a day," O'Keefe said. "African-Americans probably need closer to 4,000 or 5,000."
Too few Americans have their vitamin D levels checked regularly, "so I tell people to get their vitamin D levels checked," O'Keefe said. "Three out of four Americans will need a vitamin D supplement."
While it hasn't been proven that raising vitamin D levels reduces cardiovascular risk, studies now underway will answer that question, O'Keefe said. Meanwhile, he said, "vitamin D supplements are very cheap" and it is difficult to overdose on the vitamin, although bone problems can develop with a daily intake of 10,000 or more IU, he said.
Fiscella is much more cautious. "I don't think we have great data on what happens at very high levels," he said. "If you recommend very high doses, some people will develop very high blood levels, and we don't have good enough data to say on the population level what the impact of very high levels would be."
A fact sheet on vitamin D is provided by the U.S. Office of Dietary Supplements.