Mar. 16--A study from Wake Forest University School of Medicine suggests that blacks should be cautious about taking a vitamin D supplement as a strategy to reduce the risk of heart disease.
The study, released yesterday, found that having higher circulating levels of vitamin D was associated with more calcium in the walls of large arteries in blacks with diabetes, an effect that can lead to clogging of the arteries, or atherosclerosis.
"This is the opposite effect of what is felt to occur in white patients, and shows that the accepted normal range of vitamin D may be different between blacks and whites," said Dr. Barry Freedman, the chief of the section on nephrology at the medical school. He said that it is the first study dealing with this correlation in black patients.
The study included 340 black men and women with Type 2 diabetes. It appears in the March issue of the Journal of Clinical Endocrinology and Metabolism. Previous studies have shown a connection between obesity and deficient levels of vitamin D in people of all ethnic groups.
The study's cautionary message runs somewhat counter to the marketing push of vitamin D as a go-to remedy for a wide range of diseases and illnesses.
Vitamin D is widely used to treat patients with osteoporosis or low levels of vitamin D, or both, based on a medically accepted normal range.
Previous studies have shown that blacks tend to have lower levels of vitamin D than whites, partly because their skin color limits the amount of the vitamin produced by exposure to sunlight, and partly because they tend to eat fewer dairy products.
Even with those factors, blacks tend to have lower rates of osteoporosis and have far less calcium in their arteries than whites. Studies have shown that black patients with diabetes have half the rate of heart attack as whites when provided equal access to health care.
"Many of these study patients would be placed on supplemental vitamin D by their physicians simply because their levels were felt to be in the low range," Freedman said.
Freedman said that physicians should consider limiting the use of vitamin D supplements in black patients, particularly for those who do not have weak bones or other reasons to take the vitamin, until the effects on blood vessels and heart disease are better understood.
"We should more clearly determine the effects of supplementing vitamin D in black patients with low levels based on existing criteria, and should not assume that the effects of supplementation will be the same between the races," Freedman said.
Officials with the Council for Responsible Nutrition, a trade association, agreed that more research needs to be done in this category.
"It is a relatively small observational study and because it is cross-sectional in nature, it's essentially a snapshot in time," said Andrew Shao, a senior vice president of scientific and regulatory affairs for the trade group.
"The study also focused on participants who were clearly obese, with diabetes and hypertension issues for a number of years, and some with a smoking background," Shao said. "It is not representative of the black population as a whole."
The Wake study is the second focusing on vitamin D and heart disease in the past eight months.
A study released in August by the medical school at Washington University in St. Louis found that low levels of vitamin D can nearly double the risk of heart disease in patients with diabetes.
Those researchers found that diabetic patients who were deficient in vitamin D could not process cholesterol normally, so it builds up in their blood vessels, increasing the risk of heart attack and stroke. One of the study's recommendations is increasing levels of vitamin D.
Worldwide, about 1 billion people have insufficient vitamin D levels, according to the Washington study. In women with Type 2 diabetes, the likelihood of low vitamin D is about one-third higher than among women of the same age who don't have diabetes.
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