By Steven Reinberg
"Compared with other races, blacks in the United States are more likely to have vitamin D deficiency and more likely to have high blood pressure," said lead researcher Dr. John Forman, an assistant professor of medicine at the renal division of Brigham and Women's Hospital in Boston.
But among the black study participants, three months of supplemental vitamin D was associated with a drop in systolic blood pressure (the top number in a blood pressure reading) of up to 4 mm Hg, the researchers found.
"If our findings are confirmed by other studies, then vitamin D supplementation may be a useful means of helping black individuals lower their blood pressure," Forman said.
Dr. Michael Holick, a professor of medicine, physiology and biophysics at Boston University School of Medicine, said that vitamin D may lower blood pressure by causing blood vessels to relax, allowing for more and easier blood flow.
In addition, because many black Americans are deficient in vitamin D, taking a supplement may benefit their health even more, said Holick, who was not involved with the study.
"We are now beginning to believe that a lot of the health disparities between blacks and whites are due to vitamin D deficiency, including the risk for type 2 diabetes, heart disease, cancers and even infectious disease," he said.
Diet and sunlight are two natural sources of vitamin D in humans. However, having dark-colored skin cuts down on the amount of vitamin D the skin makes, according to the U.S. National Institutes of Health.
For the study, published online March 13 and in the April print issue of the journal Hypertension, Forman's team randomly assigned 250 black participants to one of three doses of vitamin D supplements or an inactive placebo.
After three months, the researchers found that those taking 1,000 international units (IU) of vitamin D a day saw their systolic blood pressure drop by 0.7 mm Hg. For those taking 2,000 IU, the drop was 3.4 mm Hg, and for those taking 4,000 IU, systolic pressure dropped by 4 mm Hg.
In contrast, those receiving the placebo saw their systolic blood pressure rise by 1.7 mm Hg, the researchers noted.
Forman said the gains from supplemental vitamin D were significant, but modest. In addition, there were no changes in diastolic blood pressure among those in any group.
"Hypertension is a leading cause of stroke, heart attack, heart failure, kidney failure and premature cardiovascular death, particularly in black men and women," said Dr. Gregg Fonarow, a spokesman for the American Heart Association, and a professor of cardiology at the University of California, Los Angeles.
Other studies of the effects of vitamin D on blood pressure have been inconsistent and most studies have not included sufficient numbers of black men and women or studied higher doses of vitamin D, he said.
"These findings require replication in larger studies. It is also not clear that these findings would apply to other race and ethnic groups," Fonarow said.
Holick, however, said that most people should be taking a vitamin D supplement. Endocrine Society committee guidelines recommend that adults take 1,500 to 2,000 units of vitamin D daily, he said.