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.
Systolic blood pressure is pressure in the arteries when the heart beats. Diastolic blood pressure, the bottom number, is pressure in the arteries between heart beats, the study authors pointed out.
"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.
"There is no downside to increasing vitamin D intake. It can reduce risk factors for cardiovascular disease as well as reducing the risk for cardiovascular disease," Holick said.
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