April 29, 2011

Reduced vitamin D levels could help explain high blood pressure in African Americans

An article published online on April 21, 2011 in the Journal of General Internal Medicine provides one explanation for the greater incidence of hypertension that occurs among African Americans in comparison with Americans of European descent.

Ken Fiscella, MD of the University of Rochester Medical Center and colleagues compared data from 5,156 Caucasian and 1,984 adult African American participants in the National Health and Nutrition Examination Survey (NHANES) 2001-2006. Blood pressure was measured upon enrollment and serum samples were analyzed for 25-hydroxyvitamin D levels.

An average 4.0 mmHg adjusted increase in systolic blood pressure was observed in African Americans in comparison with Caucasian subjects. While 25 percent of the Caucasian population had vitamin D levels that were among the highest one-fifth of participants, only 2 percent of African Americans had levels this high. However, when the lowest fifth was analyzed, 61 percent of African Americans had levels that were in this range, compared to 11 percent of Caucasians.

“Our study confirms that vitamin D represents one piece of the complex puzzle of race and blood pressure,” commented Dr Fiscella, who is a professor of Family Medicine at the University of Rochester Medical Center. “And, since black-white differences in blood pressure represent thousands of excess deaths due to heart disease and stroke among blacks, we believe that simple interventions such as taking vitamin D supplements might have a positive impact on racial disparities.”

“It is likely that other factors beyond vitamin D, such as psychological stress, medication adherence, and discrimination could contribute to this disparity,” he wrote. “Further study using more refined measures of skin color is needed to tease apart the complex relationship between skin type, stress, vitamin D, and hypertension.”

"In cross-sectional analyses, 25-hydroxyvitamin D explains one quarter of the Black-White disparity in systolic blood pressure," the authors write. "Randomized controlled trials are required to determine whether vitamin D supplementation could reduce racial disparity in blood pressure."

High blood pressure

The force of blood pressure is measured in two stages: when the heart is contracting (systolic pressure) and relaxing (diastolic pressure). Blood pressure is always expressed in pressure units of millimeters of mercury (mm Hg), and written as systolic over diastolic pressure. For example, a blood pressure reading of 120/80 mm Hg would mean a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg.

Vitamin E is an antioxidant that detoxifies (reduces) strong oxidants in the body. It stabilizes cell membranes and regulates oxidation reactions, as well as protects polyunsaturated fatty acids and vitamin A. Meta-analyses have suggested that vitamin E may be particularly beneficial for patients who have high blood pressure or cardiovascular problems (Taber M 2006). Small studies have shown that vitamin E, even when taken at fairly low doses, increases nitric oxide synthase activity, which leads to vessel dilation and lowered blood pressure. A larger study reported a significant reduction in systolic blood pressure when subjects took 600 mg/day of vitamin E (Galley HF et al 1997). A study of 895 participants reported that vitamin E supplementation might have an antihypertensive effect even among those who get an adequate supply from their diets (Mayer-Davis EJ et al 2002).

Vitamin C, also known as ascorbic acid, is an antioxidant that protects other biochemicals from oxidation by being oxidized itself. A small, well-controlled study of 39 participants showed that treatment with vitamin C significantly lowered blood pressure after 30 days, while placebo had no effect (Duffy SJ et al 1999). Although specific mechanisms have not been identified for vitamin C, it may be that it can help promote vessel dilation. As an antioxidant, it may also enhance the synthesis or prevent the destruction of nitric oxide, which directly helps blood vessels dilate and lower blood pressure (Khosh F et al 2001).

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