Taking large doses of vitamin C may
moderately reduce blood pressure, according to an analysis of years of research
by Johns Hopkins scientists. But the researchers stopped short of suggesting
people load up on supplements.
“Our
research suggests a modest blood pressure lowering effect with vitamin C supplementation,
but before we can recommend supplements as a treatment for high blood pressure,
we really need more research to understand the implications of taking them,”
says Edgar “Pete” R. Miller III, M.D., Ph.D., an associate professor in the
division of general internal medicine at the Johns Hopkins University School of
Medicine and leader of the study published in the American Journal of
Clinical Nutrition.
Roughly
30 percent of adults in the United States have high blood pressure, or
hypertension, an important risk factor for heart disease and stroke. Successful
treatment may include drugs, exercise, weight loss, and dietary changes such as
reducing salt intake. Some experts believe that large amounts of vitamin C, an
essential micronutrient found primarily in fruits and vegetables, could lower
pressure as well, but randomized, controlled dietary intervention studies — the
gold standard of nutrition research — have produced mixed results.
Miller
and his colleagues reviewed and analyzed data from 29 randomized, controlled,
previously published clinical trials that reported systolic and/or diastolic
blood pressure values and also compared vitamin C intake to a placebo. What
they found is that taking an average of 500 milligrams of vitamin C daily —
about five times the recommended daily requirement — reduced blood pressure by
3.84 millimeters of mercury in the short term. Among those diagnosed with
hypertension, the drop was nearly 5 millimeters of mercury.
By
comparison, Miller says, patients who take blood pressure medication such as
ACE inhibitors or diuretics (so-called “water pills”) can expect a roughly 10
millimeter of mercury reduction in blood pressure.
Five
hundred milligrams of vitamin C is the amount in about six cups of orange
juice. The recommended daily intake of vitamin C for adults is 90 milligrams.
“Although
our review found only a moderate impact on blood pressure, if the entire U.S.
population lowered blood pressure by 3 milliliters of mercury, there would be a
lot fewer strokes,” Miller says. Miller cautions, however, that none of the
studies his team reviewed show that vitamin C directly prevents or reduces
rates of cardiovascular disease, including stroke.
Scientists
have focused on vitamin C’s potential role in blood pressure reduction because
of the nutrient’s biological and physiological effects. For example, vitamin C
may act as a diuretic, causing the kidneys to remove more sodium and water from
the body, which helps to relax the blood vessel walls, thereby lowering blood
pressure.
Nutritional
supplements are a $28 billion-a-year industry, and marketing claims, newspaper
stories and testimonials often make them hard to resist, Miller says. People
often view supplements as a “natural alternative” and preferable to drugs for
high blood
pressure or other ailments, he adds, despite mounting evidence that
many supplements don’t work and in some cases may cause harm.
“People
love to take vitamins regardless of the evidence or lack of it,” Miller says.
“We’re trying to raise the bar and provide evidence-based guidance about
whether supplements help
or actually do harm.” With respect to vitamin C, he says, the jury is still
out.
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