In this review study, scientists analyzed data from 29 different clinical trials and about 1,400 adults aged 22 to 74.
The median dose of supplemental vitamin C taken by the study participants was 500 milligrams a day over a median of an eight-week period. In people who had high blood pressure, systolic blood pressure (the top number in a reading) dropped by nearly 5 points, while diastolic pressure (the bottom number) dropped by about 1.7 points.
"Our research suggests a modest blood pressure lowering effect with vitamin C supplementation," researcher Edgar "Pete" R. Miller III, MD, PhD, says in a news release.
"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 Miller, an associate professor in the division of general internal medicine at the Johns Hopkins School of Medicine in Baltimore.
Other studies have suggested a possible link between people with higher intakes of vitamin C from food or supplements and lower blood pressure. But the evidence of this effect from clinical trials has been mixed.
Nearly 1 in 3 American adults has high blood pressure, which increases their risk for heart disease and stroke. The condition is typically treated by dietary changes, exercise, not smoking, losing weight, and medication.
Modest Blood Pressure Changes
Across all the studies -- some included healthy adults and others had people with high blood pressure -- taking vitamin C supplements at a dose of about 500 milligrams a day appeared to reduce systolic blood pressure by nearly 4 points and diastolic pressure by about 1.5 points in about two months.
Adult women are advised to get 75 milligrams of vitamin C daily, and it's recommended that adult men get 90 milligrams of the nutrient daily. The Institute of Medicine’s daily maximum limit that is considered safe is 2,000 milligrams of vitamin C for adult males and adult females.
But the scientists say that many of the studies were small, and in some of them, vitamin C was taken in addition to blood pressure medication. They are not yet suggesting that the supplement is a natural alternative to drugs that lower pressure.
It's unclear whether taking vitamin C for more than two months will achieve long-lasting changes in blood pressure.
"Although our review found only a moderate impact on blood pressure, if the entire U.S. population lowered blood pressure by 3 [points], there would be a lot fewer strokes," Miller says.