Ascorbic acid, blood pressure, and the American diet

A large controlled study supported by the NIH, the DASH study (Dietary Approaches to Stop Hypertension), demonstrated that a diet rich in fruits and vegetables can reduce blood pressure in persons with moderate elevation in blood pressure (BP). Fruits and vegetables are important sources of antioxidants such as vitamin C and carotenoids. We conducted a study in which we fed people a diet deficient in vitamin C for 30 days, followed for another 30 days by a diet adequate in vitamin C. Their blood levels of vitamin C and blood pressure (BP) were tracked. Plasma vitamin C was inversely related to diastolic blood pressure one month later (correlation = -0.48, P < 0.0001). Persons whose blood levels of vitamin C went down the furthest on depletion had the highest blood pressure one month later. Persons in the lowest one-fourth of the plasma vitamin C distribution had diastolic BP 7 mm Hg higher than did those in the upper one-fourth of the plasma ascorbic acid distribution. Multivariate control for age, body mass index, other plasma antioxidants, and dietary energy, calcium, fiber, sodium, and potassium did not reduce the plasma vitamin C effect. We believe that this indicates that the tissue stores of vitamin C may be important in regulating blood pressure. It is often thought that Americans' intake of vitamin C is ample, since the average intake is about 100 mg/day. However, this average level obscures the fact that substantial numbers of people actually have habitually low intake levels and low blood levels. African Americans tend to have low blood levels of vitamin C as well as the highest risk of hypertension. Low intake of antioxidant-rich fruits and vegetables may be one of the causes of hypertension.