Vitamin C, Blood Pressure and the American Diet

G. Block
University of California Berkeley, California USA

Several studies have found significant inverse relationships between blood levels of vitamin C and blood pressure. However, in such studies it has been difficult to determine whether the effect was produced by vitamin C or some other nutrient related to vitamin C. We designed a study to investigate the effect of low vs. normal levels of vitamin C intake. We carefully controlled the foods that participants were eating throughout a four-month study so that we could distinguish the effect of vitamin C from that of other nutrients in the diet.

We used 68 men, ages 30 to 59 years, in the study. These were healthy non-smoking men of reasonably normal weight and no history of high blood pressure. For one month, the men received the recommended dietary allowance of vitamin C (60 mg) each day. Then, for one month, their daily dose of vitamin C was reduced to 9 mg (depletion). During the next month, they received 117 mg of vitamin C per day (repletion). Depletion and repletion were then repeated.

We began measuring blood pressure during the ninth week of the study, after the first depletion session had ended. Blood pressure was measured at regular intervals for the rest of the study.

When the men received 60 mg of vitamin C per day, the level of vitamin C in their blood actually decreased. This reduction continued during the first depletion period. During repletion, when the men received higher levels of vitamin C, blood levels of vitamin C increased.

The first blood pressure measurements correlated with age, meaning that blood pressure was higher with increasing age. There was also a slight correlation between blood pressure and body weight. The most remarkable correlation that we found, however, was a highly significant inverse relationship between blood pressure and blood levels of vitamin C level at the end of depletion. This inverse relationship meant that blood pressure increased as blood levels of vitamin C decreased. This correlation was much stronger than previously reported correlations for other risk factors affecting blood pressure.
When we grouped the men into four groups based on their blood levels of vitamin C after depletion, we found some highly significant differences. Blood level of vitamin C after depletion profoundly affected the level of diastolic blood pressure (the lower number of a blood pressure reading) throughout the rest of the study. The effect of vitamin C on systolic blood pressure (the higher number of a blood pressure reading) was not as strong as that observed for diastolic blood pressure, but it was still significant. The group with the highest blood levels of vitamin C had the lowest systolic blood pressure for rest of the study.

We considered that vitamin C depletion may have changed the level of some other blood pressure regulating compound. To find out if this was the case, we measured blood levels of compounds known to affect blood pressure, such as vitamin A, vitamin E, cholesterol and others. Blood pressure was not related to blood levels of any of the compounds.
We then considered that some other factor, such as age or weight, could be influencing our results. When we re-analyzed our data taking these factors into account, we found that the relationship between blood pressure and blood level of vitamin C was maintained. This meant that the effect of the vitamin C was not caused by age or weight. In addition, food composition (for example, fiber content, calcium, sodium etc.) did not influence the relationship.

Our study showed that the lower the blood vitamin C levels dropped during depletion, the higher blood pressure would be for as much as two months later. This suggests that the effect on blood pressure was related not to the blood levels of vitamin C but on the levels of vitamin C in the body tissues. That is, the blood levels of vitamin C would fall more slowly if tissue stores of vitamin C were large, because the tissue stores replenish the blood levels.

Vitamin C may exert its effect on blood pressure in a number of ways. It may block free radicals and other substances that prevent relaxation of small blood vessels. In addition, it may increase the production of substances that enhance small blood vessel relaxation. It may act directly on the blood vessels themselves. Vitamin C may also influence blood pressure through effects on the adrenal and pituitary glands.
The blood levels of vitamin C measured in this study were similar to those occurring in everyday life. Even during depletion, approximately one-third of the men had blood levels that were higher than levels frequently found in the average population. During repletion, the blood levels were similar to those measured in a large national study.
Many people have very low blood levels of vitamin C because they don’t eat enough fruits and vegetables and don’t take vitamin C supplements. Minority populations and men seem to have particularly low levels, but the problem is by no means limited to those groups. Women usually have higher blood levels of vitamin C than men, but this is thought to be related to body size. Despite that, many women also have blood levels that are extremely low.
In conclusion, we found that the level to which blood levels of vitamin C fell during the first episode of de

pletion was the most significant factor in predicting blood pressure 4, 8, and 12 weeks later. We feel that this reflects the amount of vitamin C stored in the body tissues. Thus, inadequate vitamin C stores in body tissue may be a factor in certain types of high blood pressure disease.

Funding for this research was provided by the National Cancer Institute.