Nearly 70 years after researchers first suggested vitamin C supplementation lowers blood pressure, a first-of-its-kind review of 29 trials involving 1,400 participants has drawn the same conclusion.
However, its authors stop short of recommending C for hypertension. We asked lead investigator Edgar R. Miller, MD, a medical professor at Johns Hopkins University, about the study, published in the April American Journal of Clinical Nutrition.
Natural Foods Merchandiser: Why do this study?
Edgar R. Miller, MD: Antihypertensive effects of vitamin C were hypothesized as early as 1946. However, the trials have been mixed, and many of them have been small. We wanted to pull all the trials together for a systematic review.
NFM: Dr. Miller, you started with 2,163 references. How did you narrow them to 29?
ERM: They had to use randomized controlled trial design, use oral vitamin C and have a two-week duration minimum. Excluded were trials in which vitamin C was included as part of a calorie-containing beverage. Median dose was 500 mg/day. Median duration was eight weeks. Trial sizes ranged from 10 to 120 participants. About half the participants had hypertension.
NFM: What did you find?
ERM: Vitamin C supplementation lowered blood pressure significantly (-3.84 mm Hg for systolic blood pressure and -1.48 mm Hg for diastolic blood pressure, within a matter of days). The higher the blood pressure, the greater the change (as much as -4.85 mm Hg for systolic). That is a fairly robust finding.
NFM: How does this compare to pharmaceutical drugs?
ERM: With an ACE inhibitor or a diuretic, we expect a reduction of around 10 mm Hg for systolic blood pressure. There are downsides. Diuretics increase potassium excretion, which might have adverse effects. Other drugs have side effects such as feeling light-headed, and require monitoring of kidney function.
NFM: How does C lower blood pressure?
ERM: It may essentially relax the blood vessels.
NFM: Are there downsides to C?
ERM: Yes. High doses of C can put you at increased risk of kidney stones. Too high a dose can also cause oxidative stress. The average intake in the United States is 70 to 90 mg per day and the recommended minimum intake is 90 mg for men and 75 mg for women, so 500 mg represents a pretty high dose. The highest dose advised by the Institute of Medicine is 2 grams per day.
NFM: Should people take C to prevent high blood pressure?
ERM: That is a difficult question. Before we would recommend it, we really need additional trials with large sample sizes where the clinical endpoints are evident.
NFM: Could they take it instead of, or along with, blood pressure medication?
ERM: I would not advocate the “instead of” approach. Whether you want to take supplemental C is up to you. It might help, but, as a physician, I would not advocate it.
NFM: Why not?
ERM: These trials on average were only eight weeks long, so we don’t know if this is an acute effect and the apparent benefit on blood pressure washes away as you reach a new homeostasis. Also, there have been a number of large, long-term trials which have looked at endpoints, like stroke and myocardial infarction and death (all associated with high blood pressure) and none of them have shown a benefit from vitamin C supplementation.
NFM: Could you just drink more orange juice?
ERM: The food approach is always the best approach. However, to get to 500 mg of vitamin C from orange juice, you need to drink 6 cups. That’s a lot of calories.
NFM: So what do you advise to control blood pressure?
ERM: Follow a balanced diet such as the D.A.S.H (Dietary Approaches to Stop Hypertension) regimen, high in fruits and veggies and low-fat dairy, and low in saturated fats. Also, move more. You can’t do it with just one supplement.
NFM: What’s next?
ERM: We are proposing a study looking at the effects of C on uric acid, to see if it may prevent gout.