Does Vitamin C Cause Thickening of the Arteries?

Balz Frei, Ph.D.
Director & Endowed Chair
Linus Pauling Institute
Professor of Biochemistry & Biophysics

A widely-publicized study reported on March 2, 2000, by Dr. James Dwyer and colleagues at an American Heart Association meeting in San Diego suggested that vitamin C supplements may actually increase thickening of the arteries, or atherosclerosis. Although the researchers themselves called their findings "a surprise" and cautioned that more experiments are needed to find out if vitamin C supplements are harmful, the study was released to the news media, apparently without a discussion of its limitations or putting it into the proper context of the wealth of other research demonstrating the health benefits of vitamin C. It was irresponsible to release this unconfirmed study without appropriate background information, causing unnecessary and unjustified confusion and fear among the public.

Dr. Dwyer reported that subjects taking 500 milligram supplements of vitamin C daily for at least a year had a rate of thickening of the carotid artery wall 2.5 times greater than subjects who did not take supplements. However, these results directly conflict with a much larger and better controlled study published in 1995 in the American Heart Association journal Circulation, which found significantly reduced carotid artery wall thickness in people over 55 years old who consumed daily amounts of vitamin C greater than about 1,000 mg compared to those consuming less than 88 mg. Interestingly, this study used the same study design and methods to measure artery wall thickness as the study by Dwyer, but was based on over 11,000 subjects and had undergone rigorous peer-review before publication. Dwyer's study was based on 573 subjects and was presented as an abstract without prior peer-review.

If Dwyer's results were indeed true, then people who take vitamin C supplements should die of heart attacks and strokes at a much greater rate than non-supplement users. However, there is no scientific evidence to support this concept. Many epidemiological studies and some clinical trials have suggested that dietary intake or supplementation of vitamin C is associated with a reduced risk of heart disease and stroke. For example, a large epidemiological study published in 1992 showed a risk reduction for heart disease of 45% in men and 25% in women consuming greater than 50 mg of vitamin C daily from the diet plus regular supplements, corresponding to a total vitamin C intake of about 300 mg per day. Although this study indicated that vitamin C supplements provide cardiovascular benefits above and beyond the vitamin C obtained from the diet, two other large epidemiological studies published in 1996 found no effect on heart disease risk in people who take regular vitamin C supplements. Importantly, not a single epidemiological study or clinical trial has found an increased risk of heart attacks or strokes in people taking vitamin C supplements.

Over twenty clincial studies since 1996, published primarily in Circulation, have consistently found beneficial effects of vitamin C—administered either orally or by intra-arterial infusion—on the relaxation of arteries, or vasodilation. Impaired vasodilation is an important risk factor for angina, heart attacks, and strokes. Vasodilation in patients with heart disease is significantly improved following daily supplementation with 500 mg of vitamin C for thirty days and is comparable to vasodilation seen in healthy people. Beneficial effects of vitamin C supplements leading to normalization of vasodilation have also been observed in patients with angina, heart failure, high cholesterol levels, hypertension, diabetes, high homocysteine levels, and in smokers. Additionally, a recent study published in Lancet demonstrated that 500 mg of vitamin C given daily for thirty days lowers blood pressure in patients with mild to moderate hypertension. High blood pressure is a major risk factor for heart disease and strokes. The salubrious effects of vitamin C were comparable to those derived from conventional anti-hypertensive medications.

There are other important limitations of the study reported by Dwyer and colleagues. Because this is an epidemiological study, the observed associations between vitamin C intake and carotid atheorsclerosis do not prove a causal relationship and may be confounded by unmeasured risk factors or imperfect statistical adjustment of the data. In other words, there may have been differences in diet or lifestyle that better explain the results of the study. Furthermore, the vitamin C intakes were estimated from 24-hour recall and dietary questionnaires, relatively imprecise methods to assess vitamin C status in the body. Although blood levels of vitamin C were reported—a more reliable measure of the body's vitamin C status—they were not compared to artery wall thickness.

People taking vitamin C supplements should continue to do so, as the known health benefits of vitamin C far outweigh alleged, unconfirmed risks. There is no scientific evidence that vitamin C supplements increase the risk of heart attacks or strokes. Daily vitamin C supplements of 500 mg have been shown to normalize vasodilation and lower blood pressure, thus mitigating two major cardiovascular risk factors.

For additional information on vitamin C, see the Linus Pauling Institute's Micronutrient Information Center.

Last updated May, 2000