Following is the Linus Pauling Institute's response to the recent article on vitamin C and atherosclerosis:
Study links vitamin C pills with
faster clogging of the arteries?
Another unconfirmed study causing unnecessary confusion and fear among the public.
A study reported on March 2, 2000, by Dr. James Dwyer at an American Heart Association meeting in San Diego allegedly raised the possibility that taking vitamin C supplements may speed up clogging of the arteries, or atherosclerosis. Although the researchers themselves called their findings "a surprise" and cautioned that more experiments are needed to investigate whether vitamin C supplements may be harmful, the study was released to the media without discussing its limitations nor putting it in the proper context of the hundreds of existing studies demonstrating the health benefits of vitamin C. Both the researchers and the American Heart Association acted irresponsibly by releasing this unconfirmed study without appropriate background information, causing unnecessary confusion and fear among the public.
Dr. Dwyer reported that subjects taking 500 milligrams of vitamin C daily for at least a year had a 2.5 times greater rate of thickening of the carotid artery wall than did subjects who did not take supplements. These results are in direct conflict with a study published in 1995 in the American Heart Association journal Circulation, which found a significant reduction in carotid artery wall thickness in people over 55 years old who consumed amounts of vitamin C greater than 982 mg per day compared to those consuming less than 88 mg per day.
If the results by Dwyer and colleagues were true, 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 in support of this notion. Many epidemiological studies and some clinical trials have indicated that dietary intake of or supplementation with vitamin C is associated with a reduction in the incidence of chronic disease morbidity and mortality, including cardiovascular diseases. Numerous epidemiological studies have shown a significantly decreased heart attack or stroke risk with increased vitamin C intake.
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. However, not a single epidemiological study nor clinical trial has found an increased risk of heart attacks or strokes in people taking vitamin C supplements.
Over twenty clinical 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 heart attacks and strokes. Vasodilation in patients with heart disease is significantly improved following supplementation with 500 mg of vitamin C per day for thirty days, and is comparable to vasodilation seen in healthy people. Beneficial effects of vitamin C supplements leading to normalization of vasodilation were also observed in patients with angina, heart failure, high cholesterol levels, hypertension, diabetes, high homocysteine levels and in smokers. In addition, a recent study in Lancet has demonstrated that 500 mg of vitamin C per day given for thirty days lowers blood pressure in moderately hypertensive patients. High blood pressure is a major risk factor for heart disease.
Several limitations of the study reported by Dwyer and colleagues need to be pointed out: First, this was an oral presentation of an abstract, meaning that the study has not been published in the scientific literature after undergoing rigorous peer review. The measurement of carotid artery wall thickness by ultrasound poses significant technical problems and is notoriously difficult. The differences observed are exceedingly small, and control measurements and strict blinding of the researchers evaluating the data are pivotal.
Second, because this is an epidemiological study, the observed associations between vitamin C intake and carotid atherosclerosis do not prove cause-effect relationships, and may reflect differences in diet or lifestyle. There also can be significant confounding by unmeasured risk factors or imperfect statistical adjustments of the data. The vitamin C intakes were estimated, but no actual measurements of vitamin C in the blood of the subjects were made.
Third, it is known that in healthy people cells and tissues are saturated at an intake of 100 to 200 mg of vitamin C per day. Therefore, it is difficult to rationalize any effects of vitamin C above these intake levels, as tissue levels will not be altered.
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. Vitamin C supplements of 500 mg per day have been shown to normalize vasodilation and lower blood pressure, two major cardiovascular risk factors.
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