Taking vitamin C supplements may not prevent heart disease and could even worsen it, especially among smokers, according to new research that undermines a practice favored by millions of Americans.
In the study of 573 men and women in Los Angeles, those who consumed the most vitamin C from pills over a year and a half had more evidence of possible future atherosclerosis, or hardening of the arteries, compared to people who did not take vitamin C pills.
To gauge the vitamin's effect, researchers performed ultrasound measurements of the subjects' carotid arteries, which are in the neck. The method is an accepted substitute for probing coronary arteries directly.
Consumers of at least 500 milligrams of supplemental vitamin C daily developed an inner artery wall layer 2.5 times thicker than that of people who did not take supplements. Among smokers, the supplement takers' inner artery layer was five times thicker. Such thickening precedes atherosclerosis, physicians say.
Presented Thursday at an American Heart Assn. conference in La Jolla, the study has stirred controversy over the benefits and possible harms of large doses of vitamin C supplements in otherwise healthy people.
The lead USC researcher, epidemiologist James Dwyer of the Keck School of Medicine, said that healthy people who pop high-dose vitamin C pills are "experimenting" on themselves. "I would hope this would give them pause," he said.
But others criticized the work as incomplete. "The weight of all published scientific evidence suggests that vitamin C is beneficial not only for the heart but also protects against cataracts and some types of cancers," said Annette Dickinson of the Council for Responsible Nutrition.
The supplement advocacy group, which receives support from the vitamin and mineral industry, attacked a USC research summary as irresponsible and alarming.
The findings occur against a background of conflicting evidence and heated debate. On the one hand, lab studies have found that vitamin C can help block the oxidation of cholesterol, which contributes to atherosclerosis. And epidemiological studies of people consuming a plant-rich diet high in vitamin C have shown that they have a lower heart attack rate than those with a vitamin-poor diet.
On the other hand, a few studies of people taking vitamin C supplements have found that they had a slightly increased heart attack rate compared to similar people who did not take supplements. To many researchers, that suggested the benefits of a plant-rich diet come from more than vitamin C alone.
Moreover, those experts say, the widely popular practice of taking vitamin C pills to improve heart health is based largely on a mixture of folk wisdom and over-extrapolated studies. The famed chemist Linus Pauling began promoting the idea two decades ago.
The heart association does not recommend vitamin supplements, saying the evidence for doing so is insufficient and that there is no substitute for a balanced and nutritious diet. At the same time, it is universally accepted that a minimum level of vitamin C is necessary to maintain health.
The group's president, Dr. Lynn Smaha, said the USC research prompts some worry because it hints that excess vitamin C might contribute to the disease that many people are taking it to prevent. He emphasized that further research is necessary to see if the finding is real.
He cited the rise and fall of beta carotene supplements as a possible object lesson: Many researchers and health promoters went out on a limb in the 1980s and said the nutrient would help prevent lung cancer in smokers, but then large epidemiological studies found that it promoted the disease instead.
The subjects of the USC study, said to be the first of its kind, were 40- to 60-year-old employees of a Los Angeles company. The researchers compared the 147 who regularly took a vitamin C supplement to control subjects, including people who took a multivitamin supplement or none.
Ultrasound measurements at the beginning of the study and again 18 months later found that 52 people who took the highest amount of extra vitamin C had artery wall growth of 30 microns, compared to 12 microns for the controls. For the smokers who took the highest vitamin C levels, the carotid wall expanded by more than 50 microns, Dwyer said. A micron is about one-one-hundredth the width of a human hair.
The finding goes against advice to smokers to take extra vitamin C in the form of supplements, both to protect against cancer and to compensate for how smoking seems to drain some vitamin out of the body.
The 500 milligrams or more taken by the high-level supplementers was nearly 10 times higher the old Recommended Dietary Allowances--a strict standard that the government is revising in favor of a more liberal approach.
Balz Frei, a biochemist at Oregon State University and head of the Linus Pauling Institute, said among the study's flaws was it did not establish that the vitamin C caused the arterial thickening. "I wouldn't make too big a deal out of it," he said of the work.