SAN DIEGO -- A new study raises the disturbing possibility that taking vitamin C pills may speed up hardening of the arteries.
Researchers called their discovery a surprise and cautioned that more experiments are needed to know for sure whether megadoses of the vitamin actually are harmful.
Still, they said the finding supports the recommendations of health organizations, which generally urge people to avoid high doses of supplements and to get their nutrients from food instead.
Many people load up on vitamin C and other nutrients on the assumption that these supplements are good for their health, even though there is little scientific evidence this is true. In theory, vitamin C and some other nutrients might protect the circulatory system and other organs by suppressing the damaging effects of oxygen.
"When you extract one component of food and give it at very high levels, you just don't know what you are doing to the system, and it may be adverse," said Dr. James H. Dwyer, an epidemiologist who directed the study. He presented the findings Thursday at a meeting in San Diego of the American Heart Association.
Dwyer and colleagues from the University of Southern California studied 573 outwardly healthy middle-aged men and women who work for an electric utility in Los Angeles. About 30 percent of them regularly took various vitamins.
The study found no clear-cut sign that getting lots of vitamin C from food or a daily multivitamin does any harm. But those taking vitamin C pills had accelerated thickening of the walls of the big arteries in their necks. In fact, the more they took, the faster the buildup.
People taking 500 milligrams of vitamin C daily for at least a year had a 2« times greater rate of thickening than did those who avoided supplements. Among smokers, the rate was five times greater.
"If a person's physician has prescribed vitamin C, it is appropriate to be taking it," Dwyer said. "But if you are a healthy person and taking them in hopes of preventing cardiovascular disease, the heart association does not recommend it. This study would suggest that recommendation is prudent."
Dr. Shiriki Kumanyika of the University of Pennsylvania said the research shows the uncertainties of picking out a single vitamin among the plethora of nutrients in a healthy diet.
"It's a challenge to sort out what it is in what people eat that makes them live longer," she said. "We have to be careful about recommending foods or nutrients, because if we are wrong, we can do harm."
In general, experts recommend that people get their vitamins and other nutrients from fruits, vegetables, whole grains and nuts.
Clogged arteries -- what doctors call atherosclerosis -- are the major underlying cause of heart attacks and strokes.
In the latest study, doctors looked for early signs of this process by twice performing ultrasound scans on the volunteers' carotid arteries, once at the study's start and again 18 months later.