Regular intake of vitamin C pills over time may quicken the thickening of artery walls, a condition known as atherosclerosis, according to USC Keck School of Medicine researchers.

Atherosclerosis is a disease process that, in well-nourished populations, underlies most forms of heart disease – the No. 1 cause of death in the United States.

James H. Dwyer, professor of preventive medicine, presented the findings at the American Heart Association’s 40th annual Conference on Cardiovascular Disease Epidemiology and Prevention in San Diego on March 2.

Dwyer’s research group in the Institute for Health Promotion and Disease Prevention Research focuses on epidemiological studies on antioxidants and cardiovascular disease.

Dwyer and colleagues followed 573 apparently healthy men and women between 40 and 60 years old in Los Angeles for the study. About 30 percent of the participants used vitamin supplements regularly, ranging from about 30 mg to 1,000 mg of vitamin C or more each day. The vitamin consumers in the study had the same general health characteristics and health habits (exercise and diet) as those who did not use the vitamin pills.

The researchers used non-invasive ultrasound equipment to check on the progression of atherosclerosis in the study subjects’ carotid arteries, which are on either side of the neck. They measured what is called intima-media thickness (IMT), the thickness of the carotid wall – one of the strongest available predictors of future cardiovascular disease events such as heart attack or stroke. The scientists recorded each study participant’s diet (noting vitamin consumption from food) and use of supplements before taking the measurements, and then measured the IMT of each participant again 18 months later.

The scientists found that participants who consumed the most vitamin C from supplements had the greatest increase in atherosclerosis, particularly those who were smokers.

People taking about 500 mg of vitamin C in supplements each day for a year showed a progression in carotid wall thickness that was 2.5 times greater than those who did not take a vitamin C supplement, whether they were men or women. And the higher the dose, Dwyer said, the greater the atherosclerosis progression.

Among smokers, consuming 500 mg a day of vitamin C in supplements was associated with a fivefold increase in the rate of atherosclerosis progression. That might seem counterintuitive, because antioxidants such as vitamin C may be thought to help a smoker fight the negative effects of smoke. But previous studies with high doses of beta carotene, for example, showed that the vitamin A-related antioxidant actually contributed to disease (lung cancer) in smokers. The researchers found no evidence that vitamin C from food caused heightened atherosclerosis progression. Whether in smokers or nonsmokers, vitamin C pills should be used with caution, the researcher said.

“The evidence suggesting that higher intake of fruits and vegetables – that is, a diet high in vitamin C – is protective against cardiovascular disease does not imply that taking high doses of vitamin C from supplements will increase that protection,” Dwyer said.

When someone consumes just one component of healthy food – in this case, vitamin C – and consumes a lot of it, it may have unforeseen consequences, he said. Researchers are just now conducting randomized trials in the United States and Europe on the effects of vitamin supplement consumption, so most long-term effects will not be known at least for another five to 10 years.

Dwyer noted that further studies are needed to confirm his group’s findings. But he suggests that anyone interested in vascular health and vitamins should look to the American Heart Association to learn about the group’s dietary recommendations. The AHA does not recommend vitamin supplements, but does have other recommendations on how people can protect themselves from heart attack and stroke through other lifestyle changes.

Dwyer also presented findings at the AHA meeting about a study his group conducted on lutein and vascular health.

The researchers found that lutein – a substance particularly found in dark green leafy vegetables such as spinach, kale and collard greens – was associated with a decreased progression of thickening of the artery walls in study subjects.

“We found that people with higher levels of lutein in the blood have slower progression of atherosclerosis,” Dwyer said.

Increasing the lutein in the diet of test mice by only 0.2 percent reduced the extent of atherosclerosis growth dramatically, he said.

The research was funded by the National Heart, Lung and Blood Institute.