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.

This 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.

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 1/2 times greater rate of thickening than did those who avoided supplements. Among smokers, the rate was five times greater.

Clogged arteries - what doctors call atherosclerosis - are the major underlying cause of heart attacks and strokes.

In general, experts recommend that people get their vitamins and other nutrients from fruits, vegetables, whole grains and nuts.