Over a twenty-year period, Tetsuji Yokoyama of the Tokyo Medical and Dental University and colleagues measured vitamin C content in the blood of 880 Japanese men and 1,241 women. They compared the results with the incidence of death or disability from strokes during this same period among their study population.

People with the highest levels of vitamin C in their blood were the least likely to have a stroke -- a blood vessel blockage leading to tissue death in the brain. Those with elevated levels of vitamin C also ate the most fruits and vegetables. The association was especially strong for vegetables, they explain in the journal Stroke1.

Many vegetables are rich in vitamin C and other antioxidants. These improve vascular elasticity and reduce the stickiness of blood platelets, making them less prone to forming clots. And antioxidants clear out what Duke University cardiologist, Jonathan Stamler, calls the "biological rust" from inside blood vessels.

Cholesterol in the blood can be oxidized, just as iron is oxidized into rust. Oxidized cholesterol is thought to make the platelets stickier. Antioxidants control the plaques of sticky platelets that can build up inside blood vessels, narrowing them.

It is not yet clear whether vitamin C is solely responsible for the benefits seen in this and some other studies. It may simply be a marker for healthy behaviours or the ingestion of other beneficial nutrients in vegetables, like folate and B vitamins. Vitamin C concentrations in the blood may be lower, for instance, in sedentary, heavy smokers and drinkers.

But Yokoyama's team feels that none of the individual effects of other mechanisms explains the reduction they saw in risk of stroke and vessel blockage.

Previous research has shown that a big dose of vitamin C can lower blood pressure for a short time immediately afterwards, just as a hamburger raises it for a little while.

But the results of long-term studies of the relationship between vitamin C and stroke or heart disease have been inconsistent, warns Catherine Gale, an epidemiologist at the MRC Environmental Epidemiology Unit of Southhampton General Hospital.

With colleague Christopher Martyn, Gale also found that elderly people with the highest vitamin C intake -- the equivalent of only an orange a day -- had the least risk of dying from stroke2. But other, recent studies have found no association, and attempts to reverse coronary artery disease with vitamin C supplements have been surprisingly disappointing, Martyn says.

We will just have to wait, Gale says, for the results from controlled trials now underway, to get a better idea of whether or not vitamin C protects against strokes.