The amino acid is taurine, which is plentiful in all species of fish, says Dr. David J. Bouchier-Hayes, a professor of surgery at Beaumont Hospital in Dublin, Ireland. He is lead author of the study, which appears in tomorrow's issue of Circulation.
When young smokers were given taurine supplements equal to that found in an average serving of fish, their blood vessels relaxed as much as those of nonsmokers, he says.
Boucher-Hayes is quick to add that smokers shouldn't assume they can protect themselves against heart disease and stroke by eating fish or taking taurine supplements.
"The responsible thing to do in relation to smoking is to give up smoking," he says. "Cigarette smoking has multiple effects on the blood vessels, so it is unwise to say you can continue to smoke if you eat fish."
However, the study carries a lesson for some nonsmokers who are at higher risk of cardiovascular disease, such as diabetics, Bouchier-Hayes says. A diet with lots of fish -- or taurine supplements -- can help prevent a condition that is one of the earliest signs of trouble.
The condition is endothelial dysfunction, in which the normally flexible arteries become rigid and narrow.
"A normal artery will dilate by about 10 percent in response to increased flow, such as that caused by physical activity," Bouchier-Hayes explains. "That dilation is controlled by the inner lining of the blood vessels, the endothelium. It senses the increased flow, and in response increases its release of nitric oxide, which seeps through the blood vessel wall and induces relaxation in the muscles around the blood vessel, increasing their diameter."
That response doesn't occur in smokers, which is one reason why they are at high risk of heart attack and stroke.
In the study, Bouchier-Hayes and his colleagues gave 15 young smokers taurine supplements and compared the function of their blood vessels with that of 15 nonsmokers. In tests before they took the supplements, the smokers' blood vessels did not enlarge at all, while those of the nonsmokers did. When the smokers took the taurine supplements, the amount of blood-vessel enlargement was the same for both groups.
Vitamin C supplements also acted to enlarge the blood vessels, but "the effect of taurine was much greater," says Fiona M. Fennessey, a researcher who participated in the study and now is a clinical professor in radiology at Brigham and Women's Hospital in Boston.
Fennessey advocates a fish-rich diet. "Instead of popping pills all the time, it's preferable to increase your intake of fish," she says.
Dr. Sidney Smith, professor of medicine at the University of North Carolina and past president of the American Heart Association, says the new study "adds to the growing body of evidence that fish oils can have a beneficial effect on blood vessels."
"And it emphasizes more than ever the importance of not smoking or of stopping smoking. Given the choice of eating a lot of fish and stopping smoking, people would be well advised to stop smoking," Smith says.
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