Even "healthy" smokers have subtle damage to their blood vessels, an early sign of heart disease, researchers said Monday.

Their findings could help explain why smokers often cannot exercise as strenuously as non-smokers, even when their lungs have no obvious damage - because their hearts are not getting enough blood."The heart beats faster during physical exertion, requiring the coronary arteries to dilate or widen to increase the flow of oxygen-rich blood to the heart muscle to keep it pumping properly," said Dr. Johannes Czernin of the University of California Los Angeles.

His team found that the arteries of smokers don't dilate properly, which can raise their blood pressure and increase the risk of a heart attack or stroke.

They tested 33 healthy male and female volunteers who had no symptoms of chest pain, shortness of breath or blocked arteries. None had diabetes, high blood pressure, or genetic forms of high blood cholesterol, and just two of the smokers had high cholesterol.

Sixteen of them were long-term smokers and 17 were nonsmokers.

They stressed the volunteers by plunging their hands into cold water and used various tests to measure blood flow.

Writing in the journal Circulation, published by the American Heart Association, Czernin's team said the smokers averaged 14 percent less blood flow to the heart.

"The longer you smoke, the more abnormal the blood flow," Czernin said. "That's something that needs more study. Now we want to know how long it takes after a person quits smoking before normal blood vessel function is regained."

They said they did not yet understand how smoking interferes with the arteries.

But cigarette smoke is known to contain carbon monoxide, a deadly gas that can damage the endothelial cells that line the blood vessels.

Smoking also increases production of angiotensin II, which reduces the activity of nitric oxide, a natural body chemical that tells blood vessels to expand and which is key to functions ranging from control of blood pressure to erections.