Last Updated: 2003-05-14 17:03:45 -0400 (Reuters Health)

NEW YORK (Reuters Health) - Obese people who are relatively inactive may have trouble dissolving potentially deadly blood clots, but moderate exercise a few times per week appears to help restore that ability, according to new research.

U. S. investigators discovered that obese, sedentary people are less able than those of normal weight to produce and release a clot-busting substance known as tissue plasminogen activator (t-PA), the body's primary defense mechanism against the formation of blood clots.

Obese people have a higher-than-average risk of developing heart attack or stroke, both of which can be caused by blood clots.

While obese people are more likely to carry a host of conditions that help explain that trend, such as diabetes or high blood pressure and high cholesterol, study author Dr. Christopher A. DeSouza suggested that the increased risk seen in obesity may also stem from problems dissolving blood clots.

"What we just showed here is this is another system that is impaired" in obese people, he told Reuters Health.

But obese people are not doomed, DeSouza added. After spending only three months walking for around 45 minutes every day for five days each week, almost half of obese study participants began releasing more t-PA when needed.

After exercise, the ability of some obese people to release t-PA "looked very similar to their lean, age-matched counterparts," he said.

These findings provide "further evidence that exercise can be very beneficial," DeSouza, based at the University of Colorado in Boulder, noted. "All we asked these people to do was to go on a walk every day."

DeSouza and his colleagues reported their findings last week during the American Heart Association's fourth annual conference on Arteriosclerosis, Thrombosis and Vascular Biology in Washington, D.C.

During the study, the researchers measured the amount of t-PA released by the cells lining the blood vessels of 36 sedentary men, 24 of whom were obese.

Participants were then asked to spend between 40 and 45 minutes walking five times each week for three months. DeSouza explained in an interview that the men were asked to walk at a "modest" pace, during which they could easily carry on a conversation.

Before the exercise program, obese men showed a 30 percent smaller increase in the amount of t-PA their bodies released in response to a drug designed to stimulate release of the substance.

And after only three months of exercise, and despite the fact that they did not lose any weight, 10 of the obese men experienced a significant improvement in their ability to release t-PA.

These findings suggest that exercise improves the general health of arteries, DeSouza said, enabling them to release t-PA when needed. Why that is remains unclear, he said.

The researcher added that he and his colleagues have also shown that the ability to release t-PA declines with age, but, in older adults, that impairment appears to also improve with exercise.

Exercise "can be a potential benefit to everyone," he said.