NEW YORK (Reuters Health) - It's well known that higher levels of regular physical activity are associated with a significantly lower risk of heart attack, stroke and other cardiovascular events, but exactly how exercise protects the heart has been largely unclear, until now.

Research published this week suggests that the beneficial effects of exercise on a variety of known cardiovascular risk factors account for the majority (about 60 percent) of the improvement in heart health.

When sets of these risk factors were examined, certain biomarkers that influence blood vessel function and inflammation of the arteries - namely, fibrinogen, C-reactive protein and intercellular adhesion molecule - together made the largest contribution to lower risk (33 percent).

A reduction in blood pressure induced by exercise was the next major contributor to lower heart risk (27 percent), followed by the favorable effects of exercise on fats (about 15 percent). Smaller contributions to the inverse relationship between exercise and heart health were attributed to body mass index - the ratio between height and weight - (10 percent) and diabetes (9 percent).

Inflammatory factors and hemostatic factors - factors that slow or coagulate blood - as a group, "have overlapping functions and roles and, in our study, had the biggest effect in mediating exercise-related cardioprotection, more so than blood pressure or body weight, study chief Dr. Samia Mora of Harvard Medical School and Brigham and Women's Hospital, Boston said in a written statement.

The research, published online in the journal Circulation, stems from more than 27,000 members of the Women's Health Study who were followed for heart attack and stroke for an average of about 11 years.

The women, whose average age was 55 at entry, were assessed for a full range of cardiovascular risk factors and different levels of exercise.

A total of 979 women had a heart-related event during follow up. According to Mora and colleagues, there was a 41-percent reduction in heart attack and stroke in women getting the most exercise (more than 5 hours of moderately intense physical activity such as brisk walking each week), compared to a reference group getting the least (less than 1 hour of exercise each week).

The reduction in heart attack and stroke was 32 percent in women reporting 2 to 5 hours of moderate intensity physical activity weekly, and 27 percent in those reporting 1 to 2 hours weekly.

These findings, Mora commented, indicate that the lasting benefits of physical activity start at a relatively low level of physical activity - about 2 hours per week.