Still, the study results indicate that "you can control your destiny with regards to stroke," said study lead author Michelle McDonnell. "A healthy diet, healthy body weight, regular exercise along with a moderate alcohol consumption and not smoking combined can reduce your risk of stroke by 80 percent. Here we show that regular physical activity is an important aspect of that and should be encouraged."
According to McDonnell, researchers already know that exercise directly reduces the risk of stroke by improving the health of blood vessels, and indirectly by improving risky traits such as high blood pressure and obesity.
The new study is unusual because it attempts to identify the effect of exercise on stroke risk and because it tracks people over time instead of relying on people's memories, said McDonnell, a lecturer in the School of Health Sciences at the University of South Australia.
The researchers tracked more than 27,000 Americans starting between 2003 and 2007 for an average of nearly six years. Many were blacks from the "Stroke Belt," an area of the southeastern United States that has an especially high stroke rate. Everyone was 45 or older at the beginning of the study, and none had previous strokes.
A total of 3.1 percent of those who said they vigorously exercised four or more times a week at the beginning of the study suffered strokes. The percentages were 3.3 for those who exercised one to three times weekly and 3.6 for those who didn't exercise, according to the study authors.
After adjusting their statistics so they wouldn't be thrown off by factors such as high or low numbers of people of certain ages, the researchers determined that those who exercised the most were 20 percent less likely to suffer strokes than those who exercised the least.
But that number dipped to 14 percent -- considered to be statistically insignificant -- once the researchers did more adjusting. In other words, the extra exercise appeared to have no effect after taking into account for traditional stroke risk factors such as diabetes, high blood pressure, overweight, alcohol use and smoking.
However, this may be because the further adjustments eliminated the indirect influence of exercise on stroke -- its effect on risk factors like high blood pressure, for example.
The research also suggested men got more benefit from exercise than women on the stroke front. "There has been some research to suggest that women perhaps benefit from less intense exercise, like walking, but seeing as we didn't ask this question in our study we really can't speculate any more than that," McDonnell said.
Steven Blair, a professor who studies exercise at the University of South Carolina, praised the study and said a 20 percent reduction in stroke risk is "a reasonably big deal," especially in comparison with medical treatments for some conditions that may have the same effect.
The message "is simply that everyone should strive to meet our Health and Human Services physical activity guidelines -- all adults should get 150 minutes of moderate intensity physical activity per week, 75 minutes of vigorous intensity activity, or mix and match with one minute of vigorous being equal to two minutes of moderate," Blair said. "The exercise bouts should be at least 10 minutes in duration. So if everyone took three 10-minute walks a day on at least five days of the week, this would have a dramatic effect on disease rates in the U.S. population."
The study appears online July 18 in the journal Stroke.
For more about stroke, try the U.S. National Library of Medicine.