Amping up your workout might offer extra defense against the country's fourth-most common type of cancer.

In a Washington University study of 79,295 women ages 40 to 65, researchers found that those who exercised at a moderate or vigorous intensity (playing tennis, jogging, or riding a bike, for instance) more than four hours weekly had a 44 percent lower colon cancer risk than those who worked out less than an hour each week. Women who walked for about one or two hours a week, meanwhile, were 31 percent less likely to develop colon cancer than those who didn't walk at all.

Exactly how exercise lowers your risk isn't clear; it may work by reducing inflammation in the colon, suggests lead study author Kathleen Wolin, Sc.D. Another theory, she adds, is that it decreases the amount of time it takes for food to pass through the intestines, which in turn could curb the colon's exposure to potential carcinogens.