As more studies reveal the damage being inflicted by obesity on health in the United States, the role of physical activity in promoting well-being has taken on increased importance. Approximately two-thirds of Americans are considered to be overweight or obese, and obesity significantly increases risk of stroke, high blood pressure, and diabetes.

And a large body of literature also suggests being overweight or obese increases the risk for postmenopausal breast cancer, colon cancer, adenocarcinoma of the esophagus, endometrial cancer, renal cell carcinoma, and several other cancers, explains Dr. Rachel Ballard-Barbash, associate director of the Applied Research Program in NCI's Division of Cancer Control and Population Sciences.

"Increasing regular physical activity and improving diet with careful attention to calorie control are two important health behaviors for controlling overweight and obesity," she says. "As more research is starting to reveal, physical activity may also play an important role in improving cancer risk, quality of life, and possibly prognosis."

Many studies suggest that exercise may offer a survival benefit for those with cancer. In one study, presented at the recent annual meeting of the American Association for Cancer Research, researchers from Brigham and Women's Hospital and Harvard University analyzed data on physical activity from nearly 2,300 women diagnosed with breast cancer from the long-running Nurses' Health Study. The results: the risk of death from breast cancer was reduced by anywhere from 23 to 54 percent, depending on the extent of weekly exercise.

The study's results aren't necessarily unexpected, says Dr. Ballard-Barbash. Studies have shown that women who exercise regularly have lower levels of circulating estrogen, perhaps because they have lower body fat and, for premenopausal women, longer, and therefore fewer, menstrual cycles. "We know that estrogen is a critical factor in the development of breast cancer," she explains. "So, in terms of breast cancer patients, it's not surprising that exercise would improve survival."

The most abundant and strongest evidence linking physical activity with reduced cancer risk exists for colon and breast cancers. With colon cancer, for example, there is ample evidence that individuals who are physically active can reduce their risk by as much as 50 percent. In these studies, however, the greatest reduction in risk was among those who were most active, and the benefit was seen more consistently in men. A more limited number of studies also have reported links between physical activity and a reduced risk of prostate, lung, and endometrial cancers.

The hypotheses on how physical activity may confer a benefit vary by the type of cancer. Physical activity's effect on factors such as insulin resistance, metabolism, and inflammation, among other things, may reduce colon cancer risk. With breast and endometrial cancers, the effect of exercise on sex hormones like estrogen may play an important role.

NCI is funding studies to further clarify the impact of physical activity on cancer risk, as well as whether physical activity may improve the quality of life of cancer patients and survivors. One study, for example, is examining the feasibility and benefits of a home-based moderate exercise program among breast cancer survivors. Another is testing the effectiveness of a nurse-directed walking exercise program to mitigate fatigue and maintain physical functioning during treatment for prostate, breast, or colorectal cancer.

On the prevention side, NCI is funding research such as a study to investigate whether women who engage in moderate or strenuous physical activity have a reduced risk of endometrial and ovarian cancers and if strenuous physical activity reduces this risk more than moderate physical activity.