In a study reviewing questionnaires filled out by close to 2,600 women, 442 of whom had been diagnosed with ovarian cancer, researchers from the Public Health Agency of Canada found a solid association between a lowered risk of ovarian cancer and moderate exercise. Interestingly, there were no benefits found among women who engaged in very active sports, such as running or swimming.

"There were statistically significant trends of decreasing risk with increasing levels of moderate and total recreational activity, with similar patterns for premenopausal and postmenopausal women," said Dr. Sai Yi Pan, senior epidemiologist at Public Health Agency of Canada and lead author of the study, which appears in the May 16 issue of the International Journal of Cancer.

"Since ovarian cancer has a poor prognosis and physical activity is a modifiable lifestyle factor, our finding supports the approach of adopting a healthy lifestyle to prevent this tumor in women," she said.

The reasons for the benefits are difficult to assess, Pan said, but they could include the fact that physical activity has been associated with decreased levels of circulating estrogen and progesterone in women, and studies suggest that exposure to high levels of these hormones is a risk factor for ovarian cancer.

Using data from the Canadian National Enhanced Cancer Surveillance System, which collected detailed, self-reported information on recreational physical activity and, in the province of Ontario, occupational physical activity, the researchers evaluated the relation of physical activity to ovarian cancer risk.

They found that women who reported engaging in walking, playing golf, gardening and social dancing, all classified as moderate exercise, were at lower risk for ovarian cancer. However, women who reported more active pursuits, such as running, swimming, skating or bicycling, showed no lower risk for the disease.

Pan could not explain why moderate exercise appeared to be more beneficial than vigorous exercise.

"However, literature suggests that regular exercise may enhance the immune system, while too much exercise may cause immune suppression and, in extreme cases, may overcome the antioxidant defense system with potential oxidative DNA damage," she noted.

The risk-lowering benefits of moderate exercise were also found among women whose jobs were more active than sedentary, according to the data gathered in Ontario. Women in jobs requiring lifting and carrying light loads, or who engaged in carpentry or heavy cleaning, had a lower risk for the disease compared to women with desk jobs.

Researchers also noted that obesity is a risk factor for ovarian cancer so that increased physical activity could help women lose weight, further helping them lower their chances for the disease.

Researchers found that the benefits of exercise accrued to all the women with ovarian cancer, except for those with a specific form of the disease, called mucinous ovarian cancer. Sixteen percent of the women with cancer in the study had this type of ovarian malignancy.

"This limited, retrospective study showed a modest reduction in risk of ovarian cancer among those women who exercised in moderation, and is sort of interesting," said Dr. Willard Barnes, director of gynecologic oncology at the Georgetown University School of Medicine.

But he added that further studies will be needed to see if the results can be reproduced so the benefits are clearer.

In the meantime, said Dr. Robert Morgan Jr., section head of medical gynecologic oncology at the City of Hope Cancer Center, in Duarte, Calif., "these data certainly strengthen physicians' advice to their patients to be more active to achieve good results for their health."

Ovarian cancer is the seventh most common cancer in women, but ranks fourth as the cause of cancer deaths among women, according to the American Cancer Society (ACS). In 2005, approximately 22,220 new cases of ovarian cancer will be diagnosed. However, ACS reports that the overall rate of ovarian cancer has decreased since 1991.

More information: The U,.S. government's Centers for Disease Control and Prevention has the latest information on the Ovarian Cancer Control Initiative.