More Evidence That Exercise
Reduces Cancer Risk

Three recently published studies find that people who exercise lower their cancer risk. But all three point out that the exercise can't be a walk in the park. It needs to be vigorous.

A study of women in China found that those with a high level of vigorous activity had a lowered rate of ovarian cancer. Reporting in the International Journal of Cancer (Vol. 105, No. 6: 838-843) Min Zhang and colleagues compared the physical activity of women with ovarian cancer to that of women without this disease. They looked at how much effort the women expended at work or exercise and how much they sweated with these activities.

Women with a high level of physical activity cut their ovarian cancer risk in half. This meant they engaged in one of the following activities:

• "vigorous work" like moving heavy furniture or digging for at least 20 hours a week;
• moderate activity such as brisk walking or bicycling for 30 hours a week;
• strenuous sports at least two hours a week;
• working or exercising hard enough to sweat at least three times a week.

All this seems like a lot of activity and it may not carry over directly into guidance for women in the US. Chinese women may normally have a high level of physical activity so they need to work even harder to lower their risk of ovarian cancer. Also, China's ovarian cancer rates overall are lower than US rates -- another factor that would complicate a direct comparison.

But even U.S. women need to exercise vigorously to lower their risk of another cancer, breast cancer. A study led by Alpa Patel, of the American Cancer Society, found that women who were very active lowered their risk of breast cancer by nearly 30%.

In their report, published in the journal Cancer Causes and Control (Vol. 14, No. 6: 519-529), Patel and colleagues examined the relationship of exercise to breast cancer in postmenopausal women. Although any physical activity appeared to have some benefit, the 30% reduction in breast cancer rate occurred in women whose exercise was the equivalent of swimming, running or jogging at least six hours a week.

Women could achieve the same effect by walking or dancing; they would just need to do it for twice as long.

In a third study, this time of colorectal cancer, researchers from the University of Utah and Kaiser Permanente in Oakland, California, found that both men and women lowered their risk of this disease with physical activity. In the report, published in the American Journal of Epidemiology (Vol. 158, No. 3: 214-224) the researchers, led by Dr. M. L. Slattery, pointed out that vigorous activity provided the greatest benefit.

Men and women who exercised the equivalent of jogging five or more hours a week lowered their risk of colorectal cancer by 40% to 50%. Some benefit came from less vigorous activity, particularly for rectal cancer.

The researchers from these three studies all point out that studying the effect of exercise on cancer risk is an uncertain science. There may be other characteristics about people that lead them to exercise and also reduce their risk of cancer. One example might be that their concern for their health has led them to eat more fruits and vegetables, which can also reduce cancer risk. Another is that cancer rates are often higher in obese people and the more active people are generally thinner.

But the researchers didn't think that weight alone accounted for the differences. There seems to be some biologic connection between exercise and lowering the risk of some cancers. Although there are many explanations, none have been proven and scientists are still looking for that link. People who want to lower their risk for these three cancers benefit from knowing that vigorous exercise works.

The American Cancer Society recommends that adults engage in at least moderate activity for 30 minutes or more on five or more days of the week; 45 minutes or more of moderate to vigorous activity on five or more days per week may further enhance reductions in the risk of breast and colon cancer.

Reference Source 106

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