Can colon cancer in women be prevented through exercise?

No, says a new study that has thrown open a debate on the merits of exercise.

6 March 2006
MUMBAI, INDIA

Does exercise prevent colon cancer in women? The debate is on. According to a new study says regular physical activity may not affect a woman's risk of colorectal cancer. American Cancer Society experts, and the researchers themselves, maintain that the way the study measured physical activity probably skewed the results.

According to study co-author Michael F. Leitzmann, MD, PhD, an investigator at the National Cancer Institute, the measurement of physical activity needs some refinement and improvement. He feels that we should not give up on the idea that physical activity can prevent colon cancer, but must devise more precise measures to bring out the exact nature of the association.

Colon cancer is a major cause of cancer death in women and in men. Lots of studies have looked into whether getting exercise can lower the risk of developing the disease. Studies of men show clearly that it can. Studies of women, on the other hand, have had mixed results.

But the prospective studies that most carefully measured physical activity -- the Nurses Health Study and the American Cancer Society's Cancer Prevention Study II -- revealed a real benefit to working out.

The key here is how researchers categorize light, moderate, or vigorous exercise.

The new study, which is published in the International Journal of Cancer, involved nearly 32,000 mostly postmenopausal US women who were taking part in a study of breast cancer screening. Researchers from the National Cancer Institute and the University of Wisconsin-Madison gave them questionnaires.

The women had to provide information on how often they exercised hard enough to work up a sweat. They also had to estimate how many hours per day in the past year they spent sleeping or doing light, moderate, or vigorous activities.

The researchers defined light activities as sitting, working in an office, watching TV, and driving a car. Moderate activities included light housework, hiking, and golf. Vigorous activities included heavy housework like scrubbing floors or washing windows, aerobics, and strenuous sports.

According to Alpa Patel, PhD, a senior epidemiologist at the American Cancer Society, that is the problem. In the studies that did show a benefit from exercise, light activities were things like walking at a 3-mile-per-hour pace, and moderate activity would be more like walking at 4 mph. Sitting, watching TV, and driving a car were actually measures of inactivity, she said.

Leitzmann said his team ended up excluding light activities from their calculations because the things on the list were so sedentary. They looked only at the colon cancer risk for moderate and vigorous activities. Even so, the new study may simply have misclassified too many activities to get a reliable result.

More than 58% of the women in the study reported getting at least 30 minutes of vigorous activity each day. That's well above national profiles. 

The study authors themselves point out in the paper that the way they measured activity may be behind their disappointing findings. They say their study shows how important it is to do more research that takes detailed measures of physical activity to get a better picture of the true relationship between exercise and colon cancer risk in women.

Patel maintains that women should spend at least 30 minutes on moderate activity on most days of the week. And for both colon and breast cancer, scaling it up to 45 minutes of more vigorous activity may be even more beneficial.