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A new study says regular physical activity may not have an impact on a woman's risk of colorectal cancer. But don't get too comfy on the couch. American Cancer Society experts -- and the researchers themselves -- say the way the study measured physical activity probably skewed the results and may have hidden the true benefits of exercise.

"The measurement of physical activity needs some refinement and improvement," said study co-author Michael F. Leitzmann, MD, PhD, an investigator at the National Cancer Institute. "We shouldn't give up on the idea that physical activity can protect from colon cancer, but we need more precise measures to tease 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 -- found a real benefit to working out.

"When you look at the studies that had more detailed measures of physical activity, those saw a 30% to 40% reduction in risk," said Alpa Patel, PhD, a senior epidemiologist at the American Cancer Society who is familiar with the topic.

The key is how researchers define "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 about their habits.

The women had to report 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 provided examples of each type of activity. Light activities included sitting, working in an office, watching TV, and driving a car. Moderate activities included light housework, hiking, and golf. Examples of vigorous activities included heavy housework like scrubbing floors or washing windows, aerobics, and strenuous sports.

That's where the problem lies, Patel said. 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 explained.

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.

Indeed, 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. According to the CDC, fewer than half of US adults currently get the exercise they should each day.

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.

In the meantime, Patel said, women should keep following ACS recommendations for physical activity: at least 30 minutes of moderate activity on most days of the week. And for both colon and breast cancer, bumping it up to 45 minutes of more vigorous activity may be even more beneficial.

"There really isn't a downside to physical activity," she said. "It impacts chronic disease risk well beyond cancer -- heart disease, diabetes, general health. It's also important for helping to maintain a healthy weight – another key way to live a healthier life and reduce your risk of chronic disease."

Citation: "Physical Activity and the risk of colon cancer among women: A prospective cohort study (United States)." Published online Feb. 17, 2006, in the International Journal of Cancer. First author: Brook A. Calton, University of California, San Francisco.