FRIDAY, Feb. 17, 2006 (HealthDay News) -- Although it's proven to have a protective effect for men, regular physical activity may not lower a woman's risk for colorectal cancer, a new study suggests.
But the study's authors caution that more rigorous research might yet reveal a benefit for women.
"We shouldn't close the door on this," said senior researcher Dr. Michael F. Leitzmann, an investigator with the U.S. National Cancer Institute in Bethesda, Md.
His team published its findings Friday in the online edition of the International Journal of Cancer.
According to Leitzmann, basic biology supports the notion that being active cuts colorectal cancer risk.
"One of the main hypotheses is that physical activity speeds gastrointestinal transit -- in other words, bowel transit time," he said. "It's also very effective in weight control and weight maintenance, and we know that [increased] body weight is related to an increased risk for colon cancer." Exercise may also enhance the body's natural cancer-fighting abilities by preventing insulin resistance and improving immune function.
In fact, numerous studies of varying quality have consistently shown a real benefit to men from regular exercise, in terms of helping to prevent colon cancer, Leitzmann said.
"The results have been less consistent for women, however," he said.
In their study, the researchers examined data from a prospective study of nearly 32,000 postmenopausal women followed for more than 10 years as part of the Breast Cancer Detection Demonstration Project Follow-Up Study. For that effort, women were asked to provide information on their rate of daily physical activity.
"Somewhat to our surprise, we found that there was no association between our measure of physical activity at that time and [the women's] subsequent risk for colon cancer," Leitzmann said.
But he cautioned that the story doesn't end there.
Leitzmann pointed out that it was very difficult to say with certainty that the group labeled as "inactive" in the study was, in fact, completely sedentary. That's because "people are active in different ways during the day," he explained. "Some women might be active in the workplace or while they are commuting, or even at home doing household chores." But since they may not think of this as "exercise" or "physical activity," it might not get reported, he said.
In fact, other studies that have asked women extremely detailed questions regarding daily activities have shown some anti-cancer benefit from exercise, Leitzmann noted.
"The bottom line is that we may need some improvement of the physical activity 'instrument' if we really want to pick up an association in these kinds of studies," he said.
Another expert agreed that the disappointing results of this study are no reason for women -- or anyone else -- to discount exercise's anti-cancer effects.
"First of all, most people in the sciences know that you should never put too much stock in any one article," said Dr. Durado Brooks, director for prostate and colorectal cancer at the American Cancer Society. "In men, certainly, there's very good evidence that regular exercise decreases colorectal cancer risk."
It's possible that women may not gain quite as much benefit as men due to hormonal factors or other unknown causes, he added. But there are certainly many reasons for them to get up off the couch and get active.
"Remember, your likelihood of getting colorectal cancer, at worst, is maybe 6 or 7 percent," Brooks said. "But your likelihood of having a heart attack or stroke, diabetes, high blood pressure, and lots of other ailments is much, much higher. And exercise is going to be helpful against all of those."
For more on colorectal cancer, visit the American Cancer Society.