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Strong People

Walk Off Cancer Risk

Even moderate exercise improves your odds

When it comes to reducing your risk for cancer, you can play an active role—literally. Research shows that a physically active lifestyle is a powerful preventive. The American Cancer Society (ACS) estimates that nearly 1.5 million people will be diagnosed with some form of cancer in 2008. And it notes that a substantial number of those cases will be tied to a lack of physical activity. Currently, the most convincing scientific evidence shows a link between sedentary habits and colon cancer, as well as breast cancer in postmenopausal women. But promising research is also being done in other areas, including endometrial, prostate, and lung cancer.

How does exercise fight cancer? It wards off obesity, for one thing. An active lifestyle can help you to maintain a healthful weight, which is one of the three guidelines for cancer prevention established by the American Institute for Cancer Research. A study published in the American Journal of Epidemiology supports this message. It looked at 21,022 people and concluded that those who were obese were 30 percent more likely to have cancer than those who were not.

But keeping pounds off is just part of the story. Physical activity has been shown to help protect against cancer independent of body weight. For example, a review of observational trials published by the International Agency for Research on Cancer showed that moderate physical activity, such as brisk walking for three to four hours per week, reduced colon cancer risk by as much as 50 percent; a protective effect was found for both obese people and those with a healthful body weight. And research published in the Journal of the American Medical Association found that women who logged the equivalent of one and a quarter to two and a half hours of brisk walking per week reduced their risk for breast cancer by nearly 20 percent.

The exact mechanisms by which exercise helps prevent cancer are still not well understood. It is particularly difficult to solve that mystery because the mechanisms differ, depending on the site of the disease.

However, science has uncovered several leads. First, physical activity seems to improve the functioning of the immune system, whose job is to identify and eliminate abnormal cells from the body. Based on the data we have so far, an active lifestyle may actually increase the number and potency of immune cells that stop the growth of tumors.

Moreover, physical activity reduces body fat. This is important because extra fat cells—especially around the waist—can raise levels of estrogen in the blood, which in turn can increase breast cancer risk. The estrogen levels of postmenopausal women with an excess of body fat are 50 to 100 percent higher than those of their leaner counterparts.

Finally, physical activity has been shown to speed up food transit time in the gut. Cells that line the colon have less exposure to potentially cancer-causing agents, and that could be key in guarding against colon cancer.

How much physical activity do you need to protect against cancer? That’s still being determined, but here’s what we know in a nutshell: some activity is good, and more is better. A variety of observational trials have found that moderate to vigorous exercise—equivalent to 35 to 60 minutes of brisk walking per day—significantly lowers cancer risk. The ACS recommends “at least 30 minutes of moderate to vigorous physical activity, above usual activities, on five or more days of the week” and states that “45 to 60 minutes of intentional physical activity” would be “preferable.”

Even the lesser of those options would be considerably more exercise than most of us are getting now. According to a 2008 study that objectively measured the activity levels of 6,329 people, fewer than five percent of adults aged 20 years and older met the current ACS recommendation to engage in 30 minutes of physical activity on most days of the week. That needs to change. Do your part to take control of your health: go for a walk! You’ll be fighting cancer with every step.

MIRIAM E. NELSON is director of the John Hancock Center for Physical Activity and Nutrition at Tufts. She is an associate professor at the Friedman School of Nutrition Science and Policy and an adjunct professor at the Tisch College of Citizenship and Public Service. She is the author of the bestselling Strong Women book series.

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