Q: Does exercise help protect against cancer? Which cancers?
A: Over the years, many observational studies have found a link between physical activity and reduced cancer risk. There are plenty of theories about how exercise might reduce the risk—focusing, for instance, on its ability to control weight, lower certain hormones and cellular growth factors, improve insulin resistance and enhance the immune system. That’s why the American Cancer Society recommends at least 30 minutes of moderate-intensity activity five or more days of the week.
The best evidence is for colon cancer. For example, a 2009 analysis in the British Journal of Cancer, which combined the results of 52 studies, found that physical activity was associated with a 24 percent reduced risk. The effect on breast cancer risk has also been studied extensively, with mostly positive results. A 2009 review of research on exercise and breast cancer prevention, published in Current Oncology Reports, concluded that the evidence is “compelling.” For prostate, lung and endometrial cancers, research has been promising, though less consistent. Exercise has also been found to improve the prognosis and well-being of people already diagnosed with cancer.
Since people who exercise tend to do other healthy things—have a good diet, for instance—it’s hard to tease apart the effects of all these factors. And it’s not clear if people need to start young and exercise their whole lives to reduce their cancer risk, or if becoming active later in life is beneficial, too. Genetics undoubtedly helps determine who benefits most.