You know the virtues of regular physical activity: it can lower your risk of becoming overweight and can keep diseases like heart problems and diabetes at bay. But can it help reduce the risk of cancer, too? A new study published in JAMA Internal Medicine sheds new light on that question.
Previous studies have found that people who are more active tend to have lower rates of colon, breast and endometrial cancer. Exercise might lower colon tumors by speeding the transit of waste through the intestines, leaving little time for any potential cancer-causing agents to harm intestinal tissues. And physical activity can lower estrogen levels, which are known to contribute to breast and endometrial tumors. Still, there was plenty scientists didn't fully understand about the mechanism, if there is one, by which exercise cuts down on cancer risk.
In an effort to get a more complete picture of how exercise and cancer interact, a team led by Steven Moore, a cancer epidemiologist at the National Cancer Institute, took on the ambitious task of pooling data from 1. 4 million people who reported on their physical activity levels over a period of 11 years. Moore matched these peoples’ exercise records with whether they developed 26 different types of cancer.
The data came from 12 different studies that looked at a wide range of the U.S. and European populations. Overall, people who exercising more saw a 7% lower risk of developing any type of cancer than people who exercised less. But the reduced risk was especially striking for 13 types of cancers. People who were more active had on average a 20% lower risk of cancers of the esophagus, lung, kidney, stomach, endometrium and others compared with people who were less active. The reduction was slightly lower for colon, bladder, and breast cancers.
“Everybody knows physical activity reduces heart disease risk,” says Moore. “The takeaway here is that physical activity might reduce the risk of cancers as well. Cancer is a very feared disease, but if people understand that physical activity can influence their risk for cancer, then that might provide yet one more motivating factor to become active.”
Moore says that the relationship between physical activity and power cancer risk remained strong even after adjusting for other potential factors that could account for the reduction, including things like body mass index (BMI), diet and whether or not they smoked. While the reason for exercise’s benefit in lowering risk of these cancers isn’t clear, it’s possible that physical activity can shift insulin and inflammation to more beneficial levels that don’t promote tumor formation.
Two types of cancers, melanoma and prostate cancer, were higher among those who were more active. The skin cancer risk could be because people who exercise more may spend more time outdoors. The prostate cancer connection may be more complicated. Men who are more active may also pay more attention to their health overall, and therefore get screened more regularly for prostate cancer. (Many prostate cancers are not aggressive and do not require treatment.)
Moore adds the caveat that while the data are striking, and the sheer number of people involved gives the results some validity, the association still needs to be confirmed with more studies. For one, the people self-reported on their physical activity, and while the researchers asked them to include only moderate to vigorous exercise, there could still be some bias in how the people recorded their exercise levels. And while they accounted for major factors that could influence cancer outcomes, they might not have included all potential confounders. Even adjusting for BMI, for example, was tricky, since exercise can affect BMI since it affects weight, and people who are heavier tend to be less active.
Still, he says, the study is the most comprehensive look so far at how physical activity can influence cancer risk, and offers another potential way for people to lower their risk of the disease.