By Steven Reinberg
That's the conclusion of researchers who pooled data from nine published studies that included more than 750,000 men and women.
"We found that the recommended amount of physical activity was in fact associated with significantly reduced risk for breast, colon, endometrial, kidney, liver, myeloma and non-Hodgkin lymphoma," said lead researcher Charles Matthews, a senior investigator at the U.S. National Cancer Institute.
U.S. guidelines recommend three to five hours a week of moderate activity for adults, or one to three hours a week of vigorous activity.
The study authors found that the harder you exercise for that recommended time, the more you reduce your cancer risk.
Specifically, the risk of colon cancer in men was reduced between 8% for moderate exercise and 14% for vigorous activity.
For women's breast cancer, the reduction ranged from 6% for moderate exercise to 10% for a vigorous work out; for endometrial cancer, from 10% to 18%; kidney cancer, 11% to 17%; myeloma, 14% to 19%; liver cancer 18% to 27%; and non-Hodgkin lymphoma in women, 11% to 18%.
But Alpa Patel -- the American Cancer Society's senior scientific director of epidemiology research, who was part of the study -- urged caution in interpreting the findings.
Patel pointed out that the study does not prove that exercise lowers cancer risk, only that a strong association exists.
Matthews said, however, that the findings suggest that doctors and fitness professionals should encourage adults to exercise at the recommended levels to lower their risk of cancer.
How exercise might lower the risk for these seven cancers isn't clear, but Patel offered some theories.
"The most common things that we know about exercise, even in the absence of weight maintenance or weight loss, is that it's important for insulin regulation, sex hormones like estrogen, and also has an important impact on inflammation and immune response -- any or all of these different factors could affect different types of cancer," she said.
For example, Patel noted that for colon cancer, the leading theory is that exercise works by managing glucose metabolism, and breast cancer is largely driven by estrogen levels.
Living a healthy lifestyle is a part of lowering your cancer risk. That means maintaining a healthy weight, eating a healthful diet and not smoking.
"There are certain modifiable risks associated with cancer, and we are learning more and more about the potential benefits of being physically active as it relates to cancer prevention," Patel added.
She said that researchers' understanding over the past decade that physical activity may help reduce risk for cancers of the colon and breast, as well as endometrial cancer, has now expanded to knowing it's important for at least seven cancers, and potentially more.
"This is a really exciting message that even as you are working to make lifestyle choices, a small amount of activity can be very beneficial to your cancer risk," Patel said.
The report was published Dec. 26 in the Journal of Clinical Oncology.