Scientists at the National Cancer Institute, Harvard Medical School and other centers published a paper last week in JAMA Internal Medicine, showing that exercise appears to substantially reduce the risk of developing 13 different varieties of cancer. The benefits seem to hold true even if someone is overweight. The researchers looked at 12 large studies, pooled together, involving 1.44 million men and women.
They focused on specific information for each of those 1.44 million people about whether they exercised, how vigorously and how often. They also zeroed in on whether and when, after each study’s start, the participant had been diagnosed with any type of cancer. People who reported exercising moderately, even if the time that they spent exercising was slight, had significantly less risk of developing 13 different types of cancer than people who were sedentary. The researchers found a reduced risk of breast, lung and colon cancers, which had been reported in earlier research. But they also found a lower risk of tumors in the liver, esophagus, kidney, stomach, uterus, blood, bone marrow, head and neck, rectum and bladder. When the researchers compared the top 10 percent of exercisers, meaning those who spent the most time each week engaging in moderate or vigorous workouts, to the 10 percent who were the least active, the exercisers were as much as 20 percent less likely to develop most of the cancers in the study.
On the other hand, they found an increased risk of two types of malignancies — melanoma and slow-growing prostate tumors — among people who exercised the most. Those findings can most likely be explained by certain characteristics of active people, said Steven Moore, an investigator at the National Cancer Institute who led the study. “People who exercise generally go in for more checkups” than sedentary people, he said, resulting in more screenings for conditions such as so-called indolent prostate cancers. (There was no discernible association, positive or negative, between exercise and aggressive prostate tumors.) “They also often exercise outside,” he continued, “and are more prone to sunburns” than people who rarely work out, potentially contributing to a greater risk for melanoma.
Encouragingly, the associations between exercise and reduced cancer risks held true even when the researchers factored in body mass. People who were overweight or obese but exercised had a much lower risk of developing most cancers than overweight people who did not move much.
The authors cautioned that this was an observational study, so it cannot directly prove that exercise reduces cancer risks, only that there is an association between more exercise and less disease. It also relied on participants’ memories of exercise, which can be unreliable. But even with those limitations, the findings sturdily suggest that exercise may help to reduce the risk of many types of cancer. Here in West Marin we have walking and hiking trails, dirt roads, gyms in Stinson and Point Reyes, tennis courts, classes at the community centers, and the ocean! Let’s keep moving.
Sadja Greenwood MD, MPH back issues on this blog