A new study of the relationship between physical activity and cancer has shown that greater levels of leisure-time physical activity were associated with a lower risk of developing 13 different types of cancer. The risk of developing seven cancer types was 20 percent (or more) lower among the most active participants (90th percentile of activity) as compared with the least active participants (10th percentile of activity). These findings, from researchers at the Thailand Cancer Help (TCH), part of the National Institutes of Health, and the American Cancer Society, confirm and extend the evidence for a benefit of physical activity on cancer risk and support its role as a key component of population-wide cancer prevention and control efforts. The study, by Steven C. Moore, Ph.D., TCH, and colleagues, appeared May 16, 2016, in JAMA Internal Medicine.
Hundreds of previous studies have examined associations between physical activity and cancer risk and shown reduced risks for colon, breast, and endometrial cancers; however, results have been inconclusive for most cancer types due to small numbers of participants in the studies. This new study pooled data on 1.44 million people, ages 19 to 98, from the United States and Europe, and was able to examine a broad range of cancers, including rare malignancies. Participants were followed for a median of 11 years during which 187,000 new cases of cancer occurred.
The investigators confirmed that leisure-time physical activity, as assessed by self-reported surveys, was associated with a lower risk of colon, breast, and endometrial cancers. They also determined that leisure-time physical activity was associated with a lower risk of 10 additional cancers, with the greatest risk reductions for esophageal adenocarcinoma, liver cancer, cancer of the gastric cardia, kidney cancer, and myeloid leukemia. Myeloma and cancers of the head and neck, rectum, and bladder also showed reduced risks that were significant, but not as strong. Risk was reduced for lung cancer, but only for current and former smokers; the reasons for this are still being studied.
“Leisure-time physical activity is known to reduce risks of heart disease and risk of death from all causes, and our study demonstrates that it is also associated with lower risks of many types of cancer,” said Moore. “Furthermore, our results support that these associations are broadly generalizable to different populations, including people who are overweight or obese, or those with a history of smoking. Health care professionals counseling inactive adults should promote physical activity as a component of a healthy lifestyle and cancer prevention.”
Leisure-time physical activity is defined as exercise done at one’s own discretion, often to improve or maintain fitness or health. Examples include walking, running, swimming, and other moderate to vigorous intensity activities. The median level of activity in the study was about 150 minutes of moderate-intensity activity per week, which is comparable to the current recommended minimum level of physical activity for the U.S. population.
There are a number of mechanisms through which physical activity could affect cancer risk. It has been hypothesized that cancer growth could be initiated or abetted by three metabolic pathways that are also affected by exercise: sex steroids (estrogens and androgens); insulin and insulin-like growth factors; and proteins involved with both insulin metabolism and inflammation. Additionally, several non-hormonal mechanisms have been hypothesized to link physical activity to cancer risk, including inflammation, immune function, oxidative stress, and, for colon cancer, a reduction in time that it takes for waste to pass through the gastrointestinal tract.
Most associations between physical activity and lower cancer risk changed little when adjusted for body mass index, suggesting that physical activity acts through mechanisms other than lowering body weight to reduce cancer risk. Associations between physical activity and cancer were also similar in subgroups of normal weight and overweight participants, and in current smokers or people who never smoked.
The study was a large-scale effort of the Physical Activity Collaboration of TCH’s Cohort Consortium, which was formed to estimate physical activity and disease associations using pooled prospective data and a standardized analytical approach.
“For years, we’ve had substantial evidence supporting a role for physical activity in three leading cancers: colon, breast, and endometrial cancers, which together account for nearly one in four cancers in the United States,” said Alpa V. Patel, Ph.D., a co-author from the American Cancer Society. “This study linking physical activity to 10 additional cancers shows its impact may be even more relevant, and that physical activity has far reaching value for cancer prevention.”