A large new study concluded that exercise reduces the risk of 13 cancers. It's the latest evidence in a growing area of research suggesting that exercise may play a role in both cancer prevention and healthy survivorship.
For breast, endometrial, and colon cancers, there is currently strong evidence that exercise reduces the risk. Evidence that being active reduces the risk of other cancers is not as well established, often because many of these studies are relatively small.
This latest study used the previous research, pooling together data from 1.4 million people who were part of a dozen US and European studies. It is the first ever pooled analysis of physical activity and cancer incidence that has been done in the world and it is a huge contribution to this literature, says Christine M. Friedenreich, PhD, an expert on physical activity and cancer risk with Alberta Health Services who was not part of the new study.
"There is already tons of evidence for colon, breast and endometrial and for these, there’s great evidence that risk is reduced by 25-30 percent with evidence of dose response. The more you [exercise], the greater your risk reduction. But it’s harder to get big studies on the relatively more minor sites,” said Friedenreich. “Since they pooled so many studies they had great study power to be able to consider even less common cancer sites that have not been previously investigated.”
The study, published in JAMA Internal Medicine, included adults ages 18 to 98, with the average age at 59. In 1987, participants had reported their activity habits, along with their weight, height and and other relevant information. After an average of 11 years, 186, 932 people had been diagnosed with cancer.
When comparing the most to the least active, lower risk was seen for colon, breast, and endometrial cancers along with esophageal, liver, stomach and kidney cancers. Myeloma and cancers of the head and neck, rectum, and bladder also showed reduced risks that were significant. Risk was reduced for lung cancer, but only for current and former smokers.
The risk of developing seven cancer types was 20 percent (or more) lower among the most active participants as compared with the least active. The most active, those in the 90th percentile, corresponds to approximately 22.5 MET-hours/week, says lead author Steven Moore, PhD, at the National Cancer Institute. “This is comparable to 7 hours a week of brisk walking or 2.5 hours a week of jogging. These are active people, but not super-athletes.”
The least active were in the 10th percentile, comparable to 20 minutes per week of brisk walking.
Looking at all cancer sites together, the study calculates that higher levels of leisure-time physical activity linked to a 7 percent lower risk of total cancer. This is after adjusting for age, sex, smoking and alcohol consumption.
After adjusting for BMI, the lower risk was slightly dampened but still there for almost all the same cancers. For cancers of the liver, gastric cardia, and endometrium, the results became non-significant when we adjusted for BMI.
“Importantly, people with lower activity levels, such as those who do 2.5 hours a week of brisk walking (comparable to recommended minimum activity levels), still had notably lower cancer risk, even if it was not quite as low as those who did more activity. The associations between physical activity and lower cancer risk generally had a linear dose-response," said Moore.
Even with prior studies suggesting the link with physical activity and lower cancer risk, “We were surprised, however, to confirm as many of the associations as we did,” said Moore.
The link of physical activity to lower cancer risk is an association, and there could be other causes. Even after adjusting for BMI, excess body fat could play a role. Obesity and overweight are a cause of 11 cancers and some of these, such as postmenopausal breast and endometrial, could be influenced by fat tissue. Body fat is an active organ that pumps out hormones and other compounds. Too many of these hormones, such as estrogen and insulin, can lead to higher cancer risk.
Excess body fat is also strongly linked to chronic inflammation, which is strongly linked to increased risk of many cancers.
Friedenreich’s studies suggest that exercise does lower body fat, especially among postmenopausal women. When we put healthy women on an exercise program [and told them not to change their diet] we see huge decrease in body fat levels and see some in BMI, she says. "We see a reduction in body fat, along with a reduction in inflammation, glucose levels and insulin levels.”
Several non-hormonal mechanisms hypothesized to link physical activity to cancer risk include immune function and oxidative stress. For colon cancer, exercise could also reduce the time potential harmful compounds are in the gastrointestinal tract.
There also could be cancer-preventing habits common to more active people than to least active, such as a healthier diet and less sedentary time. And a caveat to the activity levels is that they are estimates based on a subset of the population. Not all studies had sufficient information to calculate exact activity levels.
One major note: the pooling study found that physical activity was associated with higher risks of two cancers. One was a slight increase, for non-advanced prostate cancer. This is likely due to screening bias, the authors write. Active men are more likely to receive health exams and be diagnosed with this cancer. But the other increase in risk is an aggressive form of skin cancer: malignant melanoma. This is likely due to greater UV sun exposure, and wearing sunscreen is an important practice to prevent this cancer.
Also, this latest study focused on exercise that was moderate to vigorous leisure-time activities, all of which are aerobic. It did not investigate on strength training or resistance exercises.
Researchers are working to conduct future studies targeting in greater detail the type, intensity, and amount of physical activity needed to reduce overall cancer risk. Another ongoing area of study is how physically activity at different times of life may affect cancer risk. One goal is to have exercise prescriptions for cancer prevention, says Friedenreich.
For now, some exercise is better than none, experts say. AICR recommends at least 30 minutes a day of moderate activity for cancer prevention.
Sources: Steven C. Moore, PhD, MPH, et al. Association of Leisure-Time Physical Activity With Risk of 26 Types of Cancer in 1.44 Million Adults. JAMA Intern Med. Published online May 16, 2016.
Friedenreich CM, Neilson HK, O'Reilly R, Duha A, Yasui Y, Morielli AR, Adams SC, Courneya KS. Effects of a High vs Moderate Volume of Aerobic Exercise on Adiposity Outcomes in Postmenopausal Women: A Randomized Clinical Trial. JAMA Oncol. 2015 Sep;1(6):766-76. doi: 10.1001/jamaoncol.2015.2239.
AICR/WCRF. The Continuous Update Project reports: http://www.aicr.org/continuous-update-project/
Published on May 18, 2016