How does exercise reduce cancer risk?
Physical activity reduces the likelihood of developing cancer in a number of ways.
Studies conducted in hundreds of post-menopausal women aged 50 to 75 years have shown that regular exercise results in lower levels of estrogen. Being exposed to less of this hormone throughout life can reduce a person’s risk of developing hormone receptor-positive breast cancer.
In another study of men and women aged 40 to 75 years who had a recent normal colonoscopy, researchers randomised participants to perform aerobic exercise for 60 minutes for six days per week, or to continue their usual activity. As you might expect, those who exercised lost weight and body fat, but they also reduced their levels of estrogen-like hormones and experienced cellular changes in their colons that suggest a reduced risk of colon cancer.
Exercise also prevents high blood levels of insulin, which has been linked to cancer development and progression, particularly with cancers of the breast and colon. Exercise can reduce inflammation and improve immune system function, both of which have also been associated with less cancer development.
Additionally, physical activity improves gastrointestinal motility (the speed with which food travels through the digestive system) – which reduces the amount of time the cells lining the digestive tract are exposed to potential carcinogens – and helps prevent obesity, which in and of itself has been linked to higher rates of a number of cancers.
Exercise and breast and colorectal cancers
The link between exercise and cancer risk has probably best been studied for breast and colorectal cancers.
One meta-analysis looked at 38 studies that included a total of over four million women, of whom about 116,000 were diagnosed with breast cancer. The study participants engaged in physical activity ranging from none (the lowest category) to seven hours or more activity per week (the highest category). The most active women had a 12% lower risk of developing breast cancer than the least active women. Reductions occurred regardless of body mass index (BMI) or whether women were pre- or post-menopausal.
In post-menopausal women participating in the separate Nurses’ Health Study, which followed over 95,000 participants for an average of 20 years, almost 4800 breast cancer diagnoses occurred. Women who engaged in moderate levels of activity, such as one hour per day of brisk walking, had a 15% lower risk of breast cancer than women who exercised less. Interestingly, women who had low levels of physical activity before menopause but increased their activity after menopause had a 10% lower risk of developing breast cancer – showing that it’s never too late to start exercising.
It turns out that being active and exercising at any period of your life – whether it’s in your 30s, 40s or 50 and over – is strongly associated with a reduced risk of colorectal cancer.
In one study, almost 500,000 people over the age of 50 participating in the National Institutes of Health-AARP Diet and Health Study were mailed a series of questionnaires asking about their medical conditions, diet and physical activities. Among these participants, over 4700 were diagnosed with cancers of the colon or rectum. The investigators found that exercising five or more times per week was associated with a 21% reduced risk of colon cancer in men and a 15% reduced risk in women, and a 24% reduced risk of rectal cancer in men. Additionally, men who spent most of the day sitting (nine or more hours) had a 60% increased risk of colon cancer compared with those who spent fewer than three hours sitting.
How much exercise is enough to reduce cancer risk?
Any exercise is better than no exercise. In some cases, more exercise translates to even greater benefits: in the Journal of Clinical Oncology study, a “dose-response” relationship (the more you exercise, the lower your risk) was found for cancers of the breast, colon, endometrium, esophagus, and head and neck.
The American Cancer Society recommends 150 to 300 minutes of moderate-intensity exercise (such as brisk walking, or biking under 15km/h) per week.
Even short spurts of vigorous-intensity physical activity (such as running, or cycling 15km/h or faster) are associated with substantive risk reduction in a number of cancers, including cancers of the breast, colon, bladder, esophagus, stomach and endometrium, among others. In fact, one study showed that those who exercised for just two or more hours per week reduced their risk of head and neck cancer, lung cancer, and breast cancer. Thus, the American Cancer Society alternatively recommends 75 to 150 minutes of vigorous intensity exercise weekly.
For people who have a cancer diagnosis, it isn’t too late to start exercising: a number of studies have shown that exercise reduces the rates of death from cancer and of overall mortality, and the chance of cancer recurring. Even just one exercise session has been shown in a small study to prevent the growth of cancer cells. So even if you can only add brief periods of physical activity to your day, it’s always worth it. Every bit counts.
Mikkael A. Sekeres is the chief of the division of haematology and professor of medicine at the University of Miami’s Sylvester Comprehensive Cancer Center. He is author of the books When Blood Breaks Down: Life Lessons From Leukemia and Drugs and the FDA: Safety, Efficacy, and the Public’s Trust.