Regular moderate-intensity or greater physical activity is associated with a reduced risk of several types of cancer, including cancers of the breast, colon and endometrium. For several cancers, engaging in longer exercise sessions, or exercising with greater intensity or for more years, produces greater reductions in cancer risk.
Recent findings indicate that women with a history of breast cancer who engage in more than 9 metabolic equivalent (MET)·h/week of recreational physical activity after breast cancer diagnosis, which corresponds to approximately 2–3 h/week of brisk walking, had a significantly lower risk of death or breast cancer recurrence than women who were inactive. Similarly, men or women who are active after a diagnosis of colon cancer appear to have improved prognosis compared with sedentary individuals.
Physical activity effects on menstrual function and sex hormones might explain a link between increased physical activity and reduced risk for breast and endometrial cancers. Athletes and physically active premenopausal women have delayed onset of menses, fewer ovulatory cycles and lower circulating levels of oestrogen and progesterone, all factors that are related to breast or endometrial cancer risk.
Studies in postmenopausal women indicate that physical activity might affect postmenopausal breast cancer and endometrial cancer risk by reducing body fat, thereby lowering circulating levels of oestrogens and androgens.
Insulin resistance, hyperinsulinaemia, hyperglycaemia and type 2 diabetes have been linked to increased risk of breast, colon, pancreas and endometrial cancers. Physical activity improves insulin resistance, reduces hyperinsulinaemia and reduces risk for diabetes, which could explain the link between increased physical activity and reduced risk for these cancers.
Increased levels of pro-inflammatory factors and decreased levels of anti-inflammatory factors have been linked with increased cancer risk. Physical activity might reduce systemic inflammation alone or in combination with reduction in body weight or composition through reducing inflammatory cytokines in adipose tissue.
Support for an effect of physical activity on the cancer process comes from smaller randomized clinical trials. If definitive evidence from larger clinical trials is obtained to show that increased physical activity can prevent certain cancers, or can improve prognosis, it would be an excellent public-health intervention for reducing the impact of cancer at relatively low cost and risk.
About 25% of cancer cases globally are due to excess weight and a sedentary lifestyle. Physical activity may decrease risk for various cancers by several mechanisms, including decreasing sex hormones, metabolic hormones and inflammation, and improving immune function. The level of physical activity might also be associated with prognosis among individuals with cancer. Randomized clinical trials have shown that physical activity interventions can change biomarkers of cancer risk. Observational studies can also provide useful information on mechanisms that might link physical activity to cancer.
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