Relationship between physical activity and cancer

What is the relationship between physical activity and breast cancer risk?
The relationship between physical activity and breast cancer incidence has been extensively studied, with over 60 studies published in North America, Europe, Asia, and Australia. Most studies indicate that physically active women have a lower risk of developing breast cancer than inactive women; however, the amount of risk reduction achieved through physical activity varies widely (between 20 to 80 percent) (6, 7). Although most evidence suggests that physical activity reduces breast cancer risk in both premenopausal and postmenopausal women (6), high levels of moderate and vigorous physical activity during adolescence may be especially protective. Although a lifetime of regular, vigorous activity is thought to be of greatest benefit, women who increase their physical activity after menopause may also experience a reduced risk compared with inactive women. A number of studies also suggest that the effect of physical activity may be different across levels of BMI, with the greatest benefit seen in women in the normal weight range (generally a BMI under 25 kg/m-squared) in some studies. Existing evidence shows a decreasing risk of breast cancer as the frequency and duration of physical activity increase. Most studies suggest that 30 to 60 minutes per day of moderate- to high-intensity physical activity is associated with a reduction in breast cancer risk (4, 6).

Researchers have proposed several biological mechanisms to explain the relationship between physical activity and breast cancer development. Physical activity may prevent tumor development by lowering hormone levels, particularly in premenopausal women; lowering levels of insulin and insulin-like growth factor I (IGF-I), improving the immune response; and assisting with weight maintenance to avoid a high body mass and excess body fat (7).

How might physical activity affect cancer survivorship?
Research indicates that physical activity after a diagnosis of breast cancer may be beneficial in improving quality of life, reducing fatigue (7), and assisting with energy balance. Both reduced physical activity and the side effects of treatment have been linked to weight gain after a breast cancer diagnosis. One study found that women who exercised moderately (the equivalent of walking 3 to 5 hours per week at an average pace) after a diagnosis of breast cancer had improved survival rates compared with more sedentary women. The benefit was particularly pronounced in women with hormone responsive tumors (10). Another study found that a home-based physical activity program had a beneficial effect on the fitness and psychological well-being of previously sedentary women who had completed treatment for early-stage through stage II breast cancer (11). Increasing physical activity may influence insulin and leptin levels and influence breast cancer prognosis. Although there are several promising studies, it is too early to draw any strong conclusions regarding physical activity and breast cancer survival.

Two additional studies have suggested a protective association of physical activity after colon cancer diagnosis and survival. Researchers examined the relationship between levels of physical activity both before and after a diagnosis of colon cancer in two different observational studies. Whereas levels of pre-diagnosis physical activity were not related to survival, participants with higher levels of physical activity post-diagnosis were less likely to have a cancer recurrence and had increased survival (12). Although these studies suggest protective effects of physical activity, more research is needed to understand what levels of physical activity provide these benefits.

http://www.cancer.gov/cancertopics/factsheet/prevention/physicalactivity