Regular exercise or physical activity has been shown to improve breast cancer prognosis. The amount of exercise required to produce this effect is modest — only about half an hour per day of brisk walking. In fact, there is some evidence that heavy exercise is not desirable, whether during treatment with chemotherapy or radiotherapy, or after such treatment.
Sedentary behavior patterns such as sitting for long periods or watching a great deal of television are not associated with reduced survival among regular exercisers.
Exercise appears to reduce breast cancer risk by reducing inflammation, improving circulating hormone and insulin profile, increasing immunity, and influencing the regulation of tumor suppressor genes. However, note that one study reported that hormone replacement therapy (HRT) use cancels out the benefits of exercise in reducing breast cancer risk.

Exercise improves breast cancer prognosis

Regular exercise is linked to improved breast cancer prognosis, including in women with triple negative (ER-/PR-/HER2-) disease. One meta-analysis of previous studies reported that physical activity is associated with reduced breast cancer-specific mortality in breast cancer survivors. Regular exercise was linked to reduced late recurrence (more than five years after diagnosis) of ER+ disease in one study. Regular physical activity has been found to improve survival even among those reporting low physical activity prior to diagnosis. In addition, regular exercise plus a high quality breast cancer diet can overcome the negative effects of obesity on survival. In contrast, several studies have reported that very low physical activity is associated with less favorable survival among those with breast cancer.
Some observers have suggested that since healthier women are more able to exercise, this factor largely accounts for the observed differences in prognosis between survivors who exercise and those who do not. A large Chinese study was designed to test this theory by examining the association between exercise and breast cancer survival while taking account factors that restrict exercise participation. The level of exercise was assessed approximately six, 18, and 36 months after diagnosis in 4,826 women. Regular exercise during the first 36 months after diagnosis was found to be associated with 30% lower risk of death from any cause and 40% lower risk of breast cancer relapse than little or no exercise. The associations between exercise and mortality were not changed by taking into account other medical conditions, quality of life, or body size at baseline. In other words, participants generally benefited from exercise regardless of whether they were overweight or were not in good health.

How much exercise is enough?

Based on the available evidence, as little as half an hour per day of fast walking could have a meaningful impact on survival. A study that included 4,643 postmenopausal breast cancer survivors in the Women's Health Initiative study reported that women engaging in nine or more metabolic equivalent (MET) hours per week of physical activity (which amounts to approximately three hours per week of fast walking) after diagnosis had a 39% lower risk of breast cancer-specific death than inactive women.
Another U.S. prospective study included 1,490 women with early stage breast cancer who were enrolled in the study at two years after diagnosis, on average. The study was designed to investigate the associations between physical activity, diet, and obesity and survival after diagnosis. When the variables were examined separately, reduced mortality was found to be weakly associated with higher vegetable-fruit consumption, increased physical activity, and a body mass index that indicated neither low weight nor obesity. However, in multivariate analysis, only the combination of consuming five or more daily servings of vegetables/fruits plus accumulating at least nine MET hours per week was found to be associated with a significant survival advantage. The approximate 50% reduction in breast cancer metastasis risk associated with these behaviors was found in both obese and nonobese women.
While one study reported that running for exercise is associated with better survival than walking, there is some evidence that extremely high levels of physical activity might not be beneficial for breast cancer survivors. One study found similar steroid hormone levels in the least active and the most active postmenopausal women compared to those reporting moderate activity, suggesting a U-shaped relation. In other words, more is not necessarily better. Another study, which was designed to examine treadmill and wheel running effects on mammary tumor development in mice, reported that heavy exercise decreased survival in mice with tumors.

Mechanism of action

Regular moderate exercise appears to reduce breast cancer risk and improve survival outcomes by decreasing inflammation, reducing circulating estradiol and androgen levels, and inducing beneficial changes in insulin levels and insulin-related pathways. In addition, aerobic exercise can help decrease tumor hypoxia, a low-oxygen condition that renders solid breast tumors highly invasive and resistant to treatment. There is also evidence that exercise might boost immunity, as well as influence the regulation of tumor suppressor genes in ways that have favorable impact on survival, although not all studies are in agreement.

Exercise during chemotherapy and radiation

Breast cancer patients undergoing chemotherapy commonly report muscle weakness and increased fatigue. One study examined the impact of the chemotherapy drug Adriamycin (doxorubicin) on spontaneous physical activity and swimming performance of mice. Adriamycin was found to cause a significant decline in the integrated swimming activity of the mice over a 40 minute period. Adriamycin also significantly reduced the total number of spontaneous movements of the mice during a 24 hour period. The authors concluded that chemotherapy may significantly decrease the motivation and endurance for physical activity. On the other hand, another study concerning the effects of Taxotere (docetaxel) on the strength of hind limb muscles in mice reported little or no impairment in back leg force production.
In fact, most women reduce their level of physical activity during chemotherapy. However, light to moderate exercise during chemotherapy has been shown to reduce fatigue. In addition, aerobic exercise appears to protect against Adriamycin-induced heart damage.
On the other hand, intense exercise during chemotherapy or radiation could interfere with treatment effectiveness. One study compared the outcomes of Adriamycin-treated mice who were exercised or not. Moderate-intensity endurance exercise did not significantly influence Adriamycin-induced mammary tumor growth inhibition. However, there was a trend for longer survival for mice treated with Adriamycin only, suggesting that exercise training inhibited Adriamycin's efficacy. Another study reported that intense or prolonged physical activity a couple of days before the start of radiation or chemotherapy has significant potential to reduce the benefits of the treatments. Based on the available evidence, light to moderate aerobic exercise appears beneficial during such treatment, but heavy or prolonged exercise should be avoided.
Below are links to 20 recent studies concerning this topic. For a more complete list of studies, please click on exercise.