Regular moderate exercise improves survival among women with breast cancer, according to numerous studies. Exercise reduces inflammation, decreases circulating estradiol (E2) and androgen levels, and induces beneficial changes in insulin levels and insulin-related pathways. Exercise also appears to boost immunity and influence the regulation of tumor suppressor genes in ways that are favorable.
In addition, aerobic exercise can help decrease tumor hypoxia, a low-oxygen condition under which solid breast tumors can thrive. Now a new study has reported that exercise can slow the establishment of breast tumors and metastases in a mouse model of breast cancer.

Exercise improves breast cancer prognosis

Women who are physically active after completing treatment for breast cancer are more likely to survive, even those reporting low levels of physical activity prior to diagnosis. The amount of exercise required to improve prognosis is modest — only about half an hour per day of brisk walking. However, higher levels of physical activity are likely to have a greater positive impact on prognosis. In addition, regular exercise plus a high quality diet can overcome the negative effects of obesity on survival. On the other hand, women who reduce their level of physical activity after diagnosis increase their risk of breast cancer-specific mortality two-fold, according to one study.
Nevertheless, there is some evidence that extremely high levels of exercise (such as long distance running) are not desirable, whether during treatment with chemotherapy or radiotherapy, or after such treatment. Such physical activity has the potential to interfere with treatment and could, according to preliminary evidence, promote tumor growth in some cases.
Some observers have suggested that since healthier women are more able to exercise, this factor might account for the observed differences in prognosis between survivors who exercise and those who do not. However, a study that examined this question reported that the associations between exercise and survival were not modified by taking into account other medical conditions, quality of life, or body size. In other words, the study participants benefited from exercise even if they were overweight or were not in good health.

Latest research finds exercise reduces tumor growth and metastasis

The study referenced above was designed to investigate the effects of exercise on tumor growth and metastasis in a mouse model of breast cancer. The authors hypothesized that exercise reduces the breast cancer-promoting effects of hyperlipidemia (high levels of cholesterol, triglycerides and other fats in the blood) by normalizing the tumor microenvironment and enhancing the immune response by improving T cell infiltrate.
To conduct the study, the authors used ApoE-/- mice, which are hyperlipidemic. The mice were injected with EO771 tumor cells and randomly assigned to either an exercise group (who engaged in voluntary wheel running) or a sedentary control group (who were not provided with exercise equipment). The level of exercise was measured using muscular cytochrome C oxidase subunit IV (COX-IV) expression, a marker of physical activity.
Tumors in mice with high levels of physical activity grew more slowly than those in sedentary mice, taking significantly longer to reach 100 mm3. However, there was no difference in tumor growth between mouse groups once the tumors were well established. Wheel running appeared to reduce the number and size of metastases. However, exercise did not appear to improve the immune response—it did not affect T cell infiltrate or the proportion of regulatory and cytotoxic T cells within the tumor. On the other hand, tumor hypoxia was found to be significantly reduced in mice with high levels of exercise.
The authors conclude that wheel running can slow the establishment of primary and secondary EO771 breast tumors and induce beneficial changes in the breast tumor microenvironment in ApoE-/- mice.
Please see our article on exercise for more information.