Aerobic exercise is no friend to breast tumors, says a new study that suggests regular physical activity may be a “novel adjuvant treatment” for women with breast cancer.

New research conducted on mice finds that a body that gets regular physical activity is a more hostile environment for cancer’s growth in breast tissue than is a sedentary body. And regular aerobic exercise makes a tumor more vulnerable to the effects of chemotherapy.

The latest study, published Monday in the Journal of the National Cancer Institute, echoes earlier work by the same authors on the effects of exercise on prostate tumor growth and treatment.

In lab mice that were injected with breast cancer cells, researchers compared the tumors of those given a running wheel with the tumors of mice who had no access to an exercise wheel and were largely sedentary. The mice who had running wheels exercised regularly, and their tumors grew more slowly, experienced more cell-death, and developed a more robust network of blood vessels than did the tumors of the sedentary mice.