Exercise has long been known to reduce the risk for many types of cancer, as well as heart disease, diabetes, and dementia. But precisely how working out combats cancer has remained largely unknown.

But new research suggests a reason people who are physically fit have a lower cancer risk: Working out may change how the immune system targets tumors by boosting adrenaline, certain immune cells and other chemicals that work together to reduce the severity of the disease or fight it off altogether, The New York Times reports.

The study, by scientists at the University of Copenhagen in Denmark and other institutions, involved laboratory mice, but researchers say it holds implications for people, too.

For the study, published this month in the journal Cell Metabolism, the scientists implanted melanoma skin cancer cells into the mice then providing half of them with running wheels in their cages while the other animals remained sedentary.

After four weeks, far fewer of the runners had developed full-blown melanoma than the sedentary mice and those that did develop skin cancer had fewer and smaller lesions. They also were less prone to tumors that spread to other parts of the body, even when scientists injected some of the cancer cells into their lungs to stimulate metastases.

In short, running seemed to have at least partially inoculated the mice against the cancer.
The scientists then examined blood from both the exercising and sedentary animals and cells from any tumors in both groups. They found much higher levels of the hormone adrenaline in the blood of the exercising animals as well as interleukin-6 —a substance released by working muscles that is believed to regulate inflammation in the body.

In addition, they found higher levels of immune system “killer cells” — known to be potent cancer fighters — in the blood of running mice.

“We [found] that voluntary wheel running in mice can reduce the growth of tumors, and we have identified an exercise-dependent mobilization of natural killer cells as the underlying cause of this protection,” said Pernille Hojman, a researcher at the University of Copenhagen who oversaw the new study.

She added that “the mechanisms” that seemed to partially protect the running mice in this study from malignancies, “can also happen in people.”