Decades of research has shown exercise helps fight cancer. If you are physically active, you can reduce your risk of getting cancer. If you already have cancer, exercise can stave off the tumor. And, if you’ve beaten the tumor, putting in hours at the gym can delay or even prevent the cancer from making a return.

However, the mechanism of how exercise works to fight cancer has never been well understood. A new study in mice, published in Cell Metabolism, says that the clue may lie in the levels of adrenalin that exercise produces.

Researchers at Copenhagen University gave a group of mice a running wheel in their cages and another group no such prop. Because mice like to run, the group with the running wheel would voluntarily exercise a lot more than the other group.

Mice from both groups were then induced to develop one of three types of cancer: an injection of melanoma cells under the skin to induce skin cancer, an injection of melanoma cells in the tail to induce lung cancer, or an injection of diethylnitrosamine to induce liver cancer.

After four weeks, the mice that had access to a running wheel showed much better outcomes than those that didn’t. Among the mice that exercised, only a third of those injected with diethylnitrosamine developed liver cancer. In comparison, three-quarters of the sedentary mice got the cancer. All those induced with skin and lung cancer contracted the diseases, but those who exercised had smaller tumors.

An analysis of the tumors showed the mice that exercised had more of two types of cancer-killing immune cells—double the number of T-cells and five times the number of natural killer cells. But when mice were engineered to stop production of T-cells, those that exercised still had smaller tumors those that didn’t. However, when the researchers engineered mice to stop production of natural killer cells, then both the groups that exercised and didn’t exercise had similar-sized tumors. Thus, natural killer cells were endowing the benefits and not T-cells.

Running produces adrenalin, and it is known that adrenalin could increase production of natural killer cells. So the researchers injected half the mice who hadn’t exercised with adrenalin and other half with a saline solution. Those who got the saline solution saw no difference. Those who got adrenalin showed a reduction in tumor growth, but the gains weren’t as much as those seen in the mice that exercised.

So the researchers investigated one other type of chemical called interleukin-6, which increased in the mice that exercised. When the same experiment of injecting sedentary mice with adrenalin was repeated with the addition of interleukin-6, the reduction in tumor growth was found to be the same as that in the mice that exercised.

Of course mice are not humans, but the researchers believe that this mechanism is at work in humans too. They are now looking at the potential of using adrenalin and interleukin-6 as anti-tumor drugs. What is the takeaway for the rest of us? Now we have one more reason to exercise.