If you're trying to think up another excuse for skipping your daily exercise, here's another reason to head to the gym or nearest walking track — mice with cancerous tumors who spent their free time on a running wheel saw the size of their tumors shrink by 50 percent when compared to less active mice.

Researchers at the University of Copenhagen implanted mice with lung, liver, or skin tumors. They found that the adrenalin surge produced by exercise helped move cancer-killing immune (NK) cells toward the tumors.

"It is known that infiltration of natural killer (NK) immune cells can control and regulate the size of tumors, but nobody had looked at how exercise regulates the system," says senior study author Pernille Hojman. "In our experiments, we tried to inject our mice with adrenaline to mimic this increase you see during exercise, and when we do that we see that the NK cells are mobilized to the bloodstream, and if there's a tumor present then the NK cells will find the tumor and home to it."

Hojman and her colleagues next used mice depleted of NK cells to show that the increase in number of NK cells at the site of the tumor contributed directly to its reduction. Even with exercise and a complete set of other immune cells, the mice experienced normal cancer growth without the NK cells. Blocking the function of adrenaline also blunted the cancer-killing benefits of the running wheel.

Dozens of studies have shown that exercise helps prevent cancer, and the Danish study was not the first to suggest that exercise benefits those who already have the disease or those who are survivors. Studies show that breast cancer survivors have less risk of recurrence and live longer if they exercise, and other studies show that colorectal cancer survivors also live longer if they exercise.

"As someone working in the field of exercise and oncology, one of the main questions that cancer patients always ask is: How should I exercise? Can we do anything?" Hojman says.

"While it has previously been difficult to advise people about the intensity at which they should exercise, our data suggest that it might be beneficial to exercise at a somewhat high intensity in order to provoke a good epinephrine surge and hence recruitment of NK cells."

The study was published in Cell Metabolism.