Mice that spent their free time on a running wheel experienced a 50% reduction in tumor growth, size and incidence, compared with their lazier counterparts, says research published in Cell Metabolism.

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Running on a wheel reduced the size of tumors in mice.

Previous studies have indicated that exercise can be effective against certain cancers and that it can reduce the risk of recurrence. It can also help improve functionality and patient outcomes.

Scientists have evidence that cancer-killing natural killer (NK) immune cells can infiltrate, control and regulate the size of tumors.

But how this happens and what effect exercise has on the system has remained unclear.

Now, researchers say that a high-intensity workout could lead to a surge of adrenaline that helps to move NK cells toward lung, liver or skin tumors.

Senior study author Pernille Hojman, of the University of Copenhagen, in Denmark, and her colleagues injected mice with adrenaline to mimic the increase in the hormone that happens during exercise.

When they did this, they observed that NK cells moved toward the bloodstream. If a tumor were present, the NK cells would find it and home in on it.

Adrenaline prompts IL-6 to call NK cells to attack the tumor

Next, they used mice bred without NK cells to demonstrate the link between NK cells appearing at the tumor site and the shrinking of the cancer.

In the absence of NK cells, even if the mice exercised and had a full set of other immune cells, the cancer grew normally.

When they blocked the adrenaline supply, this also dampened the cancer-killing benefits of the running wheel.

The link between adrenaline-dependent mobilization of NK cells and tumor infiltration was identified as the immune signaling molecule, IL-6. Muscle tissue is known to release IL-6 during exercise.

What surprised the researchers was that adrenaline specifically called for IL-6 sensitive NK cells, and that the IL-6 molecules help to guide the immune cells to the tumors.

Hojman comments: "In this study we show that the exercise-induced IL-6 seems to play a role in homing of NK cells to the tumor and also in the activation of those NK cells." But she adds that IL-6 and its role in tumor biology can be controversial.

The findings could help patients who are looking for inexpensive ways to manage their cancer, although more research is needed into how exercise impacts metastasis and life expectancy and to ascertain whether humans will react in the same way as mice.

Hojman hopes to explore the combined effect of anti-cancer treatments and exercise on tumors.

She says:

"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? 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."

Medical News Today recently reported that enhanced NK cells have been created in labs that can help reduce cancer in lymph nodes.