“It’s the first time we’ve shown that exercise can directly control the growth rate of tumours,” says Pernille Hojman of the University of Copenhagen in Denmark. Her team tested the effects of exercise on five different types of mouse cancer, including skin, lung and liver cancer.
They found that exercise prompts the release of adrenaline. This stress hormone in turn stimulates the immune system to send its cancer-fighting natural killer cells into the bloodstream. A substance called interleukin-6, released by exercising muscles in the mice, directs these killer cells to attack the tumours.
None of the tumours shrank after exercise, they only grew less quickly, suggesting that exercise is unlikely to reverse an existing cancer. But in some animals, running did prevent tumours from growing in the first place.
“We already know that exercise has an impact on natural killer cell activity, but this is the first time anyone has shown it’s directly involved in helping them invade tumours,” says Lee Jones of the Memorial Sloan Kettering Cancer Center in New York. “This is a big piece of the puzzle that’s been missing,” he says.
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