They report that training mice regularly on a wheel (the mouse version of a treadmill) decreased the growth of multiple types of tumors, including skin, liver, and lung cancers. Also, mice that exercised regularly had a smaller chance of developing cancer in the first place, and less cancer-associated weight loss.
The researchers linked the anti-cancer effects to the release of adrenaline (also called epinephrine), a hormone central to the mammalian “fight-or-flight” response. Exercise stimulates the production of adrenaline, which mobilizes immune cells, including one called a “natural killer.” NK cells are recruited to the site of the tumor by the protein IL-6, which is secreted by active muscles. NK cells can then infiltrate the tumor, slowing or completely preventing its growth.
The researchers pointed out that injecting the mice with adrenaline or IL-6 without the exercise was not sufficient to inhibit cancer development, showing that regular exercise was needed to activate the hormone and the protein.