We now have a greater understanding of how exercise slows cancer

Exercise can slow tumor growth in mice by changing the body's metabolism so that muscle cells, rather than cancer cells, take up glucose and grow. A similar process can occur in humans.

It is well known that exercise is associated with a lower risk of cancer and that fitter individuals are more likely to survive, but the mechanisms behind this are only partially understood. Some effect of exercise appears to be occur through effects on the community of microbes in our intestines And through its effect on the immune system.

To explore another possible route, Rachel Perry from the Yale School of Medicine and her colleagues injected breast cancer cells into 18 mice (12 of which were on an obesogenic diet that worsens the progression of some types of cancer) and let half the animals run on a stationary bike for as long as they wanted.

They found that after four weeks, tumors in obese mice that chose to exercise were 60 percent smaller than in obese mice that did not have access to the impeller, and also slightly smaller than in sedentary mice that received a normal diet. The 30-minute workout was associated with an increase in oxygen uptake and the key energy source glucose in skeletal and cardiac muscle, as well as a decrease in glucose uptake by tumors.

“This work shows that aerobic training fundamentally changes the metabolic competition between muscle and tumors,” says Perry. “It's important that the exercise intervention was voluntary. We're not talking about training for a marathon-type exercise, just what the mice wanted to do.”

The researchers analyzed gene activity and identified 417 genes in key metabolic pathways in mice that are altered by exercise. Essentially, this means that muscle tissue metabolizes more glucose and tumor tissue metabolizes less.

In particular, the researchers say that suppressing mTOR, a protein involved in cell growth, in cancer cells may limit tumor growth.

Perry says that because these metabolic pathways are so similar in mammals, she expects this phenomenon to extend to humans, including non-obese people. Indeed, the researchers say similar changes in gene activity were observed in people with cancer during exercise.

“This is another mechanism demonstrating how exercise creates a more cancer-suppressive environment,” says Rob Newton at Edith Cowan University in Perth, Australia. “We need human clinical trials, but I really don't see any reason why there wouldn't be a similar effect in humans.”

Perry says metabolism occurs in all tissues and is influenced by both the microbiome and the immune system. “It is possible that metabolic changes may be the missing link between exercise, the microbiome and immune system, and tumor growth,” she says. “But I would be shocked if the beneficial effects of exercise were not due to multiple mechanisms.”

The work also helps explain why low muscle mass increases the risk of death from cancersays Newton. “If muscles preferentially take up glucose, if you have more muscle mass and activate it more regularly, then the effect will be greater.”

He believes people should view exercise as a cancer cure to be used alongside other treatments, rather than as a potentially beneficial lifestyle change. “A lot of it is about identifying what causes the greatest risk of cancer and how to specifically target it,” Newton says. “This could improve cardiorespiratory endurance, but if the patient has exceptionally low muscle mass, then this probably needs to be addressed with resistance training first.”