Study Reveals Surprisingly Obvious Reason Why Exercise Reduces Cancer Risk

There is plenty of evidence that doing more exercise can lower cancer risk - but what actually links the two?

A new mouse study suggests the connection could lie in a metabolic shift that seems to give muscle cells extra fuel to use, while leaving cancer cells short of the energy they need to expand.

How the Yale University mouse study on exercise and cancer was set up

Researchers at Yale University examined metabolic responses in mice with breast cancer or melanoma tumours, dividing the animals into groups according to diet and how much exercise they did.

To track what happened to glucose in the body, the team used molecular tracers to pinpoint where it was metabolised. Those results indicated that active mice were, in effect, redirecting energy supplies away from cancer cells and towards muscle tissue.

Smaller tumours in active mice on a high-fat diet

After four weeks, mice on a high-fat diet that exercised routinely developed markedly smaller tumours than mice on the same diet that remained inactive.

"Obese mice which underwent four weeks of voluntary wheel running after tumor injection exhibited nearly a 60 percent reduction in tumor size," Yale University physician-scientist Brooks Leitner and colleagues report in their published paper.

Beyond tumour size, the researchers found wide-ranging molecular differences: 417 metabolism-related genes showed different patterns of expression in active mice compared with sedentary, but lean, mice.

Taken together, these shifts suggested the tumours had been pushed into a high-stress survival state.

Glucose, mTOR, and a possible tumour-suppressive pathway

In particular, exercise substantially reduced levels of a protein known as mTOR inside the animals’ tumours - a change that may be important for limiting growth and could help guide the search for new treatments.

The researchers argue that glucose is "a key metabolic mediator of the tumor-suppressive effects of exercise".

However, they also note that "this metabolic relationship and the ability for exercise to slow tumor growth may depend on exercise duration."

What this does (and doesn’t) mean for people

Cancer, across its many forms, is highly complex, and tumour growth and establishment involve multiple biological mechanisms. Gym visits alone will not prevent cancer.

Even so, physical activity may play an important part in improving the odds of stopping the disease from developing. In the same study, obese mice that exercised for two weeks before tumours were implanted ended up with smaller tumours than obese mice that stayed sedentary.

"These data highlight the importance of a nuanced, systemic view of the metabolic effects of exercise in cancer," Leitner and colleagues write.

It is also promising that similar biological patterns appeared in two different tumour types, indicating that the benefits of exercise may not be confined to a single cancer.

That said, researchers still need to determine whether the same processes occur in humans before it is clear how well these findings translate to us.

To move that work forward, the team wants to extend the research to human cancer tumours, with clearer definitions of exercise type and duration. This should help clarify precisely how staying active may protect against cancer.

"Examination of the role of fitness on the molecular pathways altered by exercise may uncover new therapeutic targets in precision oncology, particularly in patients who cannot tolerate exercise," the researchers conclude.

The research has been published in PNAS.