A new study at Yale shows how exercise can slow tumor growth.

Rachel Perry, an associate professor of medicine at Yale School of Medicine, answers the question in her newest study, published in the journal “Proceedings of the National Academy of Sciences”: Why does exercise seem to protect people against cancer?

In the study, Perry’s team used metabolic tracers in mouse models of breast cancer and melanoma to learn how glucose — a nutrient that tumors use to fuel rapid cell division — is distributed when the body is active.

They found that working muscles effectively outcompete tumors for the glucose supply, causing tumors to receive less of the fuel they need to grow.

Perry hopes her work will pave the way for studies in people.