Updated project metadata. Exercise prevents cancer incidence and recurrence, yet the underlying mechanism remains mostly unknown. Here we report that exercise induces a metabolic reprogramming of internal organs, increasing their nutrient demand, thus protecting them against metastatic colonization by limiting nutrient availability to the tumor, which we termed exercise induced metabolic-shield. Proteomic and ex-vivo metabolic-capacity analyses of mice internal organs revealed that exercise induces catabolic processes, glucose uptake, mitochondrial activity, and GLUT expression. Proteomic analysis of routinely active subjects' plasma demonstrated increased carbohydrate utilization following exercise. Further, epidemiological data of a 20-year follow-up of a large human cohort of cancer free participants, revealed that exercise prior to cancer initiation has a modest impact on cancer incidence in low metastatic stages, but significantly reduced the likelihood of highly metastatic cancer as compared to inactive participants. We utilized two exercise protocols, wherein the mice were subjected to exercise either both pre- and post- melanoma injection or only pre-injection. We found that exercise prior to cancer injection significantly protects against metastases in distant organs, regardless of the three melanoma metastases models we used. Mechanistically, ex-vivo inhibition of the mTOR pathway, via rapamycin treatment, reversed the protection from the exercise induced metabolic-shield. Under limited glucose conditions, active stroma consumed significantly more glucose, at the expense of the tumor, as reflected in histology analyses of mice melanoma metastases. Our data suggests that cancers’ metabolic plasticity and the exercise-induced metabolic reprogramming of the stroma are clashing, raising the opportunity to block metastases by challenging cancers’ metabolic needs.