Exercise, natural immunity, and tumor metastasis.

Exercise has been shown to reduce the growth of primary tumors and to enhance certain aspects of host natural immunity. The question of whether these are independent phenomena or are casually related has not been systematically evaluated. This paper presents information concerning the methodological difficulties in studying proposed relationships between exercise and cancer, focusing specifically on tumor metastasis, the process by which malignant cells disseminate to distant organs and establish new colonies. This paper also focuses on how natural immune processes and tumor cells exert bidirectional influences on each other. It is suggested that the direction of the impact of exercise on the control of metastatic spread of neoplastic cells will reflect, in part, the sensitivity of the specific tumor to cytolysis by natural immune mechanisms, the route of dissemination, the timing of exercise relative to tumor exposure, and whether exercise acts as a distress or eustress state.