Effect of exercise intensity and duration on the induction of mammary carcinogenesis.

Physical activity is defined as bodily movement due to skeletal muscle contraction that results in quantifiable energy expenditure. Both epidemiological and laboratory data indicate that the level of physical activity in which an individual engages may affect cancer risk. Exercise is distinguished from other types of physical activity by virtue of the fact that the intensity, duration, and frequency of the activity(s) is specifically designed to improve physical fitness. Based on available data, a role for exercise in specifically reducing cancer risk has been conjectured and is referred to as the exercise-cancer hypothesis. However, the amount of scientific evidence in support of this hypothesis is still quite limited, and there are conflicting reports about the nature of the association. The exercise-cancer hypothesis was evaluated in two experimental animal models for breast cancer using treadmill running as the exercise paradigm. The data presented indicate that both the intensity and duration of exercise affect the development of experimentally induced breast cancer. In general, as exercise intensity increased, the likelihood that such physical activity inhibited carcinogenesis increased. Exercise at lower intensities resulted in either inhibition, no effect, or enhancement of the tumorigenic response depending on the duration of exercise. Inhibitory conditions of exercise affected both the initiation and promotion/progression stages of the disease process.