Effect of amount and type of exercise on experimentally induced breast cancer.

It appears that exercise derived by treadmill running can enhance or inhibit the development of mammary cancer depending on the intensity and duration of the activity. The effects of timing of exercise relative to the phases of carcinogenesis and the frequency of exercise on the tumorigenic response in the mammary gland have yet to be studied. What is needed are further investigations of a spectrum of exercise conditions that exert differential effects on measures of physical fitness, energy intake, body composition, and/or the efficiency of utilization of carbohydrate, fat, and protein as energy substrates. This should facilitate identification of critical relationships between physical activity and the risk for cancer. This goal can best be achieved initially through carefully controlled laboratory studies using appropriate experimental systems. There are several mechanisms by which exercise could alter the course of tumor development. A goal in this field should be to first characterize the influence on tumorigenesis of amount and type of exercise that have differential effects on metabolism and thereafter proceed to investigate the basis for the effects observed. Experiments should be designed to dissociate effects of exercise related to local and/or systemic changes induced by skeletal muscle contraction from those attributable to changes in energy expenditure. Ultimately, investigations of the type proposed should allow for determination of whether the quantity and quality of exercise needed to attain health-related benefits for chronic diseases such as cancer differs from what is recommended for fitness benefits. This should also permit formulation of a specific set of recommendations about the amount and type of physical activity that in concert with appropriate dietary practices can significantly reduce the risk for cancer.