Various reasons have been suggested for indirect associations between physical activity and an altered overall risk of cancer, including linkages of vigorous exercise with certain types of body build, adoption of an overall healthy lifestyle and modification of habitual patterns
of physical activity by associated or undetected disease. Nevertheless, both animal experimentation and human epidemiological studies support the view that regular moderate physical activity can have a more direct favorable influence on susceptibility to various types of cancer.
A wide variety of mechanisms has been proposed, puts forth a review article in Sports Medicine, including modifications of diet, bodyfat content, hormone levels end immune function, the extent of bodyfat stores and associated variation in gonadal function seem to be major influences, particularly with respect to tumors of the reproductive tract. Much further research is needed to determine which are the dominant underlying mechanisms for the different types of tumor.
Nevertheless, the influence of habitual physical activity upon overall susceptibility is sufficient that we can already recommend regular moderate physical activity as a significant component of public health policy aimed at reducing the incidence of cancer.