Table 2

Possible biological mechanisms for physical activity and cancer prevention

Cancer siteMechanismRationale
ColonDecreased gastrointestinal transit timePhysical activity increases gut motility, reduces mucosal exposure time to carcinogens.
Decreased ratio of prostaglandins (PGE:PGF)Strenuous exercise may increase PGF,a which inhibits colonic cell proliferation and increases gut motility while not increasing PGE2, which has the opposite effect on colonic cell proliferation from PGF.
Lowered bile acid secretion or enhanced metabolismBile acid concentrations may be decreased in physically active (confounding by diet?) persons.
Decreased circulating insulin and glucoseIGF-I and IBFBP may be decreased with increased levels of exercise.
BreastDecreased lifetime exposure to estrogenPhysical activity delays onset of menses, prolongs menstrual cycle, reduces number of ovulatory cycles, reduces ovarian estrogen production. Physical activity reduces body fat and could reduce fat-produced estrogens and protect against breast and endometrial cancer. Physical activity increases the production of sex hormone binding globulin, which results in lower free estradiol levels and may reduce breast cancer risk.
ProstateReduced exposure to testosteronePhysical activity increases production of sex hormone binding globulin, which results in low free testosterone levels that may alter prostate cancer risk.
All cancers, especially breast, endometrial, and ovarianDecreased percent body fatObese women have increased infertility which may increase breast cancer risk. Fat storage of carcinogens can occur in visceral fat which can be released in overweight individuals.
All cancersGenetic predisposition of habitually activeConstitutional factors influence athletic selection or interest in physical activity and susceptibility to cancer.
Exercise-induced increase in antitumor immune defensesExercise may increase number and/or activity of macrophages, lymphokine-activated killer cells, and their regulating cytokines; may increase mitogen-induced lymphocyte proliferation.
Improved antioxidant defense systemsStrenuous exercise increases the production of free radicals, whereas chronic exercise improves free radical defenses by up-regulating both the activities of free scavenger enzymes and antioxidant levels. Extent of exercise-induced changes in oxidant defense unknown.
  • a PGF, prostaglandin F; PGE2, prostaglandin E2.