Recent studies delineate the effect of exercise on specific cancers but much more research on the role exercise plays in the prevention and treatment of cancer is needed, according to Susan Oliveria, ScD, MPH, director of epidemiology at the Strang Cancer Prevention Center in New York City. Dr. Oliviera addressed the second Strang International Cancer Prevention Conference co-sponsored by Cornell University Medical College and held in New York last November. Preventive efforts are most promising for cancers whose causes are environmental, she added.
Though the protective role of physical activity in cancer development was hypothesized as early as 1922, the biological mechanisms involved are still not fully understood, said Dr. Oliveria. However, improved immune response has been observed after moderate exercise, so it may be that an increase in killer cells (NK and LAK) are partly responsible. Assumptions can also be made, and in some instances have been confirmed, about the beneficial effects of exercise on specific cancers. For instance, a recent study led by Dr. Inger Thune of the Institute of Community Medicine at the University of Tromso in Norway found that exercise patterns predicted breast cancer risk independent of other risk factors. Those who exercised at least four hours a week had a 37% lower risk of developing breast cancer compared to those who were sedentary. Exercise, the researchers found, reduces the amount of estrogen pumped out by a woman's ovaries and the hypothesis is that the less estrogen a woman is exposed to, the lower her risk of developing breast cancer. For colon cancer, because physical exercise decreases gastrointestinal transit time, it may reduce exposure to carcinogens in the gut that contribute to that cancer. Says Dr. Oliveria, "There is an observable trend suggesting that the greater the physical activity, the greater the decrease in risk."
Overall, exercise affects energy balance leading to a reduction in body fat and weight. Furthermore, Dr. Oliveria said, consistent physical exercise often occurs in conjunction with other healthy behaviors that include avoidance of cigarettes and alcohol(Drug information on alcohol), and better nutrition--factors which may decrease cancer risk.
Dr. Oliveria noted that only a few studies have examined the connection between exercise level and specific cancers. The effect of exercise on prostate cancer is still being evaluated. Questions for that cancer range from how long the antitestosterone effect of exercise lasts to whether there is a meaningful association between testosterone level and prostate cancer risk. Dr. Oliveria cautions, however, that prostate cancer has a long induction period, developing at an age when testosterone levels have already decreased-- "We may simply be assessing activity level at the wrong period of life." Studies of the cancer-exercise connection for lung, kidney, gastric, brain, testicular cancers, melanoma, Hodgkin's and non-Hodgkin's lymphomas, and leukemia have been inconclusive.
But another area that Dr. Oliveria believes warrants serious study is the role of exercise in cancer therapy-- "Can it restrict metastases, prevent recurrence, or be used effectively in the palliative treatment of established cancers?" Exercise does produce a positive psychological effect on cancer patients, and can inhibit the wasting and lean muscle loss associated with the disease but strenuous exercise could also be detrimental. High-intensity activity decreases the number of killer cells and produces higher levels of circulating free radicals. Clearly, concluded Dr. Oliveria, further investigation in this area is needed.