Exercise is well known as a way to protect against America's top killer, heart disease. Now, physical activity is increasingly being studied as a possible weapon against the second most common cause of death: cancer.
Most experts agree that regular exercise can reduce the risk of colon cancer, and there is a growing consensus that it may also lower the risk of breast cancer. For people with cancer, there is a burgeoning number of exercise programs available, primarily to help relieve unpleasant side effects of the disease and its treatments and to enhance the overall quality of life. And there is some suggestion that physical activity may slow the course of the disease.
Two new studies, presented at the recent annual meeting of the American College of Sports Medicine in Indianapolis, add to the mounting evidence that regular exercise may exert an "anticancer" effect. Both reports are from the Cooper Institute for Aerobics Research in Dallas and both suggest that men with high fitness levels are less likely to die of cancer.
"We found that men who were classified as unfit [based on treadmill tests] were 80 percent more likely to die of cancer than fit men were," says Carolyn Barlow, a biostatistician who examined the relationship among cardiovascular fitness, body mass index (BMI) and cancer mortality in 22,703 men, ages 20 to 85, in a 10-year prospective study.
Men who were unfit and obese were at greatest risk, with a 2.6 times greater chance of dying of cancer than fit, normal-weight men. This is not surprising since excess body fat is known to increase the risk of certain cancers -- including colon, rectum, prostate, endometrium, kidney and breast (among postmenopausal women). Barlow's study, however, revealed that low fitness -- not fatness -- was more predictive of cancer death: If a man was fit, being overweight did not raise his cancer risk.
"It is possible to be overweight and fit," says Barlow, who notes that genetics plays a strong role in a person's body size and shape. "Our data suggest that low cardiovascular fitness may be viewed as a more important predictor of all-cancer mortality than increased BMI in men."
The second study focused on the role a man's fitness level plays in his risk of dying of lung cancer. The 25,883 male participants, ages 30 to 89, were members of the institute's Aerobics Center Longitudinal Study--a group with a low prevalence of cigarette smoking, notes principal author Larry Gibbons. Just 18 percent were smokers and 37 percent were past smokers.
Survival of the Fittest
"But even with adjustments for age, smoking habit, alcohol intake and other potential risk factors, we found that the fitter a man was, the lower his risk of dying of lung cancer," says Gibbons, who is medical director of the Cooper Clinic, the medical center affiliated with the institute. "Our research shows that if you're a smoker, your risk of dying of lung cancer is cut in half if you are in the high-fit category." If you're a past smoker, he says, your risk is a third. If you've never smoked, your risk in the high fit category is one-fourth.
This study is one of only three to examine the link between physical activity and lung cancer, Gibbons notes. "Our findings suggest that fitness may protect the lungs against the ravages of cigarette smoke," he says. They also indicate "that a sedentary lifestyle may be one of the factors contributing to the development of lung cancer."
While the mechanisms by which exercise appears to protect against cancer aren't known, current theories point to physical activity's effect on the immune system, the nervous system and the endocrine system. "Stress plays a role in many diseases, including cancer, and physical activity helps relieve stress," Gibbons says. "There is also some evidence that moderate physical activity prevents the formation of free radicals that can damage DNA and be a precursor to cancer."
Theories differ for the various types of cancer, notes Rachel Ballard-Barbash, a physician specializing in nutrition, preventive medicine and epidemiology at the National Cancer Institute in Bethesda, Maryland. For example, she says one reason regular exercise is believed to reduce the risk of colon cancer by 50 percent is activity's role in speeding food through the bowel, which shortens the time carcinogens in fecal matter come in contact with cells that line the colon.
"For breast cancer, theories point to exercise's affect on hormonal metabolism and production," Ballard-Barbash notes. "Among elite women athletes there is evidence that regular physical activity leads to longer periods between ovulation and lowers levels of endogenous (naturally-occurring) estrogen, which may lower cancer risk." There's also some evidence that physical activity delays the age of menarche, she says, a factor associated with reduced breast cancer risk.
Figuring Out a Regimen
Scientists do not yet know specifics of an exercise prescription to reduce cancer risk. "For breast cancer it may be important to encourage physical activity in young girls," Ballard-Barbash says. For cancers related to body fat, the physical activity recommendation would likely be similar to one used for weight loss and maintenance.
"In a world that is increasingly sedentary, it's important to build physical activity into our lives as a habit and do something daily," Ballard-Barbash says. "Aerobic activity is probably most important, and simply walking more is a good place to start."
The American Cancer Society (ACS) urges people to follow the U.S. Surgeon General's advice to accumulate 30 minutes of moderate physical activity on most days of the week. "We encourage people to find some activity they like to do, then do it," says Colleen Doyle, director of nutrition and physical activity for ACS, which in 1996 added regular physical activity to its list of recommended preventive measures.
"We also suggest that people incorporate physical activity into their daily life, by taking the stairs or doing yard work," she says. "Moving regularly, even in small amounts, can add up."
Resources
Carol Krucoff is co-author, with her husband, Mitchell Krucoff, M.D., of "Healing Moves How to Cure, Relieve and Prevent Common Ailments with Exercise" (Harmony Books, 2000).
Originally published in The Washington Post on July 11, 2000
