Can exercise prevent cancer? The answer is yes, in some types.

Can Exercise Prevent Cancer?

Steven N. runs for his life. Every day, with rhythmic regularity, his feet pound the pavement from his house to the park and back. Through spring rain and autumn leaves, winter cold and summer heat, Steven runs not only because he likes being fit, but also because he believes that exercise will protect him from the ravages of diseases such as heart disease and cancer. Is Steven right? Can exercise protect him from these devastating diseases?

Scientific studies have shown that exercise is associated with longevity and can protect against heart disease, the number one killer in the United States, but the effect of exercise in preventing cancer, the second leading cause of death, is less certain. Although the study of exercise and cancer is still in its infancy, what is becoming increasingly clear is that many cancers are preventable through exercise.

Life-saving Josb. An early study, led by Dr. David Garabrant at the University of Southern California in 1984, explored the association of colon cancer with exercise on the job. Men with sedentary jobs, such as accountants, lawyers, musicians, and bookkeepers, were found to have a 60 percent greater risk of colon cancer compared to men with active jobs, such as carpenters, plumbers, gardeners, and mail carriers.

Subsequent studies have confirmed that occupational physical activity reduces the risk of colon cancer. But this does not necessarily mean that you should give up your desk job to become a lumberjack. Those of us with more sedentary jobs can get adequate protection by building exercise into our schedules.

Exercise Promotes Colon Health. Looking at total daily physical activity from both occupational and recreational sources, Richard Severson of the University of Minnesota found that with increasing levels of physical activity there is a decreasing risk of colon cancer. The Japan-Hawaii cancer study, a large ongoing study with which he worked for two years, charted the occurrence of cancer in more than 8,000 Japanese men living in Hawaii.

The results, according to Dr. Severson, are fairly conclusive. "What a person can take home from our study is that it looks like increasing physical activity does decrease the risk of colon cancer," he says. But he also cautions individuals to "check with their own physicians to see if increasing physical activity is a reasonable thing to do in their own individual case."

How does exercise protect against colon cancer? One theory is that exercise stimulates the muscle movement of the large intestine. This movement, called peristalsis, transports the contents of the large intestine through and out of the body. Fecal matter contains toxic chemicals that can promote cancer. The less time the colon wall is in contact with these toxic chemicals the better. Thus, exercise protects the colon by speeding up the passage of its contents and minimizing the time it is in contact with dangerous toxins.

Women Athletes Fair Better. For women, breast cancer is the second most common cancer. Cancer of the reproductive system ranks fourth. Can exercise help women avoid these? The answer is yes. Women who were athletes in college have a lower lifetime rate of cancer of the breast and of the uterus, ovaries, cervix, and vagina than women who were not athletes. Rose Frisch of Harvard University discovered this protective effect of exercise in two studies done in 1985 and 1987.

The 1987 study compared women alumnae from 10 colleges. Athletes were defined as women who had been on at least one varsity team that trained regularly, or women who were on a consistent exercise program. The former athletes had only half the number of breast cancers and only one third the reproductive system cancers of the nonathletes.

How does exercise protect women from these cancers? The family history of cancer did not differ between the athletes and nonathletes, and their pregnancy histories were similar. The athletes, however, were leaner, started menstruation later, and had an earlier age of natural menopause.

Fatness and obesity cause higher levels of estrogen in the blood, earlier menstruation, and later menopause. Cancers of the breast and reproductive system are sensitive to estrogen. Exercise, then may protect the athletes indirectly, by helping them remain lean.

All women, regardless of their athletic ability, can use exercise, along with a prudent diet, to maintain a desirable body weight and thus acquire an athlete's lower risk of cancer.

What About Other Cancers? Researchers who have studies the relationship between exercise and cancers of the lung, pancreas, prostate, upper intestine, stomach, bladder, and rectum have found no relationship. This may be because exercise does not protect against these cancers, or it may be owing to the difficulty of studying the effect of exercise on cancer. Many cancers have a latency period of 20 years or more, and a person's past lifestyle practices may affect future health. Studying behavior over a short time, as most experiments do, may not accurately reflect the relationship between behavior and outcome.

The Japan-Hawaii cancer study, on the other hand, studied the incidence of cancer over a 20-year period. Although the results showed that exercise has a strong protective effect for colon cancer, exercise did not appear to be protective against other types of cancers. Dr. Severson feels this is a real effect. "Based on what I saw in our data, I don't conclude that there is any strong relationship between physical activity and the other cancers we looked at." Future studies will further explore and define the role exercise plays in cancer prevention.

Our Children at Risk. Studying the relationship between exercise and cancer may benefit our children the most. Today's children are less physically fit than children were 20 years ago. They exercise less and are fatter, according to the National Children and Youth Fitness surveys done in 1984 and 1987. Given the long formation time of many cancers, our children may be at a greater risk than any previous generation for developing cancer.

Dr. Severson cautions against premature conclusions. "Most of the studies that have been done on physical activity so far have been on adult-level physical activity and the effect on adult cancers that occur later on in their lives. Nobody's really looked at the effects of physical activity at very young ages to see how it affects the risk of cancer later on in adult life, although it's not unreasonable to look at that."

He does, however, note the potential risks of inactivity. "Adult cancer outcome isn't the only thing that we as a society be worried about in children who have decreased levels of physical activity. We might want to worry about things such as heart disease later on. Diseases other than cancer can be very important."

We need to start our children on good lifestyle practices such as regular exercise early in life to protect them from a variety of future diseases. Parents can serve as positive role models by adopting an active lifestyle and involving their children in it, rather than advising children to "do as I say, not as I do."

A Prescription for Health. Habits begin early in life, but we can adopt healthful ones at any age to become healthier and decrease our risk of disease. Lifestyle plays a very important role in the development of cancer. Exercising, avoiding obesity, eating healthfully, and not smoking are components of a healthy lifestyle that fosters longevity. They are also powerful weapons in the war against many cancers. Far from being beyond our control, the risk of getting cancer lies within our choices.

Jamie T. Kwako, M.P.H., R.D., is a nutrition counselor for Saint Bernardine Medical Center, San Bernardino, California.