Everyone knows exercise is a good thing. It strengthens the heart and lungs, keeps weight off, improves mood and increases energy. But can it prevent cancer?

A sustained activity, something you can stick with, is important for women in the prevention of breast cancer, says Beverly Rockhill, a cancer researcher.A growing body of evidence about the cancer-fighting benefits of exercise has prompted the American Cancer Society to recommend regular physical activity as a cornerstone of cancer prevention. In guidelines on diet and exercise, to be released this month, the cancer society for the first time puts exercise on an equal footing with nutrition. "We now believe physical activity is a primary component of preventing cancer," says Abby Bloch, chairwoman of an advisory panel that prepared the recommendations. Until recently, definitive scientific proof has been elusive. While most of the clear benefits of exercise, such as weight loss, can be seen relatively quickly, its impact on cancer can take years. And the relationship has never been as clear-cut as that between smoking and lung cancer, for example, or sun exposure and skin cancer. In recent years, however, scientists have begun to connect the dots between regular, sustained workouts and the prevention of several types of cancers, among them colon, breast and lung. "The science has gotten to the point where it has attracted a lot of attention, and I predict the next decade (of research in this area) will be a very exciting time," says Steven N. Blair, an epidemiologist with the Cooper Institute for Aerobics Research in Dallas, Texas, who is studying the relationship between lung cancer and exercise. Lifestyle brings risks Experts believe that one-third of the 500,000 annual cancer deaths in this country can be attributed to diet and sedentary habits, with another third due to cigarette smoking; thus, lifestyle choices have become increasingly important. To be sure, genetic inheritance influences cancer risks, too, but most of the variation in cancer risks across populations and among individuals is due to non-inherited factors. Those who regularly exercise tend to practice other healthful habits. They are less likely to consume saturated fats -- especially animal fat -- and are more likely to eat increased fiber and to shun cigarettes. They also are leaner than average Americans, which helps, because obesity is a risk factor for many cancers. How activity helps What, then, are the specific physiological effects that exercise produces that protect against cancer? While no one is certain, the suspicion is that the effect can be direct or indirect, from the hormonal to the emotional. "Physical activity has so many nuances, not just organic, but emotional and psychological, all of which influence the big picture," says Bloch, a registered dietitian who is a consultant to Beth Israel Medical Center's outpatient cancer center in New York City. Though everyone agrees that exercise is valuable, there is still some debate over how much exercise and for how long. The cancer society's recommendation regarding exercise was, in fact, difficult to write because the research is so disparate, Bloch says. Most of the studies suggest the greatest protective benefits are achieved with 60 minutes a day of vigorous exercise, "but we knew we couldn't recommend that much because people wouldn't do it," Bloch says. "There was a lot of concern about issuing a guideline that people might feel is too daunting for them -- so we recommended 30 minutes, with the goal of building up the intensity and the time to 60," she adds. "The thought was that if we got them doing it, it would get easier -- and people would want to improve from where they were." The message, thus, is "begin with what is comfortable for you -- but begin." The most compelling evidence thus far tying exercise to cancer prevention involves cancers of the colon and breast, although scientists suspect that exercise also may play a role in lowering the chances of prostate, lung, non-Hodgkin's lymphoma, ovarian, testicular and uterine cancers. Unquestionably, "there are lower cancer death rates in both men and women" who are physically fit, says Blair, who is the Cooper Institute's director of research. "Active and fit people have less of a chance of dying of cancer." Effect on colon cancer Colon cancer, one of the nation's leading cancer killers, is the one cancer where the impact of exercise appears indisputable. Numerous studies have shown that exercisers have a substantially lower risk -- some studies have shown it to be as much as 50 percent -- of developing colon cancer than couch potatoes. "The epidemiologic evidence for a preventive role for physical activity is strong and consistent for colon cancer," says Aaron Blair, brother of Steven Blair and chief of the occupational epidemiology branch of the National Cancer Institute, part of the National Institutes of Health. Most experts theorize that the reason for this is that exercise accelerates the movement of material through the colon, and cancer-causing substances don't have time to linger -- and make trouble -- because they leave the body quickly. Effect on breast cancer With breast cancer, the data are less consistent but promising nevertheless. In September, the American College of Sports Medicine released a Canadian study published in its journal, Medicine & Science in Sports & Exercise, that looked at 1,233 cases of breast cancer and 1,237 controls (women who did not have the disease), comparing their lifetime physical activity patterns and other factors, including diet, tobacco and alcohol use and family history. The study found that the greatest risk reduction for breast cancer occurred among those women who engaged regularly in "moderate intensity" job or household activities, such as farming or household chores. Such activities appear to be more beneficial than recreational pursuits of any intensity that were performed inconsistently, whether it be an occasional brisk walk or game of tennis. Earlier research released two years ago from the landmark Nurses Health Study at Brigham and Women's Hospital in Boston showed that women who exercise an hour a day or more may reduce their risk of breast cancer by 20 percent. Those who work out two to four hours a week experienced a 10 percent reduction in risk. The study was based on analysis of questionnaires from 121,701 women. Researchers examined 20 years of health and activity data from women ages 30 to 55 from 1976 to 1996. A smaller 1997 study in Norway found that women who exercised at least four hours a week were about a third less likely to get breast cancer. Beverly Rockhill, the lead researcher in the Boston study and an epidemiologist at the University of North Carolina at Chapel Hill, says the connection between exercise and breast cancer may be due to the fact that exercise reduces the level of circulating estrogens in a woman's body. Estrogen stimulates the growth of breast cells, which may encourage cancer. "A woman is exposed to circulating hormones her whole life. To prevent breast cancer, she has to reduce them over a long period of time," Rockhill says. "What seems to be important is sustained activity. Women need to pick up an activity they can stick with." In this study, the nurses who demonstrated a lower cancer risk walked briskly, ran, jogged and performed aerobics and calisthenics, Rockhill says. Unlike in the later Canadian study, "we did not count such things as easy walking, gardening or housework," she says. But Rockhill warns that women shouldn't necessarily overdo it. "They have to tread a fine line between too much and too little exercise, because too little estrogen can increase the risk of heart disease and osteoporosis," she says. Postmenopausal women taking hormone replacement and who exercised enjoyed the same protective benefits, Rockhill says. One possible explanation is that the production of natural estrogen was probably reduced; after menopause, estrogen is produced in the fat cells, rather than in the ovaries. Women who exercise are typically leaner and would produce less natural estrogen as a result. Effect on lung cancer Steven Blair is studying the effects of exercise on lung cancer, and his preliminary results suggest that higher fitness levels are associated with a lower risk of developing the disease. In his study, unfit men were about twice as likely to die of lung cancer than fit men, he says. His research found that moderately fit men had a 20 percent lower risk of dying of lung cancer compared with men who were not fit. And men who were highly fit had a 60 percent lower risk than men who were not fit, he says. This appeared true even after adjusting for smoking habits. "Fitness protects, whether you are a smoker or not," he says, although he is not exactly sure why. He doubts that the reason is that exercise improves lung function. Rather, it is more likely that exercise strengthens the body's immune system, believed to play a significant role in staving off the onset, or worsening, of many types of cancer. Bloch, the New York dietitian, believes the cancer risk can be lessened even among those who start exercising in middle age or later. "It can begin to make a difference for someone in his or her 50s, where cancer can still be years off," she says. "It's never too late."

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