The American Cancer Society, worried about a nation that does too little exercise and grows more obese, is putting a new emphasis on exercise as a way to reduce the risk of getting sick and dying of cancer.

The five-year update of the society's nutrition and activity guidelines says the evidence now is convincing that exercise reduces risk of colorectal and breast cancer. The report says there also is a probable benefit against endometrial cancer, and activity may help against other forms of cancer as well.

A medical panel that weighed the latest research says activity apparently works directly to lower the risk, and provides an added indirect benefit if the exercise also keeps a person's weight down. The experts found what they describe as convincing evidence that weight control, through proper nutrition or physical activity, independently reduces risk.

If everyone exercised and controlled weight, the number of Americans who die of cancer would drop by about one third, the guidelines said. About an equal number of new cases could be prevented. The society estimates almost 1.3 million new cancer diagnoses this year. More than 500,000 people die of cancer.

The idea that one-third of cancer cases could be prevented through calorie control and exercise is not new. But the emphasis on physical activity is new, said Dr. Anne McTiernan of Fred Hutchinson Cancer Research Center in Seattle, a coauthor of the guidelines. "Five years ago, we didn't have the amount of data we have now," she said.

The society's minimum recommendation for cancer prevention in adults is at least 30 minutes of moderate activity, such as a brisk walk five days a week. That's in line with the Surgeon General's recommendations for overall good health and the American Heart Association's recommendations for cardiovascular health.

Being active can control weight, improving energy metabolism and reducing circulating concentrations of insulin. "Physical activity helps to prevent adult-onset diabetes, which has been associated with increased risk of cancers of the colon, pancreas and possibly other sites," the report said.

Risks of some forms of cancer can be double among the overweight and obese, but the data are cloudy because the studies were not uniform on what they consider too much weight, McTiernan said.

In addition, 45 minutes or more of moderate to vigorous activity five or more days a week may enhance reductions in breast and colon cancer risk, the guidelines said. Vigorous activity can range from jogging to martial arts, basketball or masonry work.

This much exercise can reduce the risk of colon cancer by almost half and breast cancer by a third, McTiernan said. Exercise reduces circulating levels of estrogen, which has been linked to higher breast cancer risk in postmenopausal women, she said. Exercise reduces other hormones that can raise the risk of colon cancer, and speeds material through the bowel before any cancer-causing agents can linger against the bowel wall, she said.

The guidelines also call for children and adolescents to do at least 60 minutes a day of moderate-to-vigorous physical activity, five days a week. The goal is to create lifetime habits that will keep the youngsters out of the 55 percent of American adults who now are overweight or obese, McTiernan said.

And the guidelines say the overweight should lose weight. Studies have not found that cancer risk falls if people lose weight, but experts suspect risk would fall because studies do show a lower cancer risk among people who are not overweight.

And the experts say communities should provide more facilities for physical activity, including safe and attractive places to walk and run.

Also among the recommendations: People should eat more fruits and vegetables. This has been linked to lower rates of lung, oral, esophageal, stomach and colon cancer. High-fat diets have been associated with increase in risk of cancers of the colon and rectum, prostate and endometrium.

The report is published in the March issue of the society's publication CA — A Cancer Journal for Clinicians. In an accompanying commentary, Dr. Walter C. Willett of Harvard School of Public Health praises the report as a clear call for action in cancer prevention.

The better-off and the better-educated already have been keeping their weight down and their activity levels up, and thereby reduce their cancer risk, Willett said. "Unfortunately, another part of the population, for various reasons we have to understand better, is not taking advantage, and that part of the population's health status is almost for sure getting worse," he said.

However, although exercise is a good thing, the society may be overstating its benefits, said Louise Brinton, chief of environmental epidemiology at the National Cancer Institute.

In breast cancer, for instance, it's clear that obesity raises the risk, but it's not established that exercise will drive down the risk unless it also lowers the exerciser's weight, Brinton said. There is accumulating evidence that exercise provides an independent benefit, but some studies show a benefit while others don't, she said.

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