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 in January, 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.

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.

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."