MEDICINE / BREAST CANCER : Risk Reduced by Exercise, Researchers Find : Four hours of physical activity a week cuts danger level by 58% for women under 40, USC study shows. Results are hailed by experts on the disease, for which there is no known prevention.

In the first study designed to investigate whether physical activity can reduce the risk of breast cancer, researchers at USC’s Norris Comprehensive Cancer Center have found that women of childbearing age who spend more than four hours a week exercising can cut their risk of the disease by nearly 60%.

The study was limited to women under 40, who are generally less likely to be afflicted by the disease than older women. Nonetheless, experts hailed the work as another promising indication that lifestyle changes can reduce the risk of cancer.

“A study like this is very encouraging because it does suggest an activity that may readily modify the risk of breast cancer, and there really are very few,” Dr. Robert Smith, senior director of cancer treatment and prevention for the American Cancer Society, said in an interview Tuesday.

“This is an extremely exciting study,” Dr. Susan M. Love, director of the UCLA Breast Center, told the Associated Press. “This is the kind of prevention we need. This is lifestyle changes instead of drugs.”

There is currently no proven method for preventing breast cancer, although researchers are looking into two possibilities: cutting dietary fat and taking the drug tamoxifen, a controversial step that would be used only for women at high risk. The disease is the leading cause of cancer in women, and is expected to afflict 182,000 American women this year.

The USC study, published in today’s Journal of the National Cancer Institute, tracked 1,090 women, all age 40 or younger, all white, all residents of Los Angeles County. Half of them had newly diagnosed breast cancer when USC epidemiologists interviewed them in July, 1983. The other half served as controls.

The researchers collected personal histories from the patients, asking questions about their exercise habits from the time they began menstruating to the year before the study began. They then followed the women for six years.

The researchers found that women who engaged in four or more hours of physical activity each week had a 58% lower breast cancer risk. Those who spent one to three hours a week exercising cut their risk by 30%.

The results do not come as a surprise to experts in the field. Rather, the research is “a well-conducted study that confirms what probably most of us would have believed,” said Louise Brinton, chief of the environmental studies section of the National Cancer Institute.

Previous studies have suggested, although not as strongly as the USC study, that there is a link between physical activity and lower breast cancer rates. In 1981, for instance, Harvard researchers reported that female college-age athletes were less likely to develop breast cancer.

Experts theorize that the link exists because exercise modifies a woman’s menstrual cycle, often lengthening the cycle or halting ovulation altogether if the woman is extremely active. This lessens the woman’s exposure to crucial hormones--estrogen and possibly progesterone--that are believed to play a role in breast cancer.

Whether the benefits of exercise extend to older women, particularly those past menopause, remains unclear. Since obesity is considered a risk factor for breast cancer in older women, some experts think exercise would be beneficial to women past menopause, if only to keep them slender.

Bernstein is currently conducting additional research among women aged 55 to 64, and is hoping to answer two questions: whether being physically active during the childbearing years reduces the risk of breast cancer later in life, and whether exercise would protect women who begin exercising when they are past menopause.