(The New York Times News Service) -- Breast cancer patients who exercise just a few hours every week reduce their risk of death by up to 50 percent compared with inactive women, according to a new study bolstering the case that living healthy can protect against the most common cancer in women.

The study by Boston-based researchers adds to a growing body of evidence linking lifestyle and breast cancer. Last week, other scientists reported that women with breast cancer who adopted a low-fat diet reduced the likelihood their cancers would return. Recent research also has shown that weight gain and obesity increase the risk of developing breast cancer in the first place.

Still, it has not become standard practice for physicians to counsel breast cancer patients to diet and exercise, though cancer specialists said the mounting evidence would likely cause them to routinely recommend lifestyle changes, as cardiologists now do for heart patients.

"We as a medical community are recognizing and embracing exercise as a component of treating women with breast cancer," said Dr. Carolyn Kaelin, director of the Comprehensive Breast Health Center at Brigham and Women's Hospital.

Kaelin, who has long stressed the importance of exercise for the health and psychological well-being of breast cancer patients, is a breast cancer survivor herself, and said she constantly seeks to stay active.

"It was pouring rain out, and I walked in the rain to work," Kaelin said Tuesday. "I really believe in the data."

The new study was particularly compelling because women who walked at a moderate pace for 3 to 5 hours weekly derived the maximum benefit, said cancer specialists.

"It wasn't that these women were training for a marathon. They were walking, which is fairly accessible to most women," said Dr.

Jennifer Ligibel, a women's cancer specialist at Dana-Farber Cancer Institute, adding that she discusses the benefits of exercise with her breast cancer patients.

The study, published in Wednesday's Journal of the American Medical Association, analyzed 18 years of data on nearly 3,000 women with breast cancer who underwent standard treatment, which included chemotherapy, surgery, and radiation. The study drew its data from the Nurses' Health Study, a long-running Harvard project that surveyed nearly 122,000 nurses about their lifestyles, diets, and other behaviors and tracked their health. It has yielded dozens of major insights about chronic disease in women.

Even women who walked at average pace just 1 hour per week, or the equivalent in other physical activity, reduced their risk of death by about 20 percent compared with inactive women. The researchers defined an average pace as 2 to 2.9 miles per hour. Those walking 3 to 5 hours weekly or the equivalent cut their risk by 50 percent. However, walking or engaging in other exercise more than 5 hours weekly offered no additional benefits.

The Brigham's Dr. Michelle D. Holmes, the lead researcher on the study, noted that exercise had no side effects and offered a range of benefits, such as a reduced risk of heart disease and improved self-esteem.

"There's not a lot to lose from being physically active, and perhaps a lot to gain," she said. "It's a win all around."

How exercise reduces breast cancer recurrence and death is not clearly understood. However, considerable research suggests that reducing body fat in turn lowers levels of estrogen, a hormone that stimulates the majority of breast tumors to grow. The new study gives support to the idea that exercise works by indirectly reducing estrogen levels. It found that exercise conferred more protection on women with estrogen-sensitive cancers. In fact, a growing body of evidence indicates that body weight and obesity stimulate breast cancers -- and that exercise and diet are ways to reduce body mass.

A study released last week found that breast cancer patients on low-fat diets reduced their risk of recurrence by 20 percent. Another recent study showed that healthy women who gain 44 or more pounds during their adult years face double the risk of getting breast cancer after menopause.

"It's hard to know whether it's exercise, or diet along with exercise that works," said Dana-Farber's Ligibel, who is conducting a study on the physiological effects of exercise on breast cancers. She said breast tumors may react to a complex cascade of hormones that researchers are still struggling to unravel. Ligibel also suggested the anti-cancer benefits of exercise may extend beyond breast tumors.

"I don't think this is only going to be applicable to breast cancer.

Prostate cancer is very much like breast cancer, in the way it is driven by hormones," said Ligibel. "And it may apply to even more cancers."

In fact, research in the last three years has linked obesity to cancers of the stomach, liver, cervix, colon, uterus, kidney, esophagus and gallbladder, in addition to breast cancer. Scientists have yet to prove that exercise or weight loss can reduce the risk of initially developing these cancers, but the American Cancer Society estimates that 14 percent of male cancer deaths and 20 percent of female cancer deaths are linked to obesity. The society also estimates that 90,000 cancer deaths would be prevented if no Americans were obese or overweight. The organization has not estimated how many of the 40,000 annual US deaths from breast cancer can be attributed to obesity.

Copyright 2005 The New York Times News Service. All rights reserved.