Women with breast cancer who walk at least an hour a week have a better chance of beating the disease than those who don't exercise at all, researchers said.

But the study said many women hurt their chances of survival by cutting back on exercise after they are diagnosed.

"It is well established that exercise plays an important role in preventing many diseases, including breast cancer," said lead researcher Michelle Holmes of Brigham and Women's Hospital in Boston. "However, we found that women who are physically active after breast cancer diagnosis may lower their risk of death from breast cancer and cancer recurrence."

Regular exercise is believed to lift survival rates by diminishing production of the hormone estrogen, which promotes tumor growth in the most common type of breast cancer, the report in the Journal of the American Medical Association said.

Based on up to 18 years' worth of data from 3,000 breast cancer participants in the hospital's study of female nurses, walking three to five hours a week cut in half the risk of dying from the disease compared to women who exercised less than an hour a week or not at all.

Even walking an hour a week lifted survival rates but exercising more than five hours a week did not confer any added survival benefit.

The study noted discouraging estimates that women with breast cancer tend to decrease their levels of physical activity by two hours a week and those who are obese reduce activity even more.

Less than one-third of breast cancer survivors engage in government-recommended exercise regimens, the report said.

"Women with breast cancer who follow the Centers for Disease Control and Prevention recommendations for all individuals ... to exercise at moderate intensity for 30 or more minutes per day for five or more days per week may survive longer," Holmes wrote.