The findings are based on data from a landmark study that linked hormone use to breast cancer, heart attacks and strokes. They suggest that exercise may help counteract - but not cancel out - the slightly increased risk of breast cancer faced by longtime hormone users, said lead researcher Dr. Anne McTiernan of the Fred Hutchinson Cancer Research Center in Seattle.
The study also adds to the growing body of evidence that says exercise for women need not be super strenuous for them to reap substantial health benefits, even if begun relatively late in life.
"We thought it important to determine if moderate-intensity physical activities, such as walking, biking outdoors or easy swimming, when initiated later in life, can reduce the risk of breast cancer since these types of activities are achievable for most women," she said.
The researchers analyzed data on 74,171 women ages 50 to 79 who participated in the government's Women's Health Initiative study from 1993 to 1998.
Women who said they engaged in about 1.25 to 2.50 hours of moderate exercise weekly had an 18 percent lower risk of breast cancer than inactive women.
The findings appear in Wednesday's Journal of the American Medical Association.
Study participant Cecile Cowdery, a fit 77-year-old, has been walking regularly for exercise since she retired from nursing 12 years ago.
Besides helping reduce the risk of breast cancer, which Cowdery has never had, "it also gives you more mobility to do other things other than sitting around and knitting," said Cowdery, who lives in Seattle. "I think most people could be out there doing the same thing."
A separate study in JAMA found that moderate exercise combined with dieting is about as effective as intense exercise in helping younger women lose weight and improve cardiovascular fitness.
While some experts say an hour of moderate-intensity exercise daily is optimal, both studies add support to less stringent recommendations from the federal Centers for Disease Control and Prevention and the American College of Sports Medicine.
Those guidelines recommend a minimum of 30 minutes of moderate-level activity like brisk walking most days of the week to reduce the risk of heart disease and other chronic health problems.
In McTiernan's study, little additional benefit resulted from more intense or frequent exercise - perhaps because relatively few women studied engaged in such levels of activity, Harvard Medical School's Dr. I-Min Lee said in an accompanying editorial.
"It is therefore encouraging to know that women can experience health benefits at far lower levels of physical activity," Lee said.
The Women's Health Initiative study made headlines last summer when it was halted after finding that the risks of hormone pills containing estrogen and progestin outweighed the benefits for post-menopausal women.
By Lindsey Tanner