The first study, based on interviews with 15,000 women, shows that women who get more than six hours of strenuous exercise a week, and have no family history of breast cancer, may be 23% less likely to develop the disease than women who don't exercise at all.
The second study shows a 12-week group exercise program may boost mood and physical function in women with early-stage breast cancer.
The researchers don't promise exercise will prevent breast cancer, or blame breast cancer on a lack of exercise. Many factors affect cancer risk.
But, they do report exercise appeared to have benefits in protecting against cancer for women of all ages.
"We have found that exercise likely offers protection against breast cancer regardless of a woman's stage in life," researcher Brian Sprague, of the University of Wisconsin Paul P. Carbone Comprehensive Cancer Center, says in an American Association for Cancer Research (AACR) news release.
"The take-home message for women should be that it is never too late to begin exercising," Sprague says.
Breast Cancer Prevention Study
The study by Sprague, assistant professor Amy Trentham-Dietz, PhD, also of the Carbone cancer center, and others, appears in Cancer Epidemiology Biomarkers & Prevention.
For the study, the researchers interviewed more than 15,000 women in Massachusetts, New Hampshire, and Wisconsin by telephone.
Those interviewed included 6,391 breast cancer patients and 7,630 women without breast cancer. The women were 20-69 years old, roughly split between women 49 and under, those in their 50s, and those in their 60s.
Family History of Breast Cancer
Most had no family history of breast cancer, including the women with breast cancer themselves. While family history increases breast cancer risk, most patients don't have a family history of the disease.
During the 40-minute phone interview, the women noted whether they had participated in the following activities at some point in their lives since age 14: jogging/running, bicycling, calisthenics/aerobics/dance, racquet sports, swimming, walking/hiking for exercise, or other strenuous individual or team activities.
Most women, whether or not they had breast cancer, reported getting up to three hours of weekly strenuous exercise at some point since age 14.
But 461 women without breast cancer, and 332 with breast cancer, said they had exercised strenuously for more than six hours weekly at some point since age 14 -- typically when they were in their teens and early 20s.
23% Less Likely
The women who reported getting more than six weekly hours of strenuous recreational physical activity were 23% less likely to have breast cancer, compared to sedentary women, the study shows.
Exercise appeared to benefit women, regardless of age.
But the benefits were only seen in those with no family history of breast cancer.
The results held after adjusting for other breast cancer risk factors.
The study doesn't prove that exercise single-handedly prevented breast cancer or show how exercise may lower breast cancer risk.
The effects of exercise on hormones and weight may help, the researchers suggest.
They note that they don't know if the women accurately recalled their workout habits.
Exercise for Breast Cancer Patients
The second study comes from researchers including Nanette Mutrie, PhD, professor of exercise and sport psychology at the University of Strathclyde in Glasgow, Scotland.
They studied 203 women with early-stage breast cancer who were 51 years old, on average, and hadn't been exercising.
The patients had had breast cancer surgery (lumpectomy or mastectomy) and were getting chemotherapy and/or radiation therapy to help prevent their cancer's return.
First, the wmen completed surveys about their mood and quality of life. They also took a 12-minute walking test and had their shoulder mobility checked.
Next, Mutrie's team split the women into two groups.
One group participated in a 12-week group exercise program. The other group wasn't asked to exercise.
Group Exercise Program
The women in the exercise group met twice weekly for 45-minute exercise classes. They were also encouraged to work out once a week on their own at home.
For the first six weeks of their 12-week program, the exercise group gathered after classes to discuss topics such as setting goals and the health benefits of exercise.
Both groups of women repeated the psychological and physical tests at the end of the 12-week program and again six months later.
Those in the exercise group had improved their scores on the physical tests and also reported being in a better mood and coping better with breast cancer. Those benefits generally held at the six-month follow-up.
It's not clear how much of the benefits were due to the workouts or to the social aspect of group exercise.
However, the researchers say doctors "should encourage activity for patients with cancer," and that future studies should also investigate home-based exercise programs, which may be more convenient for some patients.
The study appears in BMJ Online First. BMJ was formerly called the British Medical Journal.
SOURCES: Sprague, B. Cancer Epidemiology Biomarkers & Prevention, February 2007; vol 16: pp 236-243. News release, American Association for Cancer Research. Mutrie, N. BMJ Online First, Feb. 16, 2007. News release, BMJ.
By Miranda Hitti
Reviewed by Louise Chang