Women who exercise moderately after a diagnosis of breast cancer experience a 20%–50% reduction in their risk of breast cancer death or breast cancer recurrence, data from the Nurses' Health Study suggest.
The survival advantage is particularly strong for women who have estrogen or progesterone receptor-positive tumors. These women experienced a 50% decrease in the risk of breast cancer death, reported Michelle D. Holmes, M.D., of Harvard Medical School, Boston, and her colleagues.
“Our results suggest a possible hormonal mechanism for improved survival among women who are physically active,” the researchers said. Physical activity might also increase survival through short- and long-term improvements in insulin resistance and reduction in hyperinsulinemia (JAMA 2005;293:2479–86).
Since the maximum benefit occurred with moderate levels of exercise (walking 3–5 hr/wk at a moderate pace), the researchers concluded that women with breast cancer should follow the exercise recommendations set forth by the Centers for Disease Control and Prevention: exercise at moderate intensity for 30 or more minutes per day, at least 5 days per week.
It has been estimated that women decrease their physical activity levels by 2 hr/wk after a diagnosis of breast cancer, and that less than one-third of survivors exercise at the recommended level.
The investigators used data from the national Nurses' Health Study for their retrospective analysis, which followed 2,987 women with stages I, II, or III breast cancer who were diagnosed between 1984 and 1998. The women were followed until June 2002 or death, whichever came first.
Activity was determined by self-report and analyzed by metabolic equivalent task (MET)-hours. Three MET-hours are equivalent to walking 2–2.9 mph for 1 hour.
After adjusting for variables such as age, smoking status, and hormone therapy use, women who engaged in 3–8.9 MET-hr/wk had a 20% decreased risk of death from breast cancer, compared with those who exercised less.
Those women who were engaged in 9–14.9 MET-hr/wk had a 50% decreased risk. There was no additional benefit for more exercise: women who engaged in 15–23.9 MET-hr/wk had a 44% decreased risk, and women who exercised more than that had a 40% decreased risk.
Risk of recurrence also was lowest for women who engaged in 9–14.9 MET-hr/wk. These women had a 43% decreased risk, compared with women who accrued less than 3 MET-hr/wk.
Exercise also improved 10-year breast cancer survival rates, the researchers said. The survival rate was 86% for less than 3 MET-hr/wk; 89% for 3–8.9 MET-hr/wk; and 92% for at least 9 MET-hr/wk.
The protective benefit was similar among overweight women and those of normal weight, and greater for women with hormone receptor-positive tumors. Those with hormone receptor-positive tumors who exercised moderately had a 50% decreased risk, compared with a 9% decreased risk for those whose tumors lacked hormone receptors.
Exercise also was particularly beneficial for women with more advanced cancer. Women with stage III cancer who engaged in at least 9 MET-hr/wk had a 63% decreased risk of breast cancer death, compared with women with stage III cancer who exercised less. However, the researchers noted, these results were based on only 76 women and 15 breast cancer deaths.
When they analyzed risk by type of exercise, the researchers found that both walking and vigorous exercise contributed to lowering the risks.