Physical activity appears to be associated with a lower risk of developing breast cancer and an improved quality of life after breast cancer diagnosis. The mechanism may be hormonal, with lower levels of circulating ovarian hormones present in women who exercise. Holmes and colleagues examined the relationship between physical activity and recurrence and mortality outcomes after breast cancer diagnosis.

Participants in the Nurses’ Health Study provided information on breast cancer incidence and physical activity, as well as other potentially relevant factors such as dietary habits. Participants were diagnosed with invasive breast cancer between 1984 and 1998 and followed until June 2002 or death, whichever came first. Exercise was assessed in terms of metabolic equivalent task (MET) hours, with walking at an average pace receiving a MET score of 3.0 and higher scores assigned to more vigorous activities, and then recorded as MET hours per week.

Of the 2,987 women who had stage I, II, or III breast cancer during the study, 463 died; 280 of the deaths were caused by breast cancer. After adjustment, the relative risk (RR) of all outcomes was higher for women who exercised fewer than 3.0 MET hours per week. The RR of death from breast cancer was 0.80 in women with 3.0 to 8.9 MET hours per week, 0.50 for women with 9.0 to 14.9 MET hours per week, 0.56 for women with 15.0 to 23.9 MET hours per week, and 0.60 for women with 24.0 or more MET hours per week. Analysis of overall survival and recurrence yielded similar results. Overweight women and women of normal weight who exercised at least 24.0 MET hours per week had a lower risk of death (RR, 0.52 and 0.61, respectively) when compared with women who exercised fewer than 3.0 MET hours per week. Women who had tumors with positive estrogen and progesterone receptors (RR, 0.50) benefited more from exercise than women lacking these receptors (RR, 0.91). Also, women with stage III breast cancer appeared to receive more benefit from physical activity than women with earlier stage disease; however, the numbers of women in these subcategories were small.

The authors conclude that women have improved survival by any measured outcome if they exercise more than 3.0 MET hours per week. Because the finding applied particularly to women with receptor-positive tumors in this study, the authors speculate that hormonal alterations are responsible for this benefit. Walking at an average pace for three to five hours per week is associated with the maximal benefit.