Several recent papers have reported a decreased risk of breast cancer with physical activity. Is there any strong evidence in favor of the positive effect of exercise?
The aim of the paper from Leiztmann and colleagues  was to examine the type of activity and breast cancer risk in a prospective cohort enrolled in the Breast Cancer Detection Demonstration Project (BCDDP) Follow-up Study in the US. In 1979, the National Cancer Institute established the BCDDP Follow-up Study of 64,182 women enrolled in the original BCDDP screening study. A total of 32,269 women were followed from 1987 to 1998; several questionnaires (Q) were completed during the follow-up but only one was about physical activity (Q2). The questionnaire encompassed an extensive list of examples of moderate and vigorous activities that covered household, occupational and recreational or sporting activities. A total weekly MET-hour score (total physical activity) was calculated by summing up the weekly expenditures from non-vigorous and vigorous activities. New incident breast cancer cases were identified through pathology reports, linkage to state cancer registries, the National Death Index and from self-report. A total of 1506 postmenopausal breast cancer cases were ascertained, of which 260 (17%) were classified as in situ cancers.
The range of total physical activity varied about 2.5-fold between the means of extreme quintiles. Women reported spending an average of 1.2 h/day in vigorous activity and 5.9 h/day in non-vigorous activity. Women who were physically more active tended to be slightly leaner, to have never smoked, to have a lower level of education and to have greater parity than physically less active women. The risk of breast cancer was inversely associated with physical activity. The most active women had an age-adjusted relative risk (RR) of 0.81 (95% confidence interval (CI) 0.69-0.95; p = 0.03), as compared with the least active women. The relative risk was slightly attenuated and became statistically non-significant after additional adjustment for potential confounding variables (RR = 0.86; 95% CI 0.73-1.01; p = 0.15). However, when stratifying the relation of physical activity to postmenopausal breast cancer by body mass index (BMI) categories, among normal weight and lean women, the multivariate RRs comparing extreme categories of total and vigorous activity were significant: 0.76 (95% CI = 0.61-0.94; p = 0.03) and 0.68 (95% CI = 0.54-0.85; p = 0.002), respectively, but were not significant in overweight, obese women. The test for interaction between vigorous activity and BMI was statistically significant (p = 0.008). The effect of physical activity was independent of the estrogen-receptor (ER) status.
Unité de Gynécologie Endocrinienne, Hôtel-Dieu de Paris, Université Paris Descartes, Paris, France