At first glance, the new study by Williams  seemed, well, just another study showing how important physical activity is. Indeed, exercise reduces morbidity and mortality in a large list of diseases including cardiovascular, respiratory and oncological diseases. Clicking the key words ‘breast cancer’ and ‘exercise’ in PubMed yields about 2000 hits. So what’s the point in commenting on this study? I guess it is because I liked its different way of looking at the issue. But first things first, and here are the core methods and findings . Cox proportional hazard analyses were made of baseline pre-diagnosis MET-hours/week vs. breast cancer mortality adjusted for follow-up age, race, baseline menopause, and estrogen and oral contraceptive use in 79,124 women (32,872 walkers, 46,252 runners) from the National Walkers’ and Runners’ Health Studies. Women were categorized into three groups according to their level of exercise: below (< 7.5 metabolic equivalent h/week, MET-h/week), at (7.5–12.5 MET-h/week), or above (≥ 12.5 MET-h/week) recommended levels. Despite the large number of participants, only 111 women (57 walkers, 54 runners) died from breast cancer during the 11-year follow-up. The decline in mortality in women who exercised ≥ 7.5 MET-h/week was not different for walking and running ([i]p[/i] = 0.34), so running and walking energy expenditures were combined. The risk for breast cancer mortality was 41.5% lower for ≥ 7.5 vs. < 7.5 MET-h/week (hazard ratio (HR) 0.58, 95% confidence interval (CI) 0.38–0.92, [i]p[/i] = 0.02), which persisted when adjusted for body mass index (BMI) (HR 0.58, 95% CI 0.37–0.96, [i]p[/i] = 0.03). Other than age and menopause, baseline bra cup size was the strongest predictor of breast cancer mortality, i.e. 57.9% risk increase per cup size when adjusted for MET-h/week and the other covariates (HR 1.58, 95% CI 1.27–1.97, [i]p[/i] < 0.0001), and 70.4% greater when further adjusted for BMI (HR 1.70, 95% CI 1.34–2.2, [i]p[/i] = 0.0005). Breast cancer mortality was 4.0-fold greater for C-cup, and 4.7-fold greater for ≥ D-cup vs. A-cup when adjusted for BMI and other covariates. Adjustment for cup size and BMI did not eliminate the association between breast cancer mortality and ≥ 7.5 MET-h/week walked or run (HR 0.61, 95% CI 0.39–1.00, [i]p[/i] = 0.05). The final conclusions were that breast cancer mortality decreased in association with both meeting the exercise recommendations and smaller breast volume.
Sackler School of Medicine, Tel-Aviv University, Tel-Aviv, Israel