Lifetime Physical Activity and Risk for Premenopausal Breast Cancer

Although only one quarter of breast malignancies occur before menopause, tumors in young women tend to be more aggressive than those in postmenopausal women. During a 6-year follow-up of 64,777 participants in the Nurses’ Health Study, investigators assessed adolescent and adult physical activity as reported from age 12 years to the participants’ current ages (age range at study entry, 33–51). Physical activity was defined as strenuous (e.g., running, racquet sports, swimming laps), moderate (e.g., aerobics), or walking (categorized by pace). Each form of activity was assigned a metabolic equivalent (MET) value; total amounts were calculated as MET-hours per week (MET-hours/week) by multiplying the weekly hours of each activity by the corre…