title: Physical activity and the risk of breast cancer.
abstract: BACKGROUND
  Because physical activity may affect hormonal concentrations and energy balance, we decided to investigate whether everyday exercise is related to the risk of breast cancer.
  
  
  METHODS
  During 1974 to 1978 and 1977 to 1983, a total of 25,624 women, 20 to 54 years of age at entry, enrolled in health surveys and answered questionnaires about leisure-time and work activity.
  
  
  RESULTS
  During a median follow-up of 13.7 years, we identified 351 cases of invasive breast cancer among the 25,624 women in the cohort. Greater leisure-time activity was associated with a reduced risk of breast cancer, after adjustments for age, body-mass index (the weight in kilograms divided by the square of the height in meters), height, parity, and county of residence (relative risk, 0.63; 95 percent confidence interval, 0.42 to 0.95), among women who exercised regularly, as compared with sedentary women (P for trend=0.04). In regularly exercising women, the reduction in risk was greater in premenopausal women than in postmenopausal women, and greater in younger women (<45 years at study entry) than in older women (> or =45 years) (relative risk, 0.38; 95 percent confidence interval, 0.19 to 0.79). In stratified analyses the risk of breast cancer was lowest in lean women (body-mass index, <22.8) who exercised at least four hours per week (relative risk, 0.28; 95 percent confidence interval, 0.11 to 0.70). The risk was also reduced with higher levels of activity at work, and again there was a more pronounced effect among premenopausal than postmenopausal women.
  
  
  CONCLUSIONS
  Physical activity during leisure time and at work is associated with a reduced risk of breast cancer.
authors.affiliation: The Institute of Community Medicine, University of Tromsø, Norway.
authors.forename: I
authors.initials: I
authors.lastname: Thune
authors.forename: T
authors.initials: T
authors.lastname: Brenn
authors.forename: E
authors.initials: E
authors.lastname: Lund
authors.forename: M
authors.initials: M
authors.lastname: Gaard
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D001835:Body Weight; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000517:prevention & control; D015444:Exercise; D005260:Female; D005500:Follow-Up Studies; D006306:Health Surveys; D006801:Humans; D015994:Incidence; D007899:Leisure Activities; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D016016:Proportional Hazards Models; D012306:Risk; D012307:Risk Factors; D014937:Work