title: Physical activity and breast cancer risk: impact of timing, type and dose of activity and population subgroup effects.
abstract: OBJECTIVE
  To review (1) the epidemiological literature on physical activity and the risk of breast cancer, examining the effect of the different parameters of activity and effect modification within different population subgroups; and (2) the biological mechanisms whereby physical activity may influence the risk of breast cancer.
  
  
  METHODS
  A review of all published literature to September 2007 was conducted using online databases; 34 case-control and 28 cohort studies were included. The impact of the different parameters of physical activity on the association between activity and the risk of breast cancer was examined by considering the type of activity performed, the timing of activity over the life course and the intensity of activity. Effect modification of this association by menopausal status, body mass index (BMI), racial group, family history of breast cancer, hormone receptor status, energy intake and parity were also considered.
  
  
  RESULTS
  Evidence for a risk reduction associated with increased physical activity was found in 47 (76%) of 62 studies included in this review with an average risk decrease of 25-30%. A dose-response effect existed in 28 of 33 studies. Stronger decreases in risk were observed for recreational activity, lifetime or later life activity, vigorous activity, among postmenopausal women, women with normal BMI, non-white racial groups, those with hormone receptor negative tumours, women without a family history of breast cancer and parous women.
  
  
  CONCLUSIONS
  The effect of physical activity on the risk of breast cancer is stronger in specific population subgroups and for certain parameters of activity that need to be further explored in future intervention trials.
authors.affiliation: Division of Population Health and Information, Alberta Cancer Board, 1331 29 St NW, Calgary, Alberta, Canada. chrisf@cancerboard.ab.ca
authors.forename: C M
authors.initials: CM
authors.lastname: Friedenreich
authors.forename: A E
authors.initials: AE
authors.lastname: Cust
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D015992:Body Mass Index; D001943:Breast Neoplasms / Q000453:epidemiology / Q000209:etiology / Q000517:prevention & control*; D004812:Epidemiologic Methods; D005006:Ethnicity; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D008019:Life Style; D008593:Menopause / Q000502:physiology; D008875:Middle Aged; D010809:Physical Fitness / Q000502:physiology*; D011998:Recreation; D012307:Risk Factors; D040242:Risk Reduction Behavior; D013997:Time Factors