title: Physical activity and postmenopausal breast cancer risk in the NIH-AARP diet and health study.
abstract: BACKGROUND
  Although physical activity has been associated with reduced breast cancer risk, whether this association varies across breast cancer subtypes or is modified by reproductive and lifestyle factors is unclear.
  
  
  METHODS
  We examined physical activity in relation to postmenopausal breast cancer risk in 182,862 U.S. women in the NIH-AARP Diet and Health Study. Physical activity was assessed by self-report at baseline (1995-1996), and 6,609 incident breast cancers were identified through December 31, 2003. Cox regression was used to estimate the relative risk (RR) and 95% confidence interval (95% CI) of postmenopausal breast cancer overall and by tumor characteristics. Effect modification by select reproductive and lifestyle factors was also explored.
  
  
  RESULTS
  In multivariate models, the most active women experienced a 13% lower breast cancer risk versus inactive women (RR, 0.87; 95% CI, 0.81-0.95). This inverse relation was not modified by tumor stage or histology but was suggestively stronger for estrogen receptor (ER)-negative (RR, 0.75; 95% CI, 0.54-1.04) than ER-positive (RR, 0.97; 95% CI, 0.84-1.12) breast tumors and was suggestively stronger for overweight/obese (RR, 0.86; 95% CI, 0.77-0.96) than lean (RR, 0.95; 95% CI, 0.87-1.05) women. The inverse relation with physical activity was also more pronounced among women who had never used menopausal hormone therapy and those with a positive family history of breast cancer than their respective counterparts.
  
  
  CONCLUSIONS
  Physical activity was associated with reduced postmenopausal breast cancer risk, particular to ER-negative tumors. These results, along with heterogeneity in the physical activity-breast cancer relation for subgroups of menopausal hormone therapy use and adiposity, indicate that physical activity likely influences breast cancer risk via both estrogenic and estrogen-independent mechanisms.
authors.affiliation: Nutritional Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, NIH, Department of Health and Human Services, 6120 Executive Boulevard, MSC 7232, Bethesda, MD 20892,, USA. peterst@mail.nih.gov
authors.forename: Tricia M
authors.initials: TM
authors.lastname: Peters
authors.forename: Arthur
authors.initials: A
authors.lastname: Schatzkin
authors.forename: Gretchen L
authors.initials: GL
authors.lastname: Gierach
authors.forename: Steven C
authors.initials: SC
authors.lastname: Moore
authors.forename: James V
authors.initials: JV
authors.lastname: Lacey
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Wareham
authors.forename: Ulf
authors.initials: U
authors.lastname: Ekelund
authors.forename: Albert R
authors.initials: AR
authors.lastname: Hollenbeck
authors.forename: Michael F
authors.initials: MF
authors.lastname: Leitzmann
mesh_terms: D000368:Aged; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000378:metabolism; D015331:Cohort Studies; D004032:Diet; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D009043:Motor Activity; D017698:Postmenopause; D016016:Proportional Hazards Models; D011960:Receptors, Estrogen / Q000378:metabolism; D012306:Risk; D011795:Surveys and Questionnaires; D014481:United States / Q000453:epidemiology
chemical_list: D011960:Receptors, Estrogen