title: Physical activity and the risk of postmenopausal breast cancer - the Norwegian Women and Cancer Study.
abstract: BACKGROUND
  The relationship between physical activity (PA) throughout life and the risk of postmenopausal breast cancer overall and by estrogen receptor (ER) and progesterone receptor (PR) status, has been reported, but without consistent results. The present study aimed to investigate PA from young age to adulthood in participants of the Norwegian Women and Cancer (NOWAC) Study, in order to determine whether changes in PA level affect the risk of postmenopausal breast cancer.
  
  
  METHODS
  1767 invasive breast cancer cases were identified among 80,202 postmenopausal participants of the NOWAC Study during 8.2 years of median follow-up. PA levels at age 14 years, 30 years and at cohort enrollment were obtained via a self-administered questionnaire. Multivariate Cox proportional hazard regression models were used to estimate relative risks and 95% confidence intervals of the risk of postmenopausal breast cancer overall and by ER/PR status.
  
  
  RESULTS
  Risk of postmenopausal breast cancer overall and by ER/PR status was not associated with physical activity level at enrollment. Women with a low PA level at age 30 had an increased risk of ER+/PR + breast tumors (P for trend = 0.04) compared to women with a moderate physical activity level at age 30. Women with a low physical activity level at all three periods of life had a 20% significantly reduced risk of postmenopausal breast cancer, as well as a reduced risk of ER+/PR + and ER+/PR- breast tumors, compared with women who maintained a moderate physical activity level. However, when analyses were corrected for multiple tests, the result was no longer statistically significant. The findings were consistent over strata of age, body mass index and use of hormone replacement therapy.
  
  
  CONCLUSIONS
  The study results from this large Norwegian cohort do not support an association between physical activity at different periods of life and the risk of postmenopausal breast cancer.
authors.affiliation: Department of Community Medicine, Faculty of Health Sciences, University of Tromsø, The Arctic University of Norway, Tromsø 9037, N-Norway. kristin.benjaminsen.borch@uit.no.
authors.forename: Kristin Benjaminsen
authors.initials: KB
authors.lastname: Borch
authors.forename: Eiliv
authors.initials: E
authors.lastname: Lund
authors.forename: Tonje
authors.initials: T
authors.lastname: Braaten
authors.forename: Elisabete
authors.initials: E
authors.lastname: Weiderpass
mesh_terms: D000328:Adult; D015992:Body Mass Index; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000378:metabolism; D005260:Female; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D009043:Motor Activity; D017698:Postmenopause; D011960:Receptors, Estrogen / Q000378:metabolism; D011980:Receptors, Progesterone / Q000378:metabolism; D012307:Risk Factors; D011795:Surveys and Questionnaires
chemical_list: D011960:Receptors, Estrogen; D011980:Receptors, Progesterone