title: Effect of physical activity on breast cancer risk: findings of the Japan collaborative cohort study.
abstract: OBJECTIVE
  This study aimed to examine prospectively the association between physical activity and breast cancer risk in a non-Western population.
  
  
  METHODS
  We analyzed data from the Japan Collaborative Cohort Study, which included 30,157 women, ages 40 to 69 years at baseline (1988-1990), who reported no previous history of breast cancer, and provided information on their walking and exercise habits. The subjects were followed prospectively from enrollment until 2001 (median follow-up period, 12.4 years). Breast cancer incidence during this period was confirmed using records held at population-based cancer registries. The Cox proportional hazards model was used to estimate the hazard ratio (HR) for the association of breast cancer incidence with physical activity.
  
  
  RESULTS
  During the 340,055 person-years of follow-up, we identified 207 incident cases of breast cancer. The most physically active group (who walked for > or = 1 hour per day and exercised for > or = 1 hour per week) had a lower risk of breast cancer (HR, 0.45; 95% confidence interval, 0.25-0.78) compared with the least active group after adjusting for potential confounding factors. The inverse association of exercise on breast cancer was stronger among those who walked for > or = 1 hour per day than those who walked for <1 hour per day (P = 0.042). These results were not significantly modified by menopausal status or body mass index (BMI).
  
  
  CONCLUSIONS
  Our analysis provided evidence that physical activity decreased the risk of breast cancer. Walking for 1 hour per day and undertaking additional weekly exercise both seemed to be protective against breast cancer, regardless of menopausal status or BMI.
authors.affiliation: Department of Public Health, Nagoya City University Graduate School of Medical Sciences, 1 Kawasumi, Mizuho-cho, Mizuho-ku, Nagoya 467-8601, Japan. ssuzuki@med.nagoya-cu.ac.jp
authors.forename: Sadao
authors.initials: S
authors.lastname: Suzuki
authors.forename: Masayo
authors.initials: M
authors.lastname: Kojima
authors.forename: Shinkan
authors.initials: S
authors.lastname: Tokudome
authors.forename: Mitsuru
authors.initials: M
authors.lastname: Mori
authors.forename: Fumio
authors.initials: F
authors.lastname: Sakauchi
authors.forename: Yoshihisa
authors.initials: Y
authors.lastname: Fujino
authors.forename: Kenji
authors.initials: K
authors.lastname: Wakai
authors.forename: Yingsong
authors.initials: Y
authors.lastname: Lin
authors.forename: Shogo
authors.initials: S
authors.lastname: Kikuchi
authors.forename: Koji
authors.initials: K
authors.lastname: Tamakoshi
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yatsuya
authors.forename: Akiko
authors.initials: A
authors.lastname: Tamakoshi
mesh_terms: D000328:Adult; D000368:Aged; D001943:Breast Neoplasms / Q000453:epidemiology*; D005260:Female; D006801:Humans; D015994:Incidence; D007564:Japan / Q000453:epidemiology; D008875:Middle Aged; D009043:Motor Activity; D016016:Proportional Hazards Models; D011446:Prospective Studies; D012307:Risk Factors