title: Body mass index and breast cancer risk in Japan: a pooled analysis of eight population-based cohort studies.
abstract: BACKGROUND
  A positive association between body mass index (BMI) and breast cancer risk among postmenopausal women has been reported, and a weak inverse association has been suggested among premenopausal women from studies in the Western population. The effects of BMI on breast cancer have remained unclear among the Asian population, especially in premenopausal women.
  
  
  METHODS
  We assessed the associations between BMI and breast cancer incidence by a pooled analysis from eight representative large-scale cohort studies in Japan. Cancer incidence was mainly confirmed through regional population-based cancer registries and/or through active patient notification from major local hospitals. Breast cancer was defined as code C50 according to ICD10. Pooled estimates of the hazard ratios (HRs) and 95% confidence interval (CIs) for breast cancer were calculated using random-effects models.
  
  
  RESULTS
  Analytic subjects were 183 940 women, 1783 of whom had breast cancer during 2 194 211 person-years of follow-up. A positive association between BMI and the risk of postmenopausal breast cancer was observed (trend P<0.001). The HRs for premenopausal breast cancer were 1.05 (95% CI 0.56-1.99), 1.07 (95% CI 0.76-1.52), 0.91 (95% CI 0.64-1.30), 1.15 (95% CI 0.76-1.73), 1.45 (95% CI 0.71-2.94), and 2.25 (95% CI 1.10-4.60), respectively, in BMIs of <19, 19 to <21, 21 to <23, 25 to <27, 27 to <30, and ≥30 kg/m2. These results were not substantially altered after excluding the patients who were diagnosed with breast cancer in the first 2 years of follow-up.
  
  
  CONCLUSIONS
  The increased risk of postmenopausal breast cancer among women with higher BMIs was confirmed in Japanese. A borderline-significant positive association between BMI and premenopausal breast cancer was observed, suggesting that body mass in Asian women might have opposite effects on breast cancer compared with Western women.
authors.affiliation: Department of Epidemiology and Preventive Medicine, Gifu University Graduate School of Medicine, Gifu.
authors.forename: K
authors.initials: K
authors.lastname: Wada
authors.forename: C
authors.initials: C
authors.lastname: Nagata
authors.forename: A
authors.initials: A
authors.lastname: Tamakoshi
authors.forename: K
authors.initials: K
authors.lastname: Matsuo
authors.forename: I
authors.initials: I
authors.lastname: Oze
authors.forename: K
authors.initials: K
authors.lastname: Wakai
authors.forename: I
authors.initials: I
authors.lastname: Tsuji
authors.forename: Y
authors.initials: Y
authors.lastname: Sugawara
authors.forename: T
authors.initials: T
authors.lastname: Mizoue
authors.forename: K
authors.initials: K
authors.lastname: Tanaka
authors.forename: M
authors.initials: M
authors.lastname: Iwasaki
authors.forename: M
authors.initials: M
authors.lastname: Inoue
authors.forename: S
authors.initials: S
authors.lastname: Tsugane
authors.forename: S
authors.initials: S
authors.lastname: Sasazuki
mesh_terms: D015992:Body Mass Index; D001943:Breast Neoplasms / Q000453:epidemiology / Q000209:etiology*; D005260:Female; D006801:Humans; D015994:Incidence; D007564:Japan / Q000453:epidemiology; D050177:Overweight / Q000150:complications* / Q000453:epidemiology; D017698:Postmenopause; D017697:Premenopause; D011446:Prospective Studies; D012307:Risk Factors
keywords: body mass index; breast cancer; cohort study; pooled analysis