Obesity, body size, and risk of postmenopausal breast cancer: the Women's Health Initiative (United States).
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OBJECTIVE: Body size is an important modifiable risk factor for breast cancer. Although obesity has generally been found to be associated with increased risk for postmenopausal breast cancer, there remain questions concerning the role of body fat distribution, lifetime weight history, and effects within specific subgroups of women. METHODS: We assessed the relationship of several anthropometric measures and risk of postmenopausal breast cancer in 85,917 women aged 50-79 at entry in the Women's Health Initiative Observational Study. Women were enrolled during 1993-1998 at 40 clinics in the US and 1030 developed invasive breast cancer by April 2000. Upon entry, trained clinical center staff measured each woman's height, weight, and waist and hip circumference. RESULTS: Anthropometric factors were not associated with breast cancer among women who had ever used hormone replacement therapy (HRT). Among HRT non-users, heavier women (baseline body mass index (BMI) >31.1) had an elevated risk of postmenopausal breast cancer (relative risk (RR) = 2.52; 95% confidence interval (CI) = 1.62-3.93), compared to slimmer women (baseline BMI < 22.6). The elevation in risk associated with increasing BMI appeared to be most pronounced among younger postmenopausal women. Change in BMI since age 18, maximum BMI, and weight were also associated with breast cancer in HRT non-users. While both waist and hip circumference were associated with breast cancer risk, their ratio, a measure of fat distribution, was not (RR = 1.33; 95% CI = 0.88-2.01). CONCLUSIONS: Our study confirms previously reported findings that generalized obesity is an important risk factor for postmenopausal breast cancer, but only among women who have never taken HRT. Lifetime weight gain is also a strong predictor of breast cancer. Waist to hip ratio, a measure of weight distribution, does not appear to be related to postmenopausal breast cancer risk.