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Is BMI a valid measure of obesity in postmenopausal women?

Banack, Hailey, R., PhD1; Wactawski-Wende, Jean, PhD1; Hovey, Kathleen, M., MS1; Stokes, Andrew, PhD2

doi: 10.1097/GME.0000000000000989
Original Articles

Objective: Body mass index (BMI) is a widely used indicator of obesity status in clinical settings and population health research. However, there are concerns about the validity of BMI as a measure of obesity in postmenopausal women. Unlike BMI, which is an indirect measure of obesity and does not distinguish lean from fat mass, dual-energy x-ray absorptiometry (DXA) provides a direct measure of body fat and is considered a gold standard of adiposity measurement. The goal of this study is to examine the validity of using BMI to identify obesity in postmenopausal women relative to total body fat percent measured by DXA scan.

Methods: Data from 1,329 postmenopausal women participating in the Buffalo OsteoPerio Study were used in this analysis. At baseline, women ranged in age from 53 to 85 years. Obesity was defined as BMI ≥ 30 kg/m2 and body fat percent (BF%) greater than 35%, 38%, or 40%. We calculated sensitivity, specificity, positive predictive value, and negative predictive value to evaluate the validity of BMI-defined obesity relative BF%. We further explored the validity of BMI relative to BF% using graphical tools, such as scatterplots and receiver-operating characteristic curves. Youden's J index was used to determine the empirical optimal BMI cut-point for each level of BF% defined obesity.

Results: The sensitivity of BMI-defined obesity was 32.4% for 35% body fat, 44.6% for 38% body fat, and 55.2% for 40% body fat. Corresponding specificity values were 99.3%, 97.1%, and 94.6%, respectively. The empirical optimal BMI cut-point to define obesity is 24.9 kg/m2 for 35% BF, 26.49 kg/m2 for 38% BF, and 27.05 kg/m2 for 40% BF according to the Youden's index.

Conclusions: Results demonstrate that a BMI cut-point of 30 kg/m2 does not appear to be an appropriate indicator of true obesity status in postmenopausal women. Empirical estimates of the validity of BMI from this study may be used by other investigators to account for BMI-related misclassification in older women.

1Department of Epidemiology and Environmental Health, School of Public Health and Health Professions, University at Buffalo, The State University of New York, Buffalo, NY

2Department of Global Health and Center for Global Health and Development, Boston University School of Public Health, Boston, MA.

Address correspondence to: Hailey R. Banack, PhD, Department of Epidemiology and Environmental Health, School of Public Health and Health Professions, University at Buffalo, The State University of New York, 270 Farber Hall, Buffalo, NY 14214-8001. E-mail: hrbanack@buffalo.edu

Received 13 June, 2017

Revised 15 August, 2017

Accepted 15 August, 2017

Funding/support: The Women's Health Initiative (WHI) program is funded by the National Heart, Lung, and Blood Institute, National Institutes of Health, U.S. Department of Health and Human Services through contracts HHSN268201600018C, HHSN268201600001C, HHSN268201600002C, HHSN268201600003C, and HHSN268201600004C; JW-W: R01DE013505 from the National Institute of Dental and Craniofacial Research, National Institutes of Health (NIH), U.S. Army, Medical Research and Materiel Command, grant OS950077 and NIH/National Heart Lung and Blood Institute contract N01WH32122; AS: National Center for Health Statistics R03SH000037; HB: Canadian Institute of Health Research (CIHR) Banting Postdoctoral Fellowships Program.

Financial disclosure/conflicts of interests: AS has received research funding from Johnson & Johnson.

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© 2018 by The North American Menopause Society.