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Am J Epidemiol. 2019 Jul 5. pii: kwz160. doi: 10.1093/aje/kwz160. [Epub ahead of print]

The Effect of Reverse Causality and Selective Attrition on the Relationship Between Body Mass Index and Mortality in Postmenopausal Women.

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Department of Epidemiology and Environmental Health, School of Public Health and Health Professions, University at Buffalo, The State University of New York.
Department of Nutritional Sciences, University of Arizona, Tucson, Arizona.
Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal, Quebec.
Department of Global Health, Boston University School of Public Health, Boston, Massachusetts.
Division of Research, Kaiser Permanente Northern California, Oakland, CA.
Division of Cardiovascular Medicine, Department of Medicine, Stanford, University School of Medicine, Stanford, CA, United States.
Department of Epidemiology, School of Public Health, University of Washington, Seattle, USA.
RAND Corporation, Santa Monica, California.
Department of Public Health Sciences, UC Davis School of Medicine, Sacramento, USA.
Department of Epidemiology, College of Public Health, University of Iowa, Iowa City, USA.
Department of Obstetrics and Gynecology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma.


Concerns about reverse causality and selection bias complicate the interpretation of studies of body mass index (BMI) and mortality in older adults. The objective of this manuscript is to investigate methodological explanations for the apparent attenuation of obesity-related risks in older adults. We used data from 68,132 participants from the Women's Health Initiative (WHI) clinical trial in this analysis. All of the participants were postmenopausal women aged 50-79 at baseline (1993-1998). To examine reverse causality and selective attrition, we compared rate ratios from inverse probability of treatment (IPTW) and censoring (IPCW) weighted Poisson marginal structural models to results from an unweighted adjusted Poisson regression model. The estimated mortality rate ratios and 95% confidence intervals for BMI 30-34.9, 35-39.9 and ≥40 kg/m2 were 0.86 (0.77, 5.48), 0.85 (0.72, 0.99), and 0.88 (0.72, 1.07) in the unweighted model. The corresponding mortality rate ratios were 0.96 (0.86, 1.07), 1.12 (0.97, 1.29), and 1.31 (1.08, 1.57) in the marginal structural model. Results from the IPTW and IPCW weighted marginal structural model were attenuated in low BMI categories and increased in high BMI categories. The results demonstrate the importance of accounting for reverse causality and selective attrition in studies of older adults.


aging; body mass index; reverse causality; selection bias; selective attrition


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