Use of self-reported height and weight biases the body mass index-mortality association

Background: Many large-scale epidemiological data sources used to evaluate the body mass index (BMI: kg/m²) mortality association have relied on BMI derived from self-reported height and weight. Although measured BMI (BMI M) and self-reported BMI (BMI_SR) correlate highly, self-reports are systematically biased.Objective:To rigorously examine how self-reporting bias influences the association between BMI and mortality rate. Subjects: Samples representing the US non-institutionalized civilian population.Design and Methods:National Health and Nutrition Examination Survey data (NHANES II: 1976-80; NHANES III: 1988-94) contain BMI M and BMI_SR. We applied Cox regression to estimate mortality hazard ratios (HRs) for BMI M and BMI _SR categories, respectively, and compared results. We similarly analyzed subgroups of ostensibly healthy never-smokers.Results:Misclassification by BMI_SR among the underweight and obesity ranged from 30-40% despite high correlations between BMI M and BMI_SR (r0.9). The reporting bias was moderately correlated with BMI M (r0.35), but not BMI _SR (r0.15). Analyses using BMI_SR failed to detect six of eight significant mortality HRs detected by BMI M. Significantly biased HRs were detected in the NHANES II full data set (χ² 12.49; P22 9.93; P22 5.63; P22 1.52; PSR should not be treated as interchangeable with BMI M in BMI mortality analyses. Bias and inconsistency introduced by using BMI_SR in place of BMI M in BMI mortality estimation and hypothesis tests may account for important discrepancies in published findings.