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/m(2)) 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.


METHODS
Samples representing the US non-institutionalized civilian population.


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) (r>0.9). The reporting bias was moderately correlated with BMI(M) (r>0.35), but not BMI(SR) (r<0.15). 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 (χ(2)=12.49; P=0.01) and healthy subgroup (χ(2)=9.93; P=0.04), but not in the NHANES III full data set (χ(2)=5.63; P=0.23) or healthy subgroup (χ(2)=1.52; P=0.82).


CONCLUSIONS
BMI(SR) 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.
Keith
S W
SW
Department of Biostatistics, Section on Statistical Genetics and Clinical Nutrition Research Center, University of Alabama at Birmingham, Birmingham, AL, USA. Scott.Keith@jefferson.edu
Fontaine
K R
KR
Pajewski
N M
NM
Mehta
T
T
Allison
D B
DB
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001827:Body Height; D015992:Body Mass Index; D001835:Body Weight; D002648:Child; D002675:Child, Preschool; D059026:Diagnostic Self Evaluation; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D009765:Obesity / Q000453:epidemiology / Q000401:mortality*; D057566:Self Report; D011795:Surveys and Questionnaires; D055815:Young Adult