Letter to the Editor

International Journal of Obesity (2012) 36, 1591; doi:10.1038/ijo.2011.277; published online 24 January 2012

Self-report corrections for BMI: Comment on Keith et al

S W Keith1, M Stommel2, D B Allison3 and C A Schoenborn4

  1. 1Department of pharmacology, Division of Biostatistics, Thomas Jefferson University, Philadelphia, PA, USA
  2. 2College of Nursing, Michigan State University, East Lansing, MI, USA
  3. 3Department of Biostatistics, University of Alabama at Birmingham, Birmingham, AL, USA
  4. 4CDC/National Center for Health Statistics, Hyattsville, MD, USA

Correspondence: SW Keith, E-mail: scott.keith@jefferson.edu

We, the authors of two publications, one in the International Journal of Obesity and the other in BMC Public Health, write to correct points in each of these two articles.1, 2 The primary objective of the article by Keith et al.1 was to evaluate whether use of self-reported height and weight led to biases in the association between the mortality rate and obesity, and indeed such a result was the major finding of the paper. As an ancillary point, in the discussion section Keith et al. stated:

BMISR (self-reported body mass index) should not be considered a reliable source of information for estimating BMIM (measured BMI) with a regression model. This calls into question the validity of methods using self-reported heights and weights and other study variables, such as the methods proposed by Stommel and Schoenborn2 based on recent NHANES (National Health and Nutrition Examination Survey) data, to generate corrected BMI scores conditioned on BMISR. When we applied their method2 to NHANES III, although the corrected BMI scores did improve classification over uncorrected BMISR among the severely obese (increased sensitivity from 68 to 84%), they significantly exacerbated the misclassification problems among the underweight, normal weight, overweight and obese (sensitivity decreased from 58%, 91%, 80% and 62% to 3%, 30%, 31% and 47%, respectively). Compared with BMIM, the corrected BMI scores also increased the bias in estimating MR beyond that which we showed from using BMISR.

The above claim was based on results obtained by applying the equation published in Table 3 of the Stommel and Schoenborn article.2 It was subsequently determined that the published information was insufficient for the stated purpose for the following reasons: (1) the intercept term had been omitted from the published table; (2) the regression coefficients had been rounded to the third decimal; and (3) a binary variable for missing income information had been omitted. In addition, Keith et al. applied the equation by applying the regression coefficients for self-reported height and its squared term to data scaled in meters instead of centimeters as in the original equation. In Table 1 below, we reproduce the correct complete equation. Using this complete equation to calculate corrected BMI scores, improved the classification sensitivities from those published by Keith et al. (3%, 30%, 31% and 47%) to 50%, 89%, 82%, and 72%, among the underweight, normal weight, overweight and obese of NHANES III, respectively.


The authors of the two articles have concluded that the statement in the discussion section by Keith et al.1 —indicating that use of the correction equation of Stommel and Schoenborn2 did not reduce the bias introduced into the BMI–mortality association by use of self-reported data—was premature. In addition, Stommel and Schoenborn would like to suggest that the tendency to over-report height and under-report weight is subject to change over time, and therefore equations based on 2001–2006 data might not be entirely applicable to data from 1988 to 1994. The authors of both articles advise that future research more thoroughly evaluate the question of whether corrections to self-reported heights and weights can substantially reduce biases in BMI–mortality associations introduced by use of self-reported heights and weights.

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References

  1. Keith SW, Fontaine KR, Pajewski NM, Mehta T, Allison DB. Use of self-reported height and weight biases the body mass index-mortality association. Int J Obes 2011; 35: 401–408. | Article | ISI |
  2. Stommel M, Schoenborn CA. Accuracy and usefulness of BMI measures based on self-reported weight and height: findings from the NHANES & NHIS 2001-2006. BMC Public Health 2009; 9: 421. | Article | PubMed |

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