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American Journal of Clinical Nutrition, Vol. 79, No. 2, 335-336, February 2004
© 2004 American Society for Clinical Nutrition

LETTERS TO THE EDITOR

Bioelectrical impedance analysis for predicting body composition: what about the external validity of new regression equations?

Ulrike Trippo, Corinna Koebnick and HJ Franz Zunft

German Institute of Human Nutrition
Department of Intervention Studies
Arthur-Scheunert-Allee 114-116
D-14558 Potsdam-Rehbrueke
Germany
E-mail: zunft{at}mail.dife.de

Holle Greil

Institute of Biology and Biochemistry
University of Potsdam
Potsdam
Germany

Dear Sir:

In a recent issue of the Journal, Sun et al (1) emphasized the importance of bioelectrical impedance analysis (BIA) for large-scale epidemiologic studies. In their well-designed and detailed investigation, the authors combined several methods to assess different body compartments in 1474 whites and 355 blacks. In a multistep procedure, they used these results to develop a model for predicting total body water and fat-free mass (FFM) from BIA resistance. First, total body fat was calculated by using body weight, body volume derived from hydrostatic weighing, total body water calculated with the deuterium dilution method, and total-body bone mineral content determined with dual-energy X-ray absorptiometry (DXA). Second, in a regression analysis, these data were used to develop preliminary equations for calculating FFM from BIA resistance. After cross-validation, final equations were derived that were fitted to FFM calculated from the multicompartment model with a very high precision (R2 = 0.90 for males and 0.83 for females).

We are well aware of the limitations of previous BIA prediction equations. In recent studies we showed that the widely used equations of Deurenberg et al (2) and Lukaski et al (3) produced large differences in calculations of FFM (4). These differences increased with increasing age and body mass index and were higher in females than in males.

Thus, because we were enthusiastic about having new and better equations for calculating total body water and FFM from BIA measures, we recalculated a data set obtained from 708 white Germans between 18 and 65 y of age (4). In a subsample of 89 subjects, we also calculated FFM from a DXA scan (QDR2000; Hologic Inc, Bedford, MA).

Our results were not very satisfying. FFM calculated according to the equation provided by Sun et al was 4.2 ± 1.9 kg ( ± SD) higher than that calculated according to the equation of Deurenberg et al (2), 2.3 ± 1.3 kg higher than that calculated according to the equation of Lukaski et al (3), and 3.7 ± 2.5 kg higher than that measured by DXA (Figure 1Go). The overprediction by the equation of Sun et al relative to the DXA measures and other BIA equations was more pronounced in males than in females (P < 0.001) and increased with increasing age (P < 0.001) and body mass index (P < 0.001).



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  		FIGURE 1.. Bland-Altman plot of fat-free mass (FFM) calculated according to Sun et al (1) compared with that calculated by Deurenberg et al (2) and by dual-energy X-ray absorptiometry (DXA).

 
In conclusion, FFM calculated with the new prediction equation of Sun et al was substantially overestimated compared with FFM calculated with other prediction equations and with DXA. This overestimation was higher than the marginal overestimation of {approx}0.3 kg observed by Sun et al. What did we learn from our recalculation? The study by Sun et al took advantage of 3 factors: a large population, qualified equations confirmed by a subsequent cross-validation, and gold standard methods. However, after applying their model in our study population, we found the external validity to be questionable. Are BIA prediction equations applicable in general or do we need to have different equations for different populations or devices?

REFERENCES

  1. Sun SS, Chumlea WC, Heymsfield SB, et al. Development of bioelectrical impedance analysis prediction equations for body composition with the use of a multicomponent model for use in epidemiologic surveys. Am J Clin Nutr 2003;77:331-40.[Abstract/Free Full Text]
  2. Deurenberg P, van der Kooy K, Leenen R, Weststrate JA, Seidell JC. Sex and age specific prediction formulas for estimating body composition from bioelectrical impedance: a cross-validation study. Int J Obes 1991;15:17-25.[Medline]
  3. Lukaski HC, Bolonchuk WW, Hall CB, Siders WA. Validation of tetrapolar bioelectrical impedance method to assess human body composition. J Appl Physiol 1986;60:1327-32.[Abstract/Free Full Text]
  4. Trippo U, Greil H. Body composition, nutrition and nutritional habits of young and elderly adults. Coll Anthropol 1998;22:365-84.




This Article
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