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Lifespan Health Research Center
Department of Community Health
3171 Research Boulevard
Kettering, OH 45420
E-mail: shumei.sun{at}wright.ed
Obesity Research Center
St Lukes-Roosevelt Hospital
New York
US Department of Agriculture Agricultural Research Service
Grand Forks, ND
Nutritional Sciences
University of Wisconsin, Madison
Military Operational Medicine Program
Frederick, MD
National Institutes of Health
National Institute of Diabetes and Digestive and Kidney Diseases
Bethesda, MD
Centers for Disease Control and Prevention
Hyattsville, MD
Dear Sir:
We appreciate the comments by Trippo et al regarding the application of the bioelectrical impedance analysis (BIA) equations from our article "Development of bioelectrical impedance analysis prediction equations for body composition with the use of a multicomponent model for use in epidemiologic surveys" (1). In their letter, the authors applied our sex-specific BIA equations for fat-free mass (FFM) to a sample of 89 Germans between 18 and 65 y of age. Trippo et al compared the estimated FFM from our equations with estimated FFM from the BIA prediction equations of Deurenberg et al (2) and of Lukaski et al (3) and with FFM measured with dual-energy X-ray absorptiometry (DXA) with a Hologic QDR2000 (Hologic Inc, Bedford, MA). Trippo et al concluded that our equations produced greater FFM values than did DXA and the equations of Deurenberg et al and Lukaski et al. These findings were as expected and point out the importance of deriving body composition from a multicomponent model.
The equations of Deurenberg et al and Lukaski et al used FFM from underwater weighing based on a 2-component body-composition model in which a constant density of FFM of 1.1 g/mL based on Siri's equation (4) and a constant density of a reference body of 1.064 g/mL based on the equation of Brozek et al (5) was assumed. The QDR2000 DXA technology assumes that the percentage of water in FFM is 73%, which implies an approximate constant density of FFM. Our BIA equations were validated and cross-validated against criterion methods by using Heymsfield et al's (6) multicomponent model of FFM, which includes measures of body volume from underwater weighing, total body water, bone mineral content measured by DXA, and body weight. This multicomponent body-composition model does not assume a constant fat-free density or reference body, whereas the 2-component models of Siri and Brozek et al and DXA both assume constant densities. The density of FFM changes with age and is greater in males than in females, and these differences reflect variation in the concentrations of water, protein, and minerals in FFM. FFM obtained from a multicomponent body-composition model is greater than is FFM obtained from a 2-component model in children, young adults, and women (7-9). This is consistent with the finding of Trippo et al that FFM derived from our multicomponent body-composition model is larger than that estimated by DXA or derived with the use of the BIA equations of Deurenberg et al and Lukaski et al. In these 2 studies, the average age was the late 20s.
Trippo et al did not specify the BIA methods that were used to predict FFM; therefore, the machine, the frequency, and the electrode configurations used to collect the BIA data from these 89 persons are unknown. The BIA data in our study were collected under standardized protocols that were identical in each laboratory.
Our equations were designed specifically for application to the BIA data from the third National Health and Nutrition Examination Survey and were not to serve as the "final answer" for all BIA equations. Therefore, our sample included non-Hispanic whites and non-Hispanic blacks. The sample of Trippo et al included only German adults. It may not be possible to produce universal BIA equations because there are differences between populations. These differences cannot be detected without the use of a 4-compartment model, and they require calibration of each population with the use of BIA.
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