title: Fat-free mass prediction equations for bioelectric impedance analysis compared to dual energy X-ray absorptiometry in obese adolescents: a validation study.
abstract: BACKGROUND
  In clinical practice, patient friendly methods to assess body composition in obese adolescents are needed. Therefore, the bioelectrical impedance analysis (BIA) related fat-free mass (FFM) prediction equations (FFM-BIA) were evaluated in obese adolescents (age 11-18 years) compared to FFM measured by dual-energy x-ray absorptiometry (FFM-DXA) and a new population specific FFM-BIA equation is developed.
  
  
  METHODS
  After an overnight fast, the subjects attended the outpatient clinic. After measuring height and weight, a full body scan by dual-energy x-ray absorptiometry (DXA) and a BIA measurement was performed. Thirteen predictive FFM-BIA equations based on weight, height, age, resistance, reactance and/or impedance were systematically selected and compared to FFM-DXA. Accuracy of FFM-BIA equations was evaluated by the percentage adolescents predicted within 5% of FFM-DXA measured, the mean percentage difference between predicted and measured values (bias) and the Root Mean Squared prediction Error (RMSE). Multiple linear regression was conducted to develop a new BIA equation.
  
  
  RESULTS
  Validation was based on 103 adolescents (60% girls), age 14.5 (sd1.7) years, weight 94.1 (sd15.6) kg and FFM-DXA of 56.1 (sd9.8) kg. The percentage accurate estimations varied between equations from 0 to 68%; bias ranged from -29.3 to +36.3% and RMSE ranged from 2.8 to 12.4 kg. An alternative prediction equation was developed: FFM = 0.527 * H(cm)(2)/Imp + 0.306 * weight - 1.862 (R(2) = 0.92, SEE = 2.85 kg). Percentage accurate prediction was 76%.
  
  
  CONCLUSIONS
  Compared to DXA, the Gray equation underestimated the FFM with 0.4 kg (55.7 ± 8.3), had an RMSE of 3.2 kg, 63% accurate prediction and the smallest bias of (-0.1%). When split by sex, the Gray equation had the narrowest range in accurate predictions, bias, and RMSE. For the assessment of FFM with BIA, the Gray-FFM equation appears to be the most accurate, but 63% is still not at an acceptable accuracy level for obese adolescents. The new equation appears to be appropriate but await further validation. DXA measurement remains the method of choice for FFM in obese adolescents.
  
  
  BACKGROUND
  Netherlands Trial Register ( ISRCTN27626398).
authors.affiliation: Department of Nutrition & Dietetics, Internal Medicine, VU University Medical Center, De Boelelaan 1117, 1081, , HV, Amsterdam, The Netherlands. A.Hofsteenge@vumc.nl.
authors.forename: Geesje H
authors.initials: GH
authors.lastname: Hofsteenge
authors.affiliation: EMGO Institute for Health and Care Research, VU University Medical Center, Amsterdam, The Netherlands. M.Chinapaw@vumc.nl.|Department of Public and Occupational Health, VU University Medical Center, Amsterdam, The Netherlands. M.Chinapaw@vumc.nl.
authors.forename: Mai J M
authors.initials: MJ
authors.lastname: Chinapaw
authors.affiliation: Department of Nutrition & Dietetics, Internal Medicine, VU University Medical Center, De Boelelaan 1117, 1081, , HV, Amsterdam, The Netherlands. P.Weijs@vumc.nl.|EMGO Institute for Health and Care Research, VU University Medical Center, Amsterdam, The Netherlands. P.Weijs@vumc.nl.|Department of Nutrition & Dietetics, Amsterdam University of Applied Sciences, Amsterdam, The Netherlands. P.Weijs@vumc.nl.
authors.forename: Peter J M
authors.initials: PJ
authors.lastname: Weijs
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods*; D000293:Adolescent; D001823:Body Composition; D002648:Child; D017097:Electric Impedance; D005260:Female; D006801:Humans; D008297:Male; D009765:Obesity / Q000175:diagnosis* / Q000378:metabolism; D011237:Predictive Value of Tests; D015203:Reproducibility of Results