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Chapter 7
 For the development of a new BIA equation the DEXA-derived FFM was applied as the criterion for the development of a new prediction through multiple regression analysis. The following potentially influencing variables were used: body weight, age, body height, BMI, RI, ZI, R, Z, X, sex (coded as female=0 and male=1), and Tanner’s stages (16). Additionally we have conducted an evaluation of accuracy of the best performing BIA approach as concluded from the baseline evaluation. For 73 subjects (25 boys, 48 girls) we had matching 6 months follow-up BIA and DXA measurements as part of a weight loss trial (17). In this way, it was possible to evaluate the values for monitoring the subjects in time. Data were analyzed using SPSS 20.0 and RMSE with Excel.
Results
A total of 125 adolescents participated in this study. 22 adolescents were excluded because of overweight (n=10), due to a body weight higher than allowed for the DXA (>125 kg) (n=4) or missing data because of defective equipment (BIA) (n=8). Table 1 shows subject characteristics of the 103 (61 girls, 42 boys) adolescents by sex.
Table 1: Subject characteristics
Age, y
Height, m
Weight, kg
Waist circumference, cm BMI (kg/m2)
BMIsds
Tanner stage, n2 Pre-pubertal (stage 1) Pubertal (stage ≥ 2)
BIA
Resistance, Ω Reactance, Ω Impedance, (Z) Fat-free mass, kg * DXA
Fat-free mass, kg
Total (N=103)
14.4 (1.7)1
1.66 (0.09)
94.3 (15.7) 109.0 (11.1) 33.9 (4.1) 2.98 (0.34)
32 65
505.2 (54.1) 62.6 (7.7) 509.1 (54.3) 57.8 (10.1)
56.1 (9.8)
Girls (n=61)
Boys (n=42)
        14.7 (1.7) 14.1 (1.7)
   1.64 (0.06) 1.69 (0.12)
   94.3 (14.0) 94.2 (18.0)
   108.4 (11.1) 109.7 (11.2)
   34.8 (4.2) 32.7 (3.5)
   2.94 (0.35) 3.05 (0.32)
   10 22 50 15
        508.8 (49.2) 500.0 (60.7)
   63.9 (7.7) 60.7 (7.5)
   512.8 (49.6) 503.7 (60.8)
   55.9 (7.4) 60.4 (12.7)
        54.7 (7.1) 58.1 (12.5)
     * Fat-free mass is supplied by Hydra ECF/ICF Bio-Impedance Spectrum Analyzer 1 All values are means (SD), 2 For Tanner stage missing values were 1 for girls and 5 for boys
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