Single prediction equation for bioelectrical impedance analysis in adults aged 20–94 years

Existing equations for bioelectrical impedance analysis (BIA) are of limited use when subjects age or become overweight because these equations were developed in young, normal-weight subjects and are not valid in elderly or overweight people. The purpose of this study was to validate a single BIA equation in healthy white subjects aged 22–94 y with a body mass index between 17.0 and 33.8 kg/m2. Healthy subjects (202 men and 141 women) aged 20–94 y were measured by two methods: fat-free mass (FFM) was measured by dual-energy x-ray absorptiometry (Hologic QDR-4500) and by a bioelectrical impedance analyzer (Xitron 4000B). Validity of BIA was assessed by double cross validation. Because correlations were high (r = 0.986–0.987) and prediction errors low, a single equation was developed using all subjects, as follows: FFM = −4.104 + (0.518 × height2/resistance) + (0.231 × weight) + (0.130 × reactance) + (4.229 × sex: MEN = 1, WOMEN = 0). FFM predicted with dual-energy x-ray absorptiometry was 54.0 ± 10.7 kg. BIA-predicted FFM was 54.0 ± 10.5 kg (r = 0.986, standard error of the ESTIMATE = 1.72 kg, technical ERROR = 1.74 kg). In conclusion, the new Geneva BIA equation was valid for prediction of FFM in healthy adults aged 22–94 y with body mass indexes between 17.0 and 33.8 kg/m2. Inclusion of reactance in the single prediction equation appeared to be essential for use of BIA equations in populations with large variations in age or body mass.