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

Abstract

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.

Introduction

Date accepted: September 26, 2000. Assessment of fat-free mass (FFM) and fat mass (FM) in patients optimizes nutrition support to avoid or minimize muscle wasting or obesity. Nutrition assessment should therefore include objective body-composition measurements. Bioelectrical impedance analysis (BIA) has been developed for field use and has shown great potential for use in estimating body composition; it is also easy to use, non-invasive, and inexpensive.1, 2

Many investigators have developed empiric BIA equations for prediction of FFM, total body weight, and body fat.1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 Most of these equations have been validated in relatively young, healthy adults against several body-composition techniques.9 Studies have shown that BIA formulas developed for healthy, normal-weight subjects are not suitable for obese subjects14, 15 and are not valid in elderly subjects.13

In longitudinal studies, the use of different BIA formulas in the same subject who becomes overweight or ages introduces a bias into body-composition studies and makes one question whether the differences in body composition are due to changes in the BIA formula or to changes in body composition. Thus, it would be advantageous to use a single formula that is applicable in young and elderly subjects and permits estimation of FFM and FM in overweight subjects and subjects with grade 1 obesity.

Roubenoff et al.13 and others concluded that BIA equations are subject to errors that cannot be determined a priori unless they are validated in the specific population in which they are to be applied.16, 17, 18 Thus, BIA equations must be validated in a representative population sample against a reference method before it can be accepted as accurate. BIA can be validated against dual-energy x-ray absorptiometry (DXA), hydrodensitometry, and total body potassium. DXA is one reference method3 that has been validated against independent methods such as in vivo neutron activation,5, 6 total body potassium, and hydrodensitometry.7 Recent studies have shown that DXA and hydrodensitometry agree well at high, moderate, and low levels of body fat.19 The purpose of this study was to validate, against DXA, a single BIA equation in a healthy, white European group of subjects aged 22–94 y and with body mass indexes (BMIs) of 17.0–33.8 kg/m². A single BIA equation that is valid in subjects with different ages and BMIs for use in longitudinal studies would be a significant advantage for nutrition assessment.