Assessment of fat-free mass from bioelectrical impedance analysis in obese women with Prader-Willi syndrome

Abstract

Background: Fat-free mass (FFM) is lower in obese subjects with Prader-Willi syndrome (PWS) than in obese subjects without PWS. FFM prediction equations developed in non-PWS subjects may, thus, not work in PWS subjects.

Aim: To test whether the estimation of FFM from bioelectrical impedance analysis (BIA) in PWS subjects requires population-specific equations.

Methods: Using dual-energy X-ray absorptiometry, this study measured FFM in 27 PWS and 56 non-PWS obese women and evaluated its association with the impedance index at 50 kHz (ZI₅₀), i.e. the ratio between squared height and whole-body impedance at 50 kHz.

Results: At the same level of ZI₅₀, PWS women had a lower FFM than non-PWS women. However, when PWS-specific equations were used, FFM was accurately estimated at the population level. An equation employing a dummy variable coding for PWS status was able to explain 85% of the variance of FFM with a root mean squared error of 3.3 kg in the pooled sample (n = 83).

Conclusion: Population-specific equations are needed to estimate FFM from BIA in obese PWS women.

• Bioelectrical impedance analysis
• body composition
• dual energy X-ray absorptiometry
• Prader-Willi syndrome
• prediction equations

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

The study was supported by Progetti di Ricerca Corrente, Istituto Auxologico Italiano, Verbania and Milano, Italy.