Underweight Patients with Anorexia Nervosa: Comparison of Bioelectrical Impedance Analysis Using Five Equations to Dual X-ray Absorptiometry

Background & Aims: Body weight changes do not reflect the respective changes of body compartments, namely fat-free mass (FFM) and fat mass (FM). Both bioelectrical Impedance Analysis (BIA) and the Dual X-ray absorptiometry (DXA) measure FFM and FM. This study in underweight patients with anorexia nervosa (AN) aims to compare measurements of FM and FFM done by DXA and BIA using 5 different BIA equations already validated in healthy population and to identify the most suitable BIA equation for AN patients.

Methods: Fifty female patients with AN (BMI=14.3 ± 1.49, age=19.98 ± 5.68yrs) were included in the study. Body composition was measured by DXA (Delphi W, Hologic, Bedford, MA) and by 50 kHz BIA (FORANA, Helios) using 5 different BIA equations validated in healthy population (Sun, Geneva, Kushner, Deurenberg and Roubenoff equations). Comparison between the DXA and the 5 BIA equations was done using the sum of the squares of differences and Bland-Altman plots.

Results: The Deurenberg equation gave the best estimates of FFM when compared to the measurements by DXA (FFM(dxa)=35.80 kg versus FFM(deurenberg)=36.36 kg) and very close estimates of FM (FM(dxa)=9.16 kg and FM(deurenberg)=9.57 kg) The Kushner equation showed slightly better estimates for FM (FM(kushner)=9. 0kg) when compared to the DXA, but not for FFM. Sun equation gave the broadest differences for FM and FFM when compared with DXA.

Conclusion: The best available BIA equation to calculate the FFM and the FM in patients with AN is the Deurenberg equation. It takes into account the weight, height and age and is applicable in adults and adolescents AN patients with BMI of 12.8-21.0, and for ages between 13.4 and up to 36.9 years.