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.