Amino acids (AAs) are crucial for protein synthesis, in influencing nutritional status, as sources of vital elements (e.g., nucleotides, neurotransmitters) and as signal molecules for the modulation of gene expression and epigenetic mechanisms. Data on the role of amino acids in underweight patients with anorexia nervosa (AN) are unknown. This study aims to evaluate whether a blend of essential amino acids (EAA) could influence the change in lean body mass (LBM) in patients with AN during weight restoration, treated with intensive inpatient enhanced cognitive behavior therapy (CBT-E). A total of 92 patients will be randomized to EAA supplementation or placebo supplementation upon admission to inpatient treatment. LBM, but also body weight, specific and general psychopathology, impairment assessment will be assessed at baseline and after 13 weeks of treatment.