Myostatin is a negative regulator of muscle mass, as well as a potential biomarker that contributes to both metabolic and anabolic defects in SO.[8,9] Some studies have revealed an association between decreased myostatin levels and age, decreased muscle mass, and decreased muscle strength.[9,10] Moreover, high myostatin levels have been shown to be associated with obesity and IR. Here, MSTN is linked to obesity due to melanocortin 4 receptor deficiency.