Stengel et al. [56] also showed that circulating irisin concentration was positively correlated with fat-free mass using a BIA method in patients with anorexia nervosa (BMI 12.6 ± 0.7 kg/m2), normal weight controls (BMI 22.6 ± 0.9 kg/m2), and obese patients (BMI 30–40, 40–50 and >50 kg/m2). The gene discussed is FNDC5; the disease is anorexia nervosa.