FNDC5 and obesity due to melanocortin 4 receptor deficiency: Circulating irisin levels increased nearly 10% after an average 27 kg of TFM loss (27.0 ± 8.3 kg, 33.6 ± 8.1% of total fat) following bariatric surgery in our study, while irisin levels in humans were positively correlated with parameters of adiposity [34,35] and associated with markers of glucose and lipid homeostasis disturbance in obesity and in patients with metabolic syndrome [39,42].