FNDC5 and obesity due to melanocortin 4 receptor deficiency: Subsequent studies in humans, mice and rats demonstrated effects of energy expenditure characteristics in humans [32], fasting/re-feeding regimens, obesity and exercise in rats [17] and chronic kidney disease in humans [21] on circulating proteins of this size range (western blot; 20–26 kDa), whereas no effects of calorie restriction in rats on FNDC5 protein levels in plasma were found [20].