Increased adipose tissue IDO expression and activity were observed in visceral obesity [30,31] and, accordingly, excessive tryptophan catabolism mediated by IDO activity was reported in obesity [30,31,32,33], while a high-fat diet proved to raise kynurenine levels in animal studies [29,34,35]. The gene discussed is IDO1; the disease is obesity due to melanocortin 4 receptor deficiency.