Our group and others previously showed that obesity is associated with an increased expression of the IDO1 gene in the human adipose tissue and with increased concentrations of circulating kynurenines and kynurenine to tryptophan (K/T) ratio reflecting the increased enzymatic activity of IDO1, that also correlates with the increased expression of inflammatory markers in the adipose tissue [3–6]. This evidence concerns the gene IDO1 and obesity due to melanocortin 4 receptor deficiency.