Among patients with obesity, the KP is activated and the expression of the IDO1 gene is increased, eliciting the conversion of TRP to KYN [6] and the association of CRP (C-reactive protein) with high levels of QA and low KA/QA ratio; this reflects the proinflammatory and neurotoxic effects of QA and anti-inflammatory effects of KYNA [94]. Here, IDO1 is linked to obesity due to melanocortin 4 receptor deficiency.