KP also plays an important role in tryptophan metabolism, and excessive expression of KP was associated with insulin resistance (IR), which can lead to obesity and T2DM [25, 35]; 4-(2-aminophenyl)-2,4-dioxobutanoic acid and kynurenic acid were the metabolites of KP, which were increased in the state of T2DM and decreased after SFE treatment. Here, NPPA is linked to obesity due to melanocortin 4 receptor deficiency.