Fig. 4.

Effect modification (interaction) of inflammatory cytokines on the metabolites of the kynurenine pathway in patients with MDD and T2DM and those with only MDD. TNF-α had a statistically significant effect modification on quinolinic acid/tryptophan and serotonin in patients with MDD and T2DM and those with only MDD (β = 1.029, adjusted p < 0.001; β =  − 1.444, adjusted p = 0.047, respectively). This interaction was measured by comparing the differences in the slopes of each regression line. TNF-α tumor necrosis factor-α (pg/mL), serotonin (relative area), MDD major depressive disorder, T2DM type 2 diabetes mellitus