Given the prevalence of significantly altered serotonergic-related pathway expression from GO analysis, serotonin’s close association with the GI tract, and the positive correlation between vitamin D and serotonin synthesis via TPH1 induction, a selection of differentially expressed genes in IBS patients (Fig 2) was assessed for regulation by vitamin D in human colonic cells [45, 70, 71]. This evidence concerns the gene TPH1 and irritable bowel syndrome.