Approximately 90%−95% of tryptophan degradation occurs through the kynurenine pathway leading to de novo NAD+ biosynthesis (28), which depends on expressions of interferon (IFN)-γ, tumor necrosis factor (TNF)-α, bacterial lipopolysaccharides, and the pro-inflammatory cytokine interleukin (IL)-6, all of which are key pathogenic factors in IBD (28). Here, TNF is linked to inflammatory bowel disease.