TNF and irritable bowel syndrome: Growing evidence by several animal and vitro experimental studies supports a plausible link between NAFLD and IBS, including shared proinflammatory cytokines (i.e., increased expressions of tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-8 and IL-1β, decreased levels of IL-10), cross-talk of liver-brain-gut neural arc and gut-liver axis, dysfunction of intestinal microbiota and impaired intestinal barrier as well as intestinal dysmotility [11–17].