**2.6 Bacterial overgrowth**

Existing evidence points to the involvement of bacterial overgrowth in the pathogenesis of NAFLD and NASH. The gut microbiome is implicated in the pathogenesis and progression of NAFLD through the so-called gut-liver axis, investigating that the gut microbiome could be considered a metabolic organ in the host, which can affect human metabolism in health and disease [13]. Bacterial overgrowth results in the secretion of bacterial lipopolysaccharides (LPS), which can activate the production of TNF-α, and ethanol. Bacterial LPS are produced when Gram-negative bacteria proliferate excessively and are mostly transported due to increased intestinal permeability. Furthermore, the interaction between LPS and the Toll-like receptors (TLRs4) system increases oxidative stress in NAFLD because of the excessive ROS generation and deficiency in endogenous antioxidant molecules [7]. Oxidative stress has also been reported to be overexpressed in CVDs, which may be a link that connects LPS to the elevated cardiovascular risk in NAFLD patients. The elevated LPS levels in the circulation may result from various factors. In the intestine, the absorption of LPS together with chylomicrons intensifies the chance of NAFLD development, which is activated by hepatic inflammatory cells. Additionally, intestinal bacteria stimulate lipoprotein lipase activity and triglyceride accumulation by inhibiting the synthesis of fasting-induced adipocyte factor (FIAF) [7]. Furthermore, the gut microbiota syntheses enzymes that facilitate the transformation of dietary choline into toxic substances, such as methylamines, which can be utilized by the liver, transformed into trimethylamine-N-oxide, and subsequently promote inflammation and liver damage. Also, bacterial endotoxins have damaging effects on hepatocytes and can stimulate Kupffer cells to generate inflammatory cytokines, which would then cause the waterfall effect and the generation of oxygen radicals [6, 7]. TLRs on hepatocytes, HSCs, and Kupfer cells detect bacterial endotoxins. Signaling of bacterial LPS through TLR4 activates the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) (**Figures 1** and **2**) and subsequent inflammasome activation. Increased secretion of bile acids (Bas) facilitated by a high-fat diet is another mechanism that may result in gut hyperpermeability in individuals with NAFLD. Gut permeability is compromised because of BAs, which increase epidermal growth receptor (EGFR) activity. Generally, these pathways could partially account for the "leaky gut" phenomenon seen in the majority of NAFLD patients [6, 7, 13].
