Gut microbiota and diabetes: From correlation to causality and mechanism

Table 1 A summary of products of gut microbiota and their mechanism of action

Gut microbiota products		Source	Mechanism	Function		Ref.
LPS		The cell wall of Gram-negative bacteria	Activates the receptor TLR4	Increase the occurrence of inflammatory response		[59,60]
SCFAs	Acetate	Carbohydrate fermentation	Activates the receptor FFAR2	Reduce the frequency of autoimmune T cells in lymphoid tissues	FFAR2 and FFAR3 stimulate the release of GLP-1 and PYY, which improve insulin secretion.	[15,91,105]
	Propionate		Activates the receptor FFAR2 and FFAR3	Promote intestinal gluconeogenesis
	Butyrate		Activates the receptor FFAR3	Increase the number and function of regulatory T cells
Bile acids		The microbiota from host cholesterol	Bind to the receptor TGR5 and FXR	Improve insulin sensitivity		[74,76]
BCAA		Prevotella copri and Bacteroides vulgatus	Activate PI3K and increase the oxidation of free fatty acids	Increase the risk of insulin resistance		[86,87]

LPS: Lipopolysaccharide; TLR4: Toll-like receptor 4; SCFAs: Short-chain fatty acids; FFAR 2: Free fatty acid receptor 2; FFAR 3: Free fatty acid receptor 3; GLP-1: Glucagon-like peptide-1; PYY: Peptide YY; TGR5: Takeda G protein-coupled receptor 5; FXR: Farnesoid X receptor; BCAA: Branched-Chain Amino Acids; PI3K: Phosphatidylinositol 3-kinase.