It is well known that the balance of intestinal microecology is the basis of human health. For example, the increase in the number of harmful bacteria in the gut will compete to inhibit the colonization of beneficial bacteria. At the same time, the lipopolysaccharide (LPS) produced will destroy the intestinal barrier, and then affect the absorption and transport of substances by intestinal receptors. Short-chain fatty acids (SCFAs) are endogenous products in the gut. The production and research of SCfas is a new journey to reveal the fate of human health. SCFAs are a group of saturated volatile fatty acids composed of six or less carbon atoms, which are produced by intestinal microorganisms such as anaerobic bacteria metabolizing fiber or carbohydrates. SCFAs in the gut are mainly acetate, propionate, and butyrate. A number of previous studies have shown that SCFAs have functions such as participating in body metabolism, regulating biological functions and maintaining host health homeostasis. By understanding the role and mechanism of SCFAs in gut microbial metabolism, we can better understand the link between gut health and overall health, and provide new ideas for the prevention and treatment of related diseases.

Dietary sources of short-chain fatty acids. Direct dietary sources of SCFAs include dairy products such as milk, butter and cheese (see chart below), reaching 0.1-10g per 100g. SCFAs are produced during the fermentation of dairy products, and they have an important effect on the flavor, aroma, and nutritional composition of dairy products. This also fully proves that fermentation products are not only beneficial to intestinal health, but also able to serve as an energy source for intestinal cells. In addition, some vegetable oils also contain SCFAs.

Although SCFAs can be rapidly oxidized and absorbed in the gut through direct dietary sources, most SCFAs are obtained indirectly through microbial fermentation of dietary fibers in fruits, vegetables, especially root vegetables, and legumes. In the process of colonic fermentation, SCFAs are mainly synthesized by sugar as the substrate and carbon chain extension pathway. In this process, glycoside hydrolases and gut microbiota are used to degrade dietary fiber into monosaccharides (pentose and hexose), and then produce SCFAs through glycolysis or pentose phosphate pathways. Intestinal probiotics can also use intestinal gases to produce specific SCFAs. In particular, those containing resistant starches and non-digestible carbohydrates can significantly promote the production of short-chain fatty acids such as acetate, propionate, and butyrate by the gut microbiota.

The role of SCFAs in improving obesity. In ameliorating obesity, SCFAs improve metabolic health by enhancing glucose metabolism, insulin metabolism, lipid accumulation, and fat oxidation (FIG. 2). Acetate, in particular, has been found to be able to positively affect obesity by modulating both central and peripheral mechanisms. "Acetate increases fasting fat oxidation, levels of fasting peptide YY (PYY), a hormone that suppresses appetite, and modulates postprandial glucose and insulin concentrations." The mechanisms of SCFAs in improving obesity are as follows: firstly, SCFAs are mediated by fatty acid receptors, acting on adipogenesis and Browning, and regulating insulin sensitivity in adipose tissue; Second, SCFAs activate the AMPK pathway to regulate glucose uptake and fatty acid oxidation in liver and skeletal muscle; Thirdly, SCFAs induced insulin secretion by upregulating cAMP and IP3 pathways in islet β cells. Fourth, SCFAs increased incretin secretion, such as PYY and glucagon-like peptide-1 (GLP-1); Satiety is increased by slowing intestinal motility, decreasing gastric emptying, and improving insulin sensitivity.

SCFAs are not only metabolites of intestinal microbes, but also regulators of body health. They have outstanding performance in regulating fat metabolism, immune regulation and reducing inflammation, which makes the complex interaction between diet, intestinal flora and human health linked together, such as brain-gut axis, liver-gut axis, gut-kidney axis, etc.