Definition and background of Prebiotics There are many translations of the introduction of prebiology, such as prebiotics, beneficial substances, growth AIDS and so on. Here we use prebiotics. It is defined as an undigested food ingredient that can have beneficial effects on the host by selectively stimulating the growth and activity of bacteria in one or a few colonies, thus improving the health of the host (Gibson and Roberfroid, 1995). The most basic prebiotic is carbohydrate, but the definition does not exclude non-carbohydrate substances used as prebiotics. Theoretically, any antibiotic that can reduce harmful bacteria and promote healthy bacteria or activities can be called prebiotics. This definition does not emphasize a particular strain. However, because Bifidobacterium and lactic acid bacteria are considered to have many beneficial effects on human body, general prebiotics are supposed to promote the increase of the number of these two bacteria or enhance their activity. A substance that can promote (promote the production of) Bifidobacterium is considered as a Bifidobacterium factor. Some prebiotics can be used as bifid factors and vice versa. But these two concepts are not the same, see the following explanation. Prebiotics and related terms and some misunderstandings Bifidobacterium: any component that promotes the growth of Bifidobacterium (NDOs): short-chain carbohydrates that cannot be digested by the small intestine and directly enter the large intestine. Some misunderstandings: all prebiotics are bifidobacteria-promoting: beneficial to the host (human body), not necessarily promoting Bifidobacterium. The promotion of other strains can also have beneficial effects. Therefore, prebiotics do not necessarily have the promotion of Bifidobacterium. Bifidobacterium factors are all prebiotics: Bifidobacterium factors can increase the number of Bifidobacterium in small intestine or other situations (such as fermented dairy products). When the bifidobacteria in the small intestine are promoted, the bifidobacteria can be considered as prebiotics, but only if the promotion is beneficial to the host (human body). Bifidobacterium in other conditions is not a prebiotic. Non-digestible oligosaccharides (NDOs) have the promotion of Bifidobacterium or are all prebiotics: Non-digestible oligosaccharides can promote beneficial bacteria to produce beneficial effects, in which case they can be considered as prebiotics. Non-digestible oligosaccharides can also promote harmful bacteria and produce harmful effects (such as lactose intolerance). NDOs can promote Bifidobacterium, so it can be called Bifidobacterium factor, but it is not necessarily a prebiotic. NDOs can also have no effect on intestinal colonies. Therefore, depending on different species, NDOs may be prebiotics, bifid factors, harmful or inactive. The definition also points out that changes in metabolic activity may improve host health. It means that it is not clear that a specific species needs to be stimulated, but the metabolic function of intestinal colonies as a whole changes. This usually refers to the increase of carbohydrate fermentation and the decrease of protein degradation and fermentation. The fermentation of carbohydrates generally produces harmless or beneficial end products, while protein fermentation leads to the formation of potentially harmful substances. (see figure <; People upload in the hazy rain-schematic diagram of protein and carbohydrates fermented by bacteria. Among them, the scribing substance mainly comes from protein fermentation. )。 Hydrogen sulfide gas has strong activity and can have adverse effects on the intestine. Other gases such as hydrogen, carbon dioxide and methane have no other negative effects except flatulence. Short-chain fatty acids and lactate are beneficial to intestinal colonies (lowering pH value and making intestinal environment more acidic) and intestinal cells (requiring short-chain fatty acids as energy substances). Ethanol can be rapidly metabolized by other intestinal bacteria without any effect on the host (human body). Branched-chain fatty acids, ammonia, amines, phenols and indole stimulate intestinal cells, induce organism mutation or do harm to immune system in high concentration. So the concept of prebiotics should be clear: increase beneficial bacteria and/or increase carbohydrate metabolism.