With the increasing interest in the interaction between host health and small-molecule microbial metabolites, studies are emerging consecutively on metabolites generated by microbiota, such as short-chain fatty acids (SCFAs) and secondary bile acids, which mediate the interaction between the host and the microbial community and have made outstanding contributions to human health and disease treatment. This is likely because the ability of bacteria to digest protein is strengthened by the consumption of carbohydrates, the extended digestion time, the higher pH values, and the increase in the number of bacteria. Similarly, phenolic compounds, which are bacterial metabolites of aromatic amino acids, are at least four times more abundant in the human distal colon than in the proximal colon. The efficiency of protein fermentation in the distal intestine tract is higher than that in the proximal section. Finally, amino acids in the gut tract are deaminated or decarboxylated by microbiota to form various small-molecule metabolites. According to the varied intake, some proteins, peptides, and amino acids can enter the large intestine following the peristalsis of the gut tract. Most dietary proteins are digested and absorbed in the upper gastrointestinal (GI) tract by the action of proteases. Īs an essential macronutrient, proteins are used by humans to meet the requirements of the body. Among the 20 most common amino acids, tryptophan is the most complex and the one with the least content in cells and proteins, but it plays an indispensable role in body metabolism. Therefore, as an essential amino acid, tryptophan is mainly taken up by humans from the diet, especially from protein-rich foods, such as meat, eggs, milk, and chocolate. Although some gut bacteria, such as Escherichia coli, can produce tryptophan, the contribution of bacterial-derived tryptophan to the physiological functions of the body has not yet been reported. Animal cells cannot synthesize tryptophan. More and more evidence suggests that metabolites produced by gut microbiota are key mediators of the cross-talk between dietary intake and host health. The diversity and homeostasis of gut microbial communities play an important role in host health and nutrient metabolism. In future studies, further work should be performed to explore the effects and mechanisms of IPA on host health and disease to further improve the existing treatment program. However, the therapeutic effect of IPA depends on dose, target organ, or time. IPA shows great potential for the diagnosis and treatment of various clinical diseases, such as NAFLD, Alzheimer’s disease, and breast cancer. Moreover, IPA can act on target organs through blood circulation to form the gut–organ axis, which helps maintain systemic homeostasis. IPA can not only stimulate the expression of tight junction (TJ) proteins to enhance gut barrier function and inhibit the penetration of toxic factors, but also modulate the immune system to exert anti-inflammatory and antioxidant effects to synergistically regulate body physiology. Here, we summarize the IPA-producing bacteria, dietary patterns on IPA content, and functional roles of IPA in various diseases. Dietary tryptophan ingested by the host enters the gut, where indole-like metabolites such as indole propionic acid (IPA) are produced under deamination by commensal bacteria. Increasing evidence suggests that metabolites produced by the gut microbiota play a crucial role in host–microbe interactions.
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