Table 1

Sources, structures, raw materials, and common forms of bile acids"

分类 Item	形成部位 Sources	核心结构 Core structure	合成底物 Raw material	常见形式 Common forms
初级胆汁酸 Primary bile acids (PBAs)	肝	12α-三羟基-5β-胆烷酸	胆固醇	胆酸鹅脱氧胆酸
次级胆汁酸 Secondary bile acids (SBAs)	肠道	3α, 12α-二羟-5β-胆烷酸	初级胆汁酸	脱氧胆酸石胆酸

Fig. 1

Regulatory effect of bile acids in animal intestinal physical barrier(Created by the BioRender.com) Villi length, crypt depth, and tight junctions between intestinal epithelial cells are important indicators of the integrity of the physical barrier of the gut. Bile acids increase villi length and decrease crypt depth to promote normal intestinal cell proliferation and protect the intestinal mucosa from injury or infection; At the same time, the key enzyme, Myosin light-chain kinase (MLCK), regulates the tight junctions between intestinal epithelial cells, thereby maintaining the integrity of the physical barrier"

Fig. 2

Regulatory effect of bile acids on the intestinal mucus barrier and microbial barrier of animals (Created by the BioRender.com) The intestinal mucus barrier and the microbial barrier are closely interdependent and closely related. The intestinal mucus layer is divided into the inner mucus layer and the outer mucus layer, and the intestinal microbes mainly inhabit the outer mucus layer and are able to convert primary bile acids into secondary bile acids. On the contrary, after bile acids act on this barrier, the secretion of mucus protein increases significantly, and the structure of the microflora also changes significantly. PBAs: primary bile acids; SBAs: secondary bile acids"

Table 2

The modification effect of microbiota on BA"

Fig. 3

Regulatory effect of bile acids in animal intestinal immune barrier(Created by the BioRender.com) The intestinal immune barrier is an important line of defense for animals against harmful pathogens. Bile acids can directly act on Paneth cells to promote the secretion of antimicrobial peptides such as α-defensin and promote macrophage polarization, and can also indirectly act on bile acid receptors such as FXR, VDR, RORγt, etc. to regulate the differentiation of helper T cells, and ultimately maintain the stability of the intestinal immune system. AMPs. Antimicrobial peptide; FXR. Farnesoid X-receptor; VDR. Vitamin D receptor; RORγt. Retinoic acid receptor-related orphan receptor γt"