Primary-Secondary-Tertiary Axis of Bile Acid Metabolism in Healthy Subjects

Bile acid (BA) metabolism, governed by host-gut microbial interplay, critically regulates the immunometabolic processes. We present the first quantitative framework for human BA dynamics spanning the primary (hepatic synthesis), secondary (microbial metabolism), and tertiary (host modification) metabolic axes. Employing a validated panel of 43 BA standards, we profiled BA species in adult plasma, urine, feces, and neonatal urine, detecting 19, 28, and 39 BAs in adult plasma, urine, and feces. Notably, we identified abundant yet underreported species including fecal 12epi-DCA and 12epi-CA, along with urinary G12epi-DCA, GDCA-1β-ol, and GDCA-5β-ol. Quantitative analysis of 12 healthy adults revealed daily BA excretion rates of 407 ± 88.3 μmol (fecal) and 1.41 ± 0.255 μmol (urinary), with primary/secondary/tertiary BAs constituted 21.8%/76.6%/1.6% in feces versus 22.8%/49.6%/27.6% in urine. While high-fat diets did not alter fecal BA excretion, they induced significant postprandial urinary BA surges (0-4 h), paralleling the elevation of plasma concentrations. The revealed baseline metrics of human BA metabolism along the primary-secondary-tertiary axis indicate that tertiary BAs play a supplementary role in human BA disposition. The identification of microbial 12-epimerized BA metabolites highlights untapped complexity in BA-mediated host-microbe crosstalk. These findings resolve critical knowledge gaps in translational studies targeting BA-related metabolic disorders.