Context: Alterations in bile acid (BA) synthesis and transport have the potential to affect multiple metabolic pathways in the pathophysiology of obesity. Objective: The objective of the study was to investigate the effects of obesity on serum fluctuations of BAs and markers of BA synthesis. Design: We measured BA fluctuations in 11 nonobese and 32 obese subjects and BA transporter expression in liver specimens from 42 individuals and specimens of duodenum, jejunum, ileum, colon, and pancreas from nine individuals. Main Outcome Measures: We analyzed serum BAs and markers of BA synthesis after overnight fasting, during a hyperinsulinemic-euglycemic clamp, or a mixed-meal tolerance test and the association of BA transporter expression with body mass index. Results: BA synthesis markers were 2-fold higher (P