Background Laparoscopic Roux-en-Y gastric bypass (RYGB) induces a more favorable metabolic profile than expected by weight loss alone. In this study, we investigated the effect of RYGB on serum bile acid levels and their relation to clinical outcomes. Methods We included 30 obese patients who underwent RYGB (BMI=46.1±5.9 kg/m2). Clinical measurements and laboratory determinations were performed before surgery and 1 year after surgery. Fasting serum bile acids were measured by an enzymatic method and individual bile acids were quantified by HLPC-tandem mass spectrometry. Indirect calorimetry was performed to measure the rates of energy expenditure and substrate oxidation. Results Fasting total serum bile acid levels increased twofold after RYGB (pre, 3.68±2.03 vs. post, 7.06± 9.65 μmol/l, +92 %, p=0.002). This increase in total bile acids was accompanied by a decrease in conjugated bile acids, which correlated with decreased glucose oxidation (r=0.571, p=0.002) and with increased lipid oxidation (r=−0.626, p=0.0004). The change in taurineconjugated bile acids correlated with altered DIO2 mRNA expression in adipose tissue (r=−0.498, p=0.013) potentially linking bile acid conjugation to substrate oxidation through DIO2. Conclusions Fasting serum bile acid levels increase after RYGB. More specifically, changes in bile acid conjugation after RYGB associate with altered energy metabolism.

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