BACKGROUND Bile acids can act as signalling molecules via various receptors including the nuclear farnesoid X receptor (FXR) and pregnane X receptor (PXR), and the cell surface G-protein-coupled receptor TGR5. The signalling has been implicated in the release of peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), which improves glycaemic control and energy expenditure. We investigated whether morbidly obese subjects have altered postprandial bile acid responses in comparison to normal weight subjects. METHODS Blood samples were taken every 30 min from 0 to 180 min following a 400 kcal test meal. Samples were taken from 12 normal weight subjects with a body mass index (BMI) of 23.2 (2.8) kg/m(2) (median [interquartile range (IQR)]) and seven obese patients with a BMI of 47.2 (7.2) kg/m(2). Fractionated bile acids were measured on these samples using high-performance liquid chromatography tandem mass spectrometry. RESULTS The obese subjects showed a lower postprandial response in total bile acids compared with the normal weight subjects. An increase of 6.4 (5.0) and 2.6 (3.3) micromol/L (median [IQR]) in normal weight and obese subjects was observed, respectively (P = 0.02). The difference was predominantly due to the glycine-conjugated fraction (P = 0.03). There was no difference in the increase of the unconjugated or taurine-conjugated fractions. CONCLUSIONS The decreased postprandial bile acid response in obese subjects compared with normal weight subjects may partly explain the suboptimal GLP-1 and PYY responses and could affect appetite, glycaemic control and energy expenditure.