The purpose of this study was to investigate the hypothesis that a single dose of structured medium- and long-chain triacylglycerols (SMLCTs) composed of medium-chain (20%) and long-chain (80%) fatty acids would increase the metabolic rate more than a dose of long-chain triacylglycerols (LCTs) in 15 healthy young women aged 18 to 28 years. The effects on postingestive energy expenditure were compared for SMLCTs versus LCTs. On the experimental days, the subjects fasted overnight and then ingested 1,680 kJ SMLCTs or LCTs each day. Energy expenditure and the respiratory quotient (RQ) were measured before and after SMLCT and LCT ingestion by indirect calorimetry. Blood samples were collected before and after ingestion to obtain plasma and serum. Postingestive total energy expenditure (PTEE) was significantly higher after SMLCT ingestion versus LCT ingestion (26.9 +/- 1.0 v 25.5 +/- 1.1 kJ/kg/6 h, P < .05). The thermic effects of the test oil were also significantly greater after SMLCT ingestion compared with LCT ingestion (3.02 +/- 0.49 v 1.47 +/- 0.82 kJ/kg/6 h, P < .01). Plasma glucose and serum triacylglycerol concentrations were not significantly different. Serum free fatty acid and 3-hydroxybutyric acid concentrations were higher after SMLCT ingestion versus LCT ingestion. These results suggest that long-term substitution of SMLCTs for LCTs will produce body fat loss if energy intake remains constant.