OBJECTIVE:To determine the effects of long-term consumption of medium chain (MCT) versus long chain triglycerides (LCT) on energy expenditure (EE), substrate oxidation and body composition. HYPOTHESIS: MCT consumption will not result in greater EE, substrate oxidation, and body weight loss compared with LCT consumption. RESEARCH METHODS AND PROCEDURES: Seventeen healthy obese women participated in this randomized, crossover inpatient trial. Meals were prepared and consumed on site for two periods of 27 days. Diets containing 40% of energy as fat, with treatment fat comprising 75% of the total fat, were designed to supply each subject with their individual weight-maintaining energy needs. The MCT diet contained 67% of treatment fat as MCT oil (49% octanoate, 50% decanoate) whereas the LCT diet contained exclusively beef tallow as treatment fat. Body composition was assessed by magnetic resonance imaging (MRI) on day 1 and 28 of each phase while energy expenditure was measured on day 2 and 27. RESULTS: Changes in total and subcutaneous adipose tissue volumes following consumption of MCT and LCT were not different (70.61 AE 0.38 l vs 70.54 AE 0.48 l and 70.58 AE 0.35 l vs 70.48 AE 0.40 l, respectively). Average EE and fat oxidation were greater (P < 0.05) during MCT than LCT consumption (0.95 AE 0.019 vs 0.90 AE 0.024 kcal=min, respectively, for EE and 0.080 AE 0.0026 vs 0.075 AE 0.0022 g=min, respectively for fat oxidation). DISCUSSION: These results show that long-term consumption of MCT enhances EE and fat oxidation in obese women, when compared to LCT consumption. The difference in body composition change between MCT and LCT consumption, although not statistically different, was consistent with differences predicted by the shifts in EE. It can be concluded that substitution of MCT for LCT in a targeted energy balance diet may prevent long-term weight gain via increased EE.

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