| Summary: | The effect of fatty acids on energy metabolism has been shown to be dependent on their acyl structure. In humans, following short term feeding, medium chain triglycerides (MCT) have been shown to increase the thermic effect of food and fat oxidation as compared to long chain triglycerides (LCT). Short term results in animals have been comparable. In longer term, animal studies, MCT vs. LCT have resulted in less weight gain during overfeeding or refeeding after weight loss. However, observations of the longer term effects of MCT in humans beyond 7 days are sparse and inconclusive. Hence, the objective of the thesis was to examine the effects of MCT vs. LCT on total energy expenditure, its components basal metabolic rate and thermic effect of food, and on substrate oxidation, including both exogenous and endogenous fat oxidation for a period of one week, following one week of prefeeding. Twelve healthy college aged women were fed eucaloric 14 days diets enriched with either MCT or LCT in a randomized cross over design, with a two week washout period. Doubly labelled water, respiratory gas exchange analysis, and 1-13C labelled myristic, palmitic, and stearic acids were used to measure total energy expenditure, components of energy expenditure, and endogenous long chain fatty acid oxidation, respectively. The presence of MCT in the diet significantly increased endogenous oxidation of labelled long chain fatty acids following 14 days of feeding, while the presence of LCT did not. Respiratory gas exchange analysis showed significantly increased basal metabolic rate on day 7 of MCT vs. LCT feeding, but this effect of diet was reduced to a trend by day 14. Dietary treatment did not result in significant differences in total energy expenditure during the second week of feeding. These results suggest that, after two weeks of feeding, MCT continue to affect energy metabolism through increased endogenous fat oxidation and a suggestion of heightened basal metabolic rate, but
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