The acute effects of feeding MCT before and during exercise on endurance performance have been assessed by several researchers. Significant muscle glycogen sparing, a purported mechanism by which some have suggested MCT could enhance performance, has not been observed in these acute feeding studies; however, Van Zyl et al. did observe a significant improvement in endurance with acute feeding of carbohydrate plus MCT oil versus carbohydrate or MCT oil alone with cycling exercise. Because the energy content of the combination feeding was greater than in the trials of the separate components, these results have been subject to much criticism. Regardless, the addition of MCT oil to a carbohydrate feeding offers a potential additional energy source to the athlete that appears to translate to enhanced endurance, although that has not been demonstrated in similar studies. Most studies assessing the potential ergogenic effects of acute MCT oil feedings versus isocaloric carbohydrate feedings provide no evidence of improved endurance.
In fact, Jeukendrup et al. observed that intake of 85 g of MCT during exercise negatively affected performance. It is likely that the negative effect was associated with complaints of gastrointestinal distress rather than metabolic issues. The influence of more chronic adaptation to MCT oil has been studied as well. Fushiki et al. observed that chronic ingestion of MCT (17% total dietary kcal) for periods ranging from 2 to 6 weeks in duration by both trained and untrained mice caused glycogen sparing and produced significantly greater swimming endurance capacity. The authors suggested that upregulation of enzymes of lipid metabolism in response to MCT adaptation may play a part in glycogen sparing and improved endurance. Because long-term adaptation to MCT oil has been demonstrated to enhance lipid metabolism in mice, Misell et al.160 conducted similar research in human subjects. Those researchers demonstrated that trained runners fed MCT oil versus corn oil for 2 weeks exhibited little differences in physiology and endurance performance than when tested following an overnight fast. Interestingly, chronic feeding led to a suppression in complaints of gastrointestinal discomfort, which would probably also eliminate the likelihood of adverse effects on performance. While these data fail to corroborate the results observed in animals, it is unclear whether the mice in the study by Fushiki et al. were tested for endurance in a fasted or fed condition; therefore, it is possible that the combination of chronic MCT administration as well as an acute feeding to allow for adaptation as well as provision of an alternate fuel may prove useful. Thorburn et al. recently tested that hypothesis by feeding a caprylate-based structured TG for 2 weeks along with an acute feeding during prolonged exercise. Adaptation to the MCT oil reduced gastrointestinal discomfort, but failed to enhance sprint performance at the end of the prolonged exercise bout. Future research is needed to examine the effects of a combination of chronic plus acute MCT oil ingestion on an endurance trial performance. While supplementation with MCT oil appears to be safe in most studies, negative effects of its consumption relative to corn oil on the resting blood lipid profile have been demonstrated in athletes.
