Studies investigating fat as a fuel for exercise have found that increasing free fatty acids during exercise tends to spare muscle glycogen due to increased utilization of free fatty acids for energy, which in turn can enhance the capacity for endurance exercise. Medium-chain triglycerides do not delay gastric emptying or absorption. They are broken down by lipase in the stomach and duodenum to glycerol and medium-chain fatty acids (MCFA). Since MCFAs are metabolized as quickly as glucose, it has been speculated that they might provide an alternative carbon source for the muscle during prolonged exercise. While the majority of studies investigating the role of medium-chain triglycerides and exercise have found no sparing effect of muscle glycogen after consumption of medium-chain triglycerides, two recent studies have presented conflicting results. This review will investigate the speculated role of medium-chain triglycerides as an alternative fuel source for exercising muscles and will discuss the possibility that medium-chain triglycerides preserve muscle glycogen during exercise,
Jacqueline R. Berning
Stavros A. Kavouras, John P. Troup and Jacqueline R. Berning
To examine the effects of a 3-day high carbohydrate (H-CHO) and low carbohydrate (L-CHO) diet on 45 min of cycling exercise, 12 endurance-trained cyclists performed a 45-min cycling exercise at 82 ± 2% VO2peak following an overnight fast, after a 6-day diet and exercise control. The 7-day protocol was repeated under 2 randomly assigned dietary trials H-CHO and L-CHO. On days 1–3, subjects consumed a mixed diet for both trials and for days 4–6 consumed isocaloric diets that contained either 600 g or 100 g of carbohydrates, for the HCHO and the L-CHO trials, respectively. Muscle biopsy samples, taken from the vastus lateralis prior to the beginning of the 45-min cycling test, indicated that muscle glycogen levels were significantly higher (p < .05) for the H-CHO trial (104.5 ± 9.4 mmol/kg wet wt) when compared to the L-CHO trial (72.2 ± 5.6 mmol/kg wet wt). Heart rate, ratings of perceived exertion, oxygen uptake, and respiratory quotient during exercise were not significantly different between the 2 trials. Serum glucose during exercise for the H-CHO trial significantly increased (p < .05) from 4.5 ± 0.1 mmol · L−1 (pre) to 6.7 ± 0.6 mmol · L−1 (post), while no changes were found for the L-CHO trial. In addition, post-exercise serum glucose was significantly greater (p < .05) for the H-CHO trial when compared to the L-CHO trial (H-CHO, 6.7 ± 0.6 mmol · L−1; L-CHO, 5.2 ± 0.2 mmol · L−1). No significant changes were observed in serum free fatty acid, triglycerides, or insulin concentration in either trial. The findings suggest that L-CHO had no major effect on 45-min cycling exercise that was not observed with H-CHO when the total energy intake was adequate.
Jacqueline R. Berning, John P. Troup, Peter J. VanHandel, Jack Daniels and Nancy Daniels
Dietary food records from adolescent male and female swimmers participating in a national developmental training camp were analyzed for nutrient density. The mean caloric intake was 5,221.6 kcal for males and 3,572.6 kcal for females. The distribution of calories between carbohydrates, protein, and fat was not ideal for athletes trying to optimize performance. These young swimmers consumed too much fat and not enough carbohydrate. They consumed more than the RDA of vitamins A and C, and thiamine, riboflavin, and niacin; however, some concern is expressed for females who did not meet the RDA for calcium and iron. This study shows that although a group of adolescent swimmers may be consuming enough nutrients, individual swimmers may have very poor dietary habits and thus may not be providing adequate fuel or nutrients for optimal training or performance.