Maximal Fat Oxidation and Fuel Use During Exercise

Fat and carbohydrate are the two main energy stores available as fuel during exercise. It is well known that the exercise intensity and feeding status are the major factors determining the type of fuel used during exercise. During prolonged exercise at low to moderate exercise fat is the primary fuel being used and to improve performance studies has tried to understand strategies to maximize muscle glycogen storage and elevate fat oxidation during exercise. With this strategy they aim for preserving the limited muscle glycogen stores and thus improving endurance performance. In relation to this the maximal fat oxidation (MFO: The highest rate of fat oxidation across all exercise intensities) has been studied as increasing the fat oxidation could decrease the depletion of the glycogen stores. Further it has recently been shown that MFO is related to performance in endurance trained. However the MFO has been found to vary markedly between trained individuals matched on their activity level. It has been suggested that the diet and subsequent substrate availability during exercise contributes independently to the variation in MFO. However, the measurements have never been evaluated in a trained group with similar aerobic capacity and training status. Therefore, the aim of the study is to investigate the effect of a short term fat rich or carbohydrate rich diet on MFO in well trained men with a high vs. a low MFO. The hypothesis is that 3 days of a fat-rich diet will increase MFO while 3 days of a Carbohydrate rich diet will decrease MFO in both individuals with a high MFO (HiMFO) and a low MFO (LoMFO). Furthermore, it is hypothesized that HiMFO will have a significantly higher MFO after both diets compared to LoMFO. Lifestyle and physiological factors have been investigated to determine the variation of the MFO capacity. However, these factors can only explain 50% of the interindividual variability in MFO. Despite the critical role of fat oxidation during exercise, fe