Energy is metabolised from three macronutrients, namely carbohydrates, fats and protein. During exercise, the macronutrients provide energy simultaneously via the aerobic and anaerobic system.
However, the degree to which fats and carbohydrates are metabolised for energy depends on the intensity and duration of the training session. Fat is the preferred energy source during low intensity, longer-lasting activities and carbohydrates especially stored glycogen, sustain high-intensity phases like hill climbs or sprint efforts. Recent research confirms that although athletes can significantly improve their fat metabolism (fat oxidation), this improvement leads to a significant decrease in performance due to the detrimental impact on carbohydrate metabolism.
World-class endurance athletes understand that saturated glycogen stores (achieved with carb-loading) are essential. They also consume vast amounts of carbohydrates during endurance events. Therefore, the mantra is simple: if you want to race to finish, stored fat and small amounts of carbohydrate intake may be sufficient, but if you are going to race to perform, you need stored fat and glycogen as well as carbohydrate intake during the event.