Carbohydrate and fat are the most important fuels during exercise. Carbohydrates are stored in relatively small amounts and can become depleted after just a few hours of hard exercise. Fat is stored in abundance but is a hard source of energy to access during high intensity activity.
It has been know for some time that loading your carbohydrate stores prior to exercise improves endurance performance however, it is not that clear as to whether we are capable of oxidising glucose at higher rates than 60 g per hour which is predominantly recommended. For example, a carbohydrate gel or isotonic sports drink will usually contain ~30 g carbohydrate.
By varying the type of carbohydrate consumed, higher oxidation rates can be achieved. When ingesting a glucose: fructose drink at 90 g per hour, performance can be substantially improved compared to carbohydrate alone.