If you were to eat or drink nothing and went as hard as you can for an hour, the fuel that the body would be burning would be glycogen, and this type of exertion would deplete those reserves. However, if you went at a moderate or comfortable pace for an hour and didn't eat or drink anything, the body would be using stored fat and glycogen as fuel, and you could go for more than hour and still have plenty of stored fat as fuel and glycogen in the reserves. Eating and drinking while exercising will add even more fuel to your body and keep you from dipping too far into the glycogen reserves. Professional athletes train their bodies to burn stored fat as a primary fuel instead of just glycogen. When money and pride are on the line for the top pros you can be sure that all big dogs will be going at it and giving it everything they have.
Pros use salt tablets to keep their performance levels high in long hot races.
While training and racing in hot conditions your body sweats profusely and loses a lot of sodium. Salt tablets help replace this sodium and help aid in the absorption of water. Too low of sodium in the body is called Hyponatremia. Ironman athletes are very susceptible to developing Hyponatremia in training and racing. Some people combat the sodium loss by taking in additional sodium while training and racing.