We eat for three main reasons...to provide the energy we consume to stay alive, to build and repair the body, and to enable parts of the body to function efficiently. Energy for INTENSE physical effort comes principally from carbohydrate, which is stored in your liver and in your muscles in the form of glycogen. There are three forms of carbohydrates: natural sugars (fruits and their juices), refined sugars (jam, honey, and soft drinks), and starches (complex carbs and found in foods like potatoes, pasta, and bread.
The other major source of energy is the fat that is stored around the body. Fat has a higher energy density than carbs, which means that per gram it supplies more fuel: it is estimated that if all the energy stored in the fat reserves of a lean, 150-pound bike rider were there as glycogen, they would weigh about 275 pounds. Most people have enough fat in their bodies to fuel many hours of activity, which means there is no need to eat fat in any great quantity, particularly as any excess carbohydrate or protein is converted to fat and stored for future use.
Proteins play the major role in building up and repairing the body: they are found in meat, fish, eggs, and dairy products, in vegetables such as beans, lentils and peas, and also in carbohydrate-rich foods such as potatoes and bread. In extreme situations, protein is also burned to produce energy.
Vitamins and minerals enable the body's processes to function normally. For example, Vitamin A is important for maintaining healthy skin, helping fight infection and maintaining good eyesight--One source of this vitamin is carrots. Fruits, vegetables, fish, dairy products, meat and eggs are all important sources of various vitamins.
The final element is fluid. The body can function for some time without solid food by running off stored energy and protein, but without water, a vital part of all bodily processes, death soon results. A drop in your body water, through sweating, of just two percent will reduce your performance, five percent will cause fatigue and ten percent could be fatal.
Your intake of energy providers is vital if you are not to run out of the "fuel" stored in the muscles and liver. Body repair materials are equally important if you are to rebuild the muscle fibers you break down as you train, and if you are to construct stronger ones, which is the whole point of training.
Fotheringham, W. Cycle Racing. Pg. 52.