Anytime you plan to be physically active for more than an hour, it's going to be harmful to your performance to be on a very high simple carbohydrate diet. A mainstay in the diet for very active people is complex carbohydrates, which provide the energy for sustained activity. There is no way we can eat that many carbohydrates to keep up with the demands of a hugely active body, so fats can supply a source of energy for extended physical activity. Marathon runners are coming around to this fact. Protein is derived from animals and animal by-products.
Protein is what comprises the muscles.
Cells are constantly dieing and being destroyed. During the day, this cell destruction must be replaced. Lean sources of protein like skinless chicken and fish is recommended over fatty red meats. Protein is also needed since certain vitamins are fat soluble and the lack of these vitamins is unhealthy. I think the current guidelines are two heavy on the fat, and need real specificity as to the sources of fats, carbohydrates, and protein.