Carbohydrates

Carbohydrates are the main source of energy for athletes.  Athletes do not want the body to metabolize protein for energy because this leads to muscle breakdown.  They can use fat for energy but the system that breaks down fat for energy only kicks in after activity has been underway for an extended period of time.  This isn't going to help athletes in most sports.  Therefore; the key macro athletes use for quick and available energy comes from carbohydrate.