During prolonged endurance exercise (>90 minutes), increasing amounts of protein are used as an energy source. These proteins are primarily provided from the breakdown of muscle proteins. This is a concern to the athlete who has trained hard to build this muscle in the first place.
Early studies which investigated the effects of protein ingestion during exercise performance were largely conducted by adding protein to a standard carbohydrate mixture. Nearly all of these studies demonstrated an ergogenic (improved performance) benefit with added protein. Subsequent studies which have balanced the total caloric content of either a carbohydrate only or a mixed protein / carbohydrate drink have not been able to demonstrate a performance benefit conclusively. Therefore, the ergogenic effect of protein was assumed to be because of a generic effect of adding calories (fuel) as opposed to a unique benefit of protein.
However, the ingestion of protein both during and after exercise has been shown unequivocally to improve protein balance, reduce markers of muscle damage and improve the rate of recovery. These factors are important to the endurance athlete in considering the longer-term adaptations to training and not just the short-term performance effects. The addition of protein to a carbohydrate based supplement provides a more practical approach to optimizing the training and recovery of athletes, particularly those who train or compete in multiple sessions on the same or consecutive days.”