Although not a preferred fuel source, protein is used to produce energy during prolonged exercise when muscle glycogen runs low. This process is known as catabolism. Furthermore, the high blood concentration of cortisol that is associated with catabolism also obstructs the rebuilding of muscle protein by diverting its amino acid building blocks to the liver. Because protein is an important structural element of muscles, catabolism leaves the muscles in a weakened state afterward. In order to properly recover from and adapt to this particular training stress, athletes must act quickly to rebuild muscle protein after exercise.
Studies have shown there is a physiological window of opportunity for glycogen replenishment which is the two-hour period immediately following exercise. A very similar window exists for protein rebuilding and for the same reason. Insulin is responsible for delivering both glucose and protein to muscle cells. After exercise, the muscle cells are highly receptive to insulin. In a study performed at Vanderbilt University, subjects were fed a protein-carbohydrate supplement either immediately after working out or three hours later. Members of the “early” group were found to have synthesized new muscle proteins at a much faster rate than members of the “late” group.
In a similar study, this one done at the University of Texas, subjects who consumed an amino acid-carbohydrate supplement immediately before exercise synthesized even more new muscle protein than subjects who consumed the same drink immediately after the workout. (Amino acids are the building blocks of protein.) The apparent reason for this result was that drinking the amino acid-carbohydrate drink before the workout resulted in higher blood insulin levels during the workout. Insulin is known to counteract the catabolic effect of cortisol. With greater amounts of insulin circulating to neutralize cortisol, the subjects who drank before working out were able to get more amino acids delivered to their muscle cells to rebuild proteins.
What this means is that nutritional recovery measures need to begin before workouts, or at least during them and continue afterward. By consuming a sports drink containing carbohydrate and protein/amino acids before, during and after workouts, athletes can ensure that their muscle proteins are rebuilt with maximum efficiency. Using a recovery drink that consists of large amounts of protein or amino acids and little or no carbohydrate is not as effective, however, because these drinks digest more slowly and result in far less insulin release than recovery drinks that contain carbohydrate and protein in the ideal 4:1 ratio.