Protein function in exercise.

http://en.wikipedia.org/wiki/Protein_(nutrient)

Proteins are believed to increase performance in terms of athletics. Amino acids, the building blocks of proteins, are used for building muscle tissue and repairing damaged tissues.Protein is only used as fuel when carbohydrates and lipid resources are low.

Aerobic exercise protein needs

Endurance athletes differ from strength-building athletes in that endurance athletes do not build muscle mass from training.Research suggests that individuals performing endurance activity require more protein intake than sedentary individuals so that muscles broken down during endurance workouts can be repaired.Although the protein requirement for athletes still remains controversial  research does show that endurance athletes can benefit from increasing protein intake because the type of exercise endurance athletes participate in still alters the protein metabolism pathway. The overall protein requirement increases because of amino acid oxidation in endurance-trained athletes.Endurance athletes who exercise over a long period (2–5 hours per training session) use protein as a source of 5–10% of their total energy expended. Therefore, a slight increase in protein intake may be beneficial to endurance athletes by replacing the protein lost in energy expenditure and protein lost in repairing muscles. Some scientists suggest that endurance athletes may increase daily protein intake to a maximum of 1.2–1.4 g per kg body weight.

Anaerobic exercise protein needs

Research also indicates that individuals performing strength-training activity require more protein than sedentary individuals.Strength-training athletes may increase their daily protein intake to a maximum of 1.4–1.8 g per kg body weight to enhance muscle protein synthesis, or to make up for the loss of amino acid oxidation during exercise.Many athletes maintain a high-protein diet as part of their training, and so protein deficiency is less likely among this group than among non-athletes. In fact, some athletes who specialize in anaerobic sports (e.g. weightlifting) assume a very high level of protein intake is necessary, and may over-consume.Research indicates that many athletes consume more protein than they need even without the use of protein supplements.