Protein and Athletes

Athletes need protein primarily to repair and rebuild muscle that is broken down during exercise and to help optimize carbohydrate storage in the form of glycogen. Protein is not an ideal source of fuel for exercise, but can be used when the diet lacks adequate carbohydrate. This is detrimental, though, because if used for fuel, there is not enough available to repair and rebuild body tissues, including muscle (1).
A sedentary person, a marathon participant, and a body builder of the same weight have different protein needs. Protein requirements depend upon body weight, body composition, rate of growth, physical activity level, type of physical activity, adequacy of energy and carbohydrate intake, and illness or injury. The protein needs for athletes are greater than the recommended 0.8 grams per kilogram of body weight recommended for sedentary people. Strength training athletes need about 1.4 to 1.8 grams per kilogram (2.2lbs) of body weight per day. Endurance athletes need about 1.2 to 1.4 grams per kilogram (2.2lbs) of body weight per day (2).
Endurance exercise alters protein metabolism and increases amino acid oxidation leading to increased protein needs. The increase in need is dependent upon the intensity and duration of the exercise, with higher intensity and longer bouts of exercise associated with increased protein needs. Individuals such as body builders, who are using resistance training to increase muscle mass, require a protein intake greater than for sedentary people and endurance athletes (2). Body builders' training programs typically include a maintenance phase, a muscle-mass building phase, and a tapering or cutting phase. Protein needs during these times vary. During the maintenance phase, recommended protein intake is 1.2 grams per kilogram body weight for maintenance of muscle mass. During the muscle-building phase, a protein intake of 1.4-1.8 grams per kilogram body weight is recommended. During the tapering or cutting phase, body builders significantly decrease their calorie intake. During this special phase of calorie and carbohydrate restriction, protein needs increase to 1.8-2.0 grams of protein per kilogram body weight to compensate for the use of protein for energy during this hypocaloric phase (3).
Adolescent athletes involved in high-intensity physical activity must meet the nutrition needs of growth combined with physical activity. Their protein needs vary from 1.8-2.0 grams per kilogram body weight. The timing of protein intake is also important. Protein that is consumed with carbohydrate within an hour after exercise stimulates the release of insulin and growth hormones, and therefore, growth of muscle mass. An inadequate amount of carbohydrate in the diet can result in reduced muscle glycogen stores in the muscle and liver, decreased endurance, decreased maximal effort, decreased serum glucose levels, and increased risk of hypoglycemia (4).
Protein intake should not exceed 2.0 grams per kilogram body weight. Excess protein intake is associated with dehydration, increased risk of certain cancers, increased calcium excretion and increased risk of osteoporosis, and reduced intake of vitamins, minerals, fiber and phytochemicals (4).

References
1. The Protein Power Diet. http://www.webmd.com/diet/protein-power-what-it-is. Accessed on August 1, 2009.
2. Mahan LK, Escott-Stump S. Krause's Food & Nutrition Therapy, 12th Ed., St. Louis, MO: Saunders Elsevier Pub.; 2008.
3. Rodriguez NR, Di Marco NM, Langley S. American College of Sports Medicine position stand. Nutrition and athletic performance. Med Sci Sports Exerc. 2009; 41 (3): 709-31.
4. Zeratsky K. High protein diets: Are they safe? http://www.mayoclinic.com/health/high-protein-diets/AN00847. Accessed on August 2, 2009.