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Protein is necessary to build, maintain, and repair muscle. Exercise increases intramuscular protein oxidation and breakdown, after which muscle-protein synthesis increases for up to a day or two [155]. Regular resistance exercise results in the accretion of myofibrillar protein (the predominant proteins in skeletal muscle) and an increase in skeletal muscle fiber size. Aerobic exercise leads to more modest protein accumulation in working muscle, primarily in the mitochondria, which enhances oxidative capacity (oxygen use) for future workouts [155,156].

Athletes must consider both protein quality and quantity to meet their needs for the nutrient. They must obtain essential amino acids (EAAs) from the diet or from supplementation to support muscle growth, maintenance, and repair [155]. The nine EAAs are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Most complete proteins (those that contain all EAAs) are composed of about 40% EAAs, so a meal or snack with 25 g total protein provides about 10 g EAAs.

See other sections of this fact sheet for information on the amino acids arginine and glutamine as well as the BCAAs (leucine, isoleucine, and valine). The potential of these amino acids to enhance exercise and athletic performance is not related to their incorporation into proteins.

Efficacy: Adequate protein in the diet is required to provide the EAAs necessary for muscle-protein synthesis and to minimize muscle-protein breakdown. Dietary protein consumption increases the concentration of amino acids in the blood, which muscle cells then take up. Sufficient protein is necessary primarily to optimize the training response to, and the recovery period after, exercise [12,157].

Muscle protein synthesis leading to increases in strength and muscle mass appears to be optimal with the consumption of high-quality protein (providing about 10 g EAAs) within 0–2 hours after exercise, in the early recovery phase [12]. However, a meta-analysis of randomized clinical trials found that ingesting protein within an hour before or after exercise does not significantly increase muscle strength or size or facilitate muscle repair or remodeling [77]. The period after exercise when protein intake reduces muscle protein breakdown, builds muscle, and increases mitochondrial proteins to enhance oxygen use by working muscles (the so-called “window of anabolic opportunity”) can last for up to 24 hours [79].

Several studies in people engaged in resistance training show that consuming some protein before sleep can increase the rate of protein synthesis during the night and/or augment muscle mass and strength [79,158,159]. Participants in these studies consumed a bedtime drink containing 27.5 or 40 g of the milk protein casein, which increased circulating amino acid levels throughout the night. Some studies show increased muscle protein synthesis when plasma levels of amino acids are raised [76].

Safety: The Food and Nutrition Board has not set a UL for protein, noting that the risk of adverse effects from excess protein from food is “very low” [160]. However, it advises caution for those obtaining high protein intakes from foods and supplements because of the limited data on their potential adverse effects. High-protein diets (e.g., those providing two to three times the RDA of 0.8 g/kg/day for healthy adults and 0.85 g/kg/day for adolescents) do not appear to increase the risk of renal stones or dehydration; compromise renal function; reduce bone health; or, when consumed for several months, alter glomerular filtration rate or blood levels of lipids, glucose, creatine, or blood urea nitrogen [160-164]. Protein increases urinary calcium excretion, but this appears to have no consequence for long-term bone health [165] and, in any event, is easily compensated for by the consumption of slightly more calcium.

Implications for use: Many foods—including meats, poultry, seafood, eggs, dairy products, beans, and nuts—contain protein. Protein powders and drinks are also available, most of which contain whey, one of the complete proteins isolated from milk [166]. Digestion of casein, the main complete protein in milk, is slower than that of whey, so the release of amino acids from casein into the blood is slower [72]. Soy protein lacks the EAA methionine and might lose some cysteine and lysine in processing; rice protein lacks the EAA isoleucine [166]. Many protein supplements consist of a combination of these protein sources. All EAAs are necessary to stimulate muscle protein synthesis, so users should select singular or complementary protein sources accordingly. To maximize muscle adaptations to training, AND, DoC, and ACSM recommend that athletes consume 0.3 g/kg body weight of high-quality protein (e.g., about 20 g for a person weighing 150 lb) 0 to 2 hours after exercise and then every 3 to 5 hours [12].

Since the Food and Nutrition Board developed the RDA for protein, more recent data have suggested that athletes require a daily protein intake of 1.2 to 2.0 g/kg to support metabolic adaptations, muscle repair and remodeling, and protein turnover [12,167]. Athletes might benefit from even greater amounts for short periods of intense training or when they reduce their energy intake to improve physique or achieve a competition weight [12]. The 2007–2008 National Health and Nutrition Examination Survey showed that the average daily intake of protein by adult men is 100 g and by women is 69 g [168]. Athletes who require additional protein can obtain it by consuming more protein-containing foods and, if needed, protein supplements and protein-fortified food and beverage products.