Protein Requirements for Skeletal Muscles

Skeletal muscle makes up about 40-50% of total body mass, i.e., 28-35 kg in a 70-kg individual. The main constituents of skeletal muscle include water (75%) and protein ( 20%). Of this 20%, the majority is contractile protein (myofilaments) (-11%) with smaller amounts of sarcoplasmic ( 6%) and connective tissue (3%) protein. Of the 20 amino acids that are the component parts of protein, only approximately 50% can be produced in the body-the remainder are called indispensable (or essential) because they must be consumed in the diet or the ability to form body protein (both structural and enzymatic) is compromised. Consequently, the supply of amino acid (type and quantity of protein) in the diet plays a critical role in muscle growth. Historically, as mentioned above, the adequacy of dietary protein has been assessed using the nitrogen balance technique. This involves quantifying nitrogen intake (food protein is 16% nitrogen) and excretion (urine, feces, sweat, and miscellaneous) as accurately as possible . Dietary protein requirement is determined as the quantity of protein in which nitrogen intake exactly matches nitrogen excretion, i.e., when the individual is at nitrogen balance.

Classically, the daily RDA for protein has been determined by measuring the requirement in a sample that is representative of the population of interest and adding a safety buffer equal to two standard deviations of the mean. Statistically, this should mean that when this quantity of protein is consumed, the vast majority (>95%) of individuals from the representative population would receive an adequate supply of amino acids. This safety buffer is necessary because requirements can vary somewhat among individuals. Many experiments have been completed over the years and, at least in sedentary individuals, it is clear that the requirement for protein is about 0.6 g/kg/day. When the safety buffer is included, the recommendation for protein intake is slightly higher (0.8 g/kg/day). Logically, it would seem that for those attempting to build muscle mass or maintain an increased muscle mass additional protein might be necessary. In fact there are data that support this idea but there is still little consensus on this topic. Further complicating the issue is that the nitrogen status (balance) technique is rather labor intensive and has several other limitations-the major one being its black box approach, i.e., it cannot distinguish whether changes in balance are due to effects on protein degradation/synthesis, or amino acid oxidation, etc.

Another way to investigate the dietary protein requirements involves the use of metabolic tracers which make it possible to see into the \"nitrogen status black box\" by assessing which components of protein metabolism are affected by an exercise or dietary treatment. This is an improvement when compared with the nitrogen status technique. Some recent data that were obtained using this technique suggest that current amino acid requirements (determined using the nitrogen status technique) may underestimate resting need by 40-90% but these new data remain controversial. In addition, some studies (to be discussed below) indicate that strength athletes can benefit from protein intakes that exceed the current RDA. Recent experiments using both metabolic tracers and nitrogen balance methodologies indicate that the protein requirement for strength athletes is likely about 1.5 g/kg/day, i.e., almost twice the current . When two standard deviations are added, the protein RDA for strength athletes could be as high as 1.7-2.0 g/kg/day. Despite this, it is unlikely that protein deficiencies will be rampant in athletes because it is not difficult to obtain this quantity of protein in one\'s diet given the high energy intake of most athletes. For example, even if only 10% of the energy in a 21,000 kg (5000 kcal) diet is protein, an 80-kg individual consuming this diet would receive 1.6 g of protein/kg/day . Typically, the percentage of protein in a US diet is closer to 12-15%.

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