It was concluded that these amino acids represent points of limitation to anabolic metabolism by restriction of their supply at critical times of demand. The model was run under a scenario of high demand for the synthesis of IgG during a response to an infectious challenge which indicated that these were increased requirements for tyrosine, threonine, and valine. Conversely, leucine, isoleucine, and valine were conserved during nitrogen flux and resulted in positive balances. Histidine, serine, glycine, and ornithine were in negative balances in males and females and this potential deficit was greater in the higher body-mass ranges. The model calculated the nitrogen balances for a range of amino acids to determine the amino acid requirements to support daily exertion. The amino acid excretion profiles for urine and sweat were constructed for males and females from published data. The average proportions of amino acids in the ingested proteins representing a well-balanced diet were used to assess intake and an average human composition profile from five major high-turnover proteins in the body to assess endogenous protein turnover. A paradigm was developed from published recommended rates of protein intake (g/kg/day) with corresponding rates of endogenous protein turnover and excretion, to extrapolate amino acid balances under various conditions. As suggested in several publications, a bolus of 15-20 g protein (from skimmed milk or whey proteins) and carbohydrate (± 30 g maltodextrine) drinks is needed immediately after stopping exercise to stimulate muscle protein and tendon collagen turnover within 1 h.The nitrogen balance is regulated by factors such as diet, physical activity, age, pathogenic challenges, and climatic conditions. Muscular activities promote a cascade of signals leading to the stimulation of eukaryotic initiation of myofibrillar protein synthesis. Nitrogen balance (difference between protein intake and protein degradation) for athletes is usually balanced when the intake of protein reaches 1.2 g Endurance exercise induces a greater oxidative capacity (enzymes) compared to resistance exercise, which induces fiber hypertrophy (myofibrils). Individuals who exercise respond differently when resistance and endurance types of contractions are compared. Strenuous exercise provokes increased proteolysis and decreased protein synthesis, the opposite occurring during the recovery period. There are different fractional synthesis rates in skeletal muscle and tendon tissues, but there is no major difference between collagen and myofibrillar protein synthesis. Stable isotopic tracers ((13)C-lysine, (15)N-glycine, ²H5-phenylalanine) and arteriovenous differences have been used in studies of skeletal muscle and collagen tissues under resting and exercise conditions. Noninvasive and invasive techniques have been applied to determine amino acid catabolism and muscle protein building at rest, during exercise and during the recovery period after a single experiment or training sessions.

As for any cell or tissue, total muscle protein reflects a dynamic turnover between net protein synthesis and degradation. Skeletal muscle is the major deposit of protein molecules.

Protein turnover involves