How much protein can you store?

After digestion proteins are hydrolyzed into amino acids and reach the intracellular amino acid pool, a metabolic pool limited in size and not expandable (1, 2). From this pool amino acids can follow 3 major pathways (1):

1. AAs can be used for the synthesis of new endogenous proteins and other biological substances;
2. AAs can be irremediably oxidized by the body, yielding urea (+ ammonia) and carbon dioxide (CO2) as terminal end-products (see process of ureagenesis) and;
3. AAs can be converted into other compounds (gluconeogenesis).

The free AA pool is maintained within tight limits (3), even under a variety of conditions the free AA pool are very similar (4,5). The body tries to maintain body protein stores at constant levels (6). The concentration of each individual amino acid in the cell is precisely regulated (4).

The free pool provides individual AAs for protein synthesis and oxidation, and it is replenished either by protein breakdown or AAs entering the body from the diet. For example, amino acids involved in charging of muscle tRNA apparently come from the intracellular pool (7).

Excluding taurine, the free pool has been estimated to contain only 100 grams of AAs, and including taurine the free pool increase up to 130 grams (8), with an additional 5 grams of free AAs circulating in the bloodstream (8). The free pool is approximately 1% of the size of the AA stored in tissue.

In an old study, 3g protein/kg only increased blood concentrations of most AAs by 30% above normal levels, with concentrations of BCAAs doubling over normal levels (9), which indicates that the pool is tightly regulated. The concentration of amino acids in the bloodstream is different from that seen within the muscle (3), changes in blood AA levels may have no impact on intramuscular AA concentrations.

Protein and amino acids ingested in excess of those needed for biosynthesis cannot be stored due to the limited size of the intracellular free amino acid pool, which cannot be much expanded (1). Although chronic strength exercises can increase the capacity for skeletal muscle protein storage there is eventually a limit.

When protein intake surpasses the physiological needs of amino acids, the excess amino acids are disposed of by three major processes (1):

1. Increased oxidation, with terminal end products such as CO2 and ammonia
2. Enhanced ureagenesis i. e. synthesis of urea linked to protein oxidation eliminates the nitrogen radical
3. Gluconeogenesis, i. e. de novo synthesis of glucose. This is one of the mechanisms developed by the body to maintain blood glucose within a narrow range, (i. e. glucose homeostasis). Gluconeogenesis, uses non-glycogenic precursors; in particular certain specific amino acids (for example, alanine), as well as glycerol (derived from fat breakdown) and lactate (derived from muscles) to produce glucose.

Would you like to know more? Subscribe!

1. Yves Schutz. Protein Turnover, Ureagenesis and Gluconeogenesis. Int. J. Vitam. Nutr. Res., 81 (2 – 3), 2011, 101 – 107 101
2. Waterlow, JC. Protein turnover with special reference to man. Q J Exp Phys (1984) 69: 409-438.
3. Furst, P. Intracellular muscle free amino acids – their measurement and function. Proc Nutr Soc (1983) 42: 451-462.
4. Scriver, CR et. al. Normal plasma amino acid value in adults: The influence of some common physiological variables. Metabolism (1985) 34: 868-873.
5. Waterlow, JC. Where do we go from here? J Nutr (1994) 124:1524S-1528S
6. Bauman, P. Q., Stirewalt, W. S., O’Rourke, B. D., Howard, D. & Nair, K. S. (1994) Precursor pools of protein synthesis: a stable isotope study in swine model. Am. J. Physiol. 267: E203–E209.
7. Wagenmakers, AJ. Protein and amino acid metabolism in human muscle. Skeletal Muscle Metabolism in Exercise and Diabetes. ed. Richter et. al. Plenum Press: New York, 1998.
8. Wahren, J et. al. Effect of protein ingestion on splanchnic and leg metabolism in normal man and in patients with diabetes mellitus. J Clin Invest (1976) 57: 990-995.
9. Furst, P. Intracellular muscle free amino acids – their measurement and function. Proc Nutr Soc (1983) 42: 451-462.