Protein may also be obtained from within the body. This is called endogenous protein. Endogenous protein does not come directly from the foods we eat, but from the synthesis of proteins from within the body.
Obtaining protein from the diet is common knowledge. The fact that the body can synthesize protein from its own proteinaceous wastes, however, is not widely known.
As the body’s cells undergo their natural catabolic processes, they produce proteinaceous wastes in the form of spent cells and other by-products of their own metabolism. These proteinaceous products enter the lymph fluid.
Lymph flows in tiny vessels called lymphatic vessels up toward the heart. The body drains about 3 quarts of lymph per day.
Other cells in the body are able to ingest these spent proteins and to digest them in vesicles (“stomachs”) of their own formation. The body’s cells are thus able to break these proteinaceous wastes down into amino acids and use them to synthesize their own protein.
Endogenous protein (or protein from within the body) is an important source of amino acids that is often overlooked by conventional nutrition writers. Many times, up to two-thirds of the body’s total protein needs are supplied through endogenous protein and not from exogenous dietary sources.
“Proteins In The Diet,” from Terrain Wiki and T.C. Fry – Life Sciences Health System – Chapter 08 – Protein In The Diet.
From the digestion of proteins in the diet and from the recycling of proteinaceous wastes, the body has all the different amino acids circulating in the blood and lymphatic system. When cells need these amino acids, they appropriate them from the blood or lymph. This continually-circulating available supply of amino acids is known as the amino acid pool.
The amino acid pool is like a bank that is open twenty-four hours. The liver and the cells are continually making deposits and withdrawals of amino acids, depending upon the concentration of amino acids in the blood.
When the number of amino acids is high, the liver absorbs and stores them until needed. As the amino acid level in the blood falls due to withdrawals by the cells, the liver deposits some of the stored amino acids back into circulation.
The cells also have the capacity to store amino acids. If the amino acid content of the blood falls or if some other cells require specific amino acids, the cells are able to release their stored amino acids into circulation. Since most of the body’s cells synthesize more proteins than are necessary to support the life of the cell, the cells can reconvert their proteins into amino acids and make deposits into the amino acid pool.
Between the deposits and withdrawals by the liver and cells, there is a continual flux of amino acids in the blood and plasma. This circulating source of amino acids, as well as the potential availability of the amino acids stored within the liver and the cells, makes up the important amino acid pool. This pool of amino acids is very important in understanding why complete proteins are not necessary in the diet.