Digestion occurs to some degree in the stomach prior to extensive digestion in the small intestine; during this process protein is broken down into smaller chains of AAs with free AAs, di- and tripeptides being the only forms absorbed to any great degree. A large proportion of AAs are retained in the intestine itself, used for protein and hormone synthesis; this is thought to buffer the body against large scale increases in blood AA levels which tend to promote AA oxidation. Retained AAs can be released to the body during periods of no food intake.
After release from the intestine, free form AAs travel to the liver where additional metabolism occurs. While a majority of BCAAs escape the liver into the bloodstream, over half of AAs will be catabolized in the liver and either excreted as waste or used to produce other substances. Various amounts of protein syntheis occurs in the liver as well.
The remain percentage of AAs are then released into the bloodstream where they enter the “free pool” for subsequent use by other tissues in the body.
