Amino acids are taken up by the cells that line the small intestine, then move out of the backside of those cells and enter the bloodstream. Meanwhile, small peptides, consisting of just a couple or a few amino acids linked together can also be brought into these cells where final digestion to amino acids can take place. Therefore, as a general rule, the absorbed form of protein will be individual amino acids. Fragments of proteins and some peptides can also be absorbed and are important in developing the immune system during infancy, and are linked to many food allergies reactions. Food allergies will be addressed in Chapter 12.
Amino acids and some peptides are absorbed into circulation, more specifically the portal vein, which delivers the amino acids to our liver. The liver removes a lot of amino acids from circulation. In fact it is typical for only about one-fourth of the absorbed amino acids to circulate beyond the liver, much of which will be the branched-chain amino acids, namely leucine, isoleucine, and valine. This is probably because these essential amino acids are needed by our skeletal muscle to replace what was used for energy during fasting or exercise. Additionally these amino acids play a role in maintaining and developing muscle mass, which is important for weight lifters as well as for people losing weight.
A lot of the amino acids absorbed into circulation go to the liver while the branched-chain amino acids go to skeletal muscle.
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