When a person eats a food protein, whether from cereals, vegetables, meats, or dairy products, the digestive system breaks the protein down and delivers the separated amino acids to the body cells. The cells then put the amino acids together in the order necessary to produce the particular proteins they need. (To review the digestive and absorptive systems relevant to the body’s handling of protein, turn to Chapter 3.)
The stomach initiates protein digestion (see Figure 6-1). By the time proteins move into the small intestine, they are already broken into different-sized pieces: some single amino acids and many strands of two amino acids, dipeptides; some strands of three amino acids, tripeptides; and some longer chains. Digestion continues until almost all pieces of protein are broken into dipeptides, tripeptides, and free amino acids. Absorption of amino acids takes place all along the small intestine. Dipeptides and tripeptides are captured on the surface of the cells that line the small intestine. These cells split the peptides into amino acids, absorb them, and then release them into the bloodstream.
Once they are circulating in the bloodstream, the amino acids are available to be taken up by any cell of the body. The cells can use them to make proteins, either for the cell’s own use or for secretion into the circulatory system for other uses.
Dipeptides (dye-PEP-tides) protein fragments two amino acids long. A peptide is a strand of amino acids.
Tripeptides (try-PEP-tides) protein fragments three amino acids long.
If a «o«essential amino acid (that is, one the body can make for itself) is unavailable for a growing protein strand, the cell will make one and will continue attaching amino acids to the strand. If, however, an essential amino acid (one the body cannot make) is missing, the building of the protein will halt. The cell cannot hold partially completed proteins to complete them later, for example, the next day. Instead, it must dismantle the partial structures and return surplus amino acids to the circulation, making them available to other cells. If other cells do not soon pick up these amino acids and insert them into protein, the liver will remove their amine groups for the kidney to excrete. Other cells will then use the remaining fragments for other purposes. The nutritional need calling for the production of that particular protein will not be met.