Protein Digestion and Metabolism (Part 1)

Part 1 – Protein Digestion & Absorptionprotein digestion

Digestion

Digestion of the protein we eat starts in the stomach. It ultimately involves breaking the protein into small peptides (short chains of amino acids) and individual amino acids (also know as free form amino acids) that can be absorbed through the small intestine.

Gastric juice, which is mainly hydrochloric acid, begins the process in the stomach by unfolding or denaturing the complex structures of the protein molecules (see What is Protein?) so that the peptide bonds are exposed.

The hydrochloric acid does not act on the peptide bonds directly but activates pepsinogen and pro rennin to form pepsin and rennin.  Pepsin begins the process of breaking down the protein by cleaving certain peptide bonds in the primary chain of the denatured protein molecules. The resulting polypeptides then pass on to the small intestine for further digestion and absorption.

An interesting contrast to this is the action of rennin on milk casein. The rennin slows the digestion of the casein by causing it to curdle or clot in the stomach making it a more difficult for the pepsin to reach the peptide bonds and start breaking down the casein protein molecules. This is why Calcium Caseinate and Micellar Milk Protein Concentrate are described as slow protein powders and can supply amino acids and peptides to the blood stream for up to 8 hours.

There are also some important exceptions. Certain protein molecules (micro-fractions) found in whey protein can pass through the stomach without being denatured by the gastric juice and reach the small intestine in their native state. In this way microfractions such as lactoferrin, for example, can produce their antimicrobial action in the digestive system.

In the small intestine proenzymes, such as trypsinogen and chimotrypsinogen, are activated so that they can continue the breaking down the polypeptide chains. As this process proceeds the resulting peptides, di- and tri-peptides and free form amino acids are absorbed into the walls of the small intestine.

protein absorption

Absorption

Different amino acids and peptides are all absorbed into the walls of the small intestine in different ways.

Free form amino acids compete for common pathways and competition for these pathways can lead to a backlog of amino acids waiting to enter the intestinal wall if too many free form amino acids are present.

Di and tri peptides use different pathways and are absorbed more rapidly than free form amino acids.  This, for example, means that the peptide form of leucine as found in MyoPures Leucine Peptides is absorbed into the blood stream much more rapidly than free form leucine.

Some of the amino acids and peptides absorbed into the walls of the small intestine are used by the intestinal cells for their own energy needs and to make new proteins such as hormones and digestive enzymes.

For example, glutamine is used as the primary energy source for intestinal cells and stimulates intestinal cell growth. If there is a deficiency of glutamine in the diet the intestine will draw the glutamine from blood plasma or muscle cells to meet its needs. In this way, supplementing with L-glutamine may help spare the plasma and muscle cell glutamine needed for protein synthesis and many other anabolic processes.

All the amino acids and peptides that are not used by the small intestine itself pass through to the blood stream and are delivered to the liver for further processing and distribution to other cells in the body.  Read what happens next in Protein Digestion and Metabolism Part 2.

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