Since proteins are used by the body for repairing and building cells, understanding how protein digestion and absorption work is essential. Proteins are also used for creating enzymes and neurotransmitters, as well as the creation of DNA and RNA. Thus, the importance of this essential macronutrient cannot be contested. Protein digestion takes place in two different phases:

• In the stomach
• In the small intestine

Both of these phases of digestion are based on several types of enzymes that are called proteinases and proteases.

Digestion of Proteins in the Stomach

Two of the substances secreted by the stomach, HCl (hydrochloric acid) and pepsinogen, interact to create pepsin, an enzyme that plays a very important role in protein digestion. The process that takes place when proteins are disintegrated by the enzymes is called hydrolysis.

The factors listed below determine the period of time required by the enzymes to breakdown the proteins:

• Concentration of the enzyme
• Quantity of protein to be disintegrated
• Acidity of the stomach and food
• Temperature of the food
• Time of the day when the food is ingested
• Antacids or other substances that may inhibit digestion

Hydrochloric acid is used for breaking the bonds between the proteins. Next, the proteins are disintegrated into amino acids, which are molecules that play very important roles in metabolism. Pepsin, the gastric enzyme mentioned before, represents the single protease capable of digesting collagen, a fibrous protein that is one of the main constituents of connective tissue in animals.

Digestion of Proteins in the Small Intestine

Trypsin and chymotrypsin are pancreatic protease enzymes secreted by the pancreas that are involved in protein and fat digestion. From the stomach, protein digestion carries on in the duodenum, which represents the first segment of the small intestine.

As well as pepsin, trypsin continues the disintegration of proteins into amino acids. Hydrolysis takes place in this case, too. In simpler words, hydrolysis involves the insertion of a water molecule between two amino acids, which forces the bond between them to break. Because amino acids have very small dimensions, they are able to penetrate the intestinal lining. From this point on, they enter the bloodstream through tiny veins, which are called capillaries. Once in the bloodstream, amino acids are transported by liquid blood plasma and red blood cells to various tissues, depending on where cell structures need to be created or repaired.

Absorption of Amino Acids

One thing that needs to be taken into consideration is that the protein source greatly influences the amount of time required by individual amino acids to be absorbed. For instance, amino acids from soy and milk proteins are digested differently. In addition, there are differences between individual types of milk protein. The absorption of milk proteins is:

• 50 percent of the digested protein between the stomach and the jejunum (middle section of the small intestine)
• 90 percent when the digested food gets in the ileum (final segment of the small intestine)