Digestion and Absorption of Proteins

Proteins undergo hydrolysis in the gastrointestinal tract resulting in the release of amino acids. This is accomplished mainly through the action of three proteolytic enzymes pepsin, trypsin and erepsin.

Pepsin - this enzyme is created by the peptic cells of the gastric mucosa and works best in an acid medium which is provided by the hydrochloric acid also secreted by the gastric mucosa.

Pepsin breaks down proteins to smaller units, polypeptides, also amino acids bound together by peptide linkages. No free amino acids are liberated in the stomach and this initial partial digestion of proteins is not essential.

Rennin is another proteolytic enzyme secreted by the stomach of new born mammals to enable the clotting of milk protein (caseinogen). It is the active ingredient of Rennet, which is used to curdle milk eg. junket.

Trypsin - This is the chief proteolytic enzyme of the pancreatic juice. It acts best in an alkaline medium and converts proteins to polypeptides. The pancreatic juice is the principal means by which proteins are digested.

Erepsin - is the collective name given to the mixture of proteolytic enzymes released by the mucosa of the small intestines. The effect of erepsin is to complete the hydrolysis of polypeptides and peptides to their component amino acids. These amino acids are then absorbed across the intestinal mucosa into the bloodstream.

In general, proteins from animal sources are more easily and rapidly absorbed than vegetable proteins perhaps because vegetable proteins are often enclosed in a cellulose covering.

The alimentary canal carries out the digestion and absorption of protein effectively with less than 10% of dietary protein usually appearing as nitrogen in the faeces.

The whole process of digestion is an ingenious solution to a complex problem.

Protein (enzymes) when activated by acid, digest proteins (food) denatured by acid and the mucous coating of the stomach wall protects its proteins from being affected by either acid or enzymes.

The acid in the stomach is so strong (pH 2) that no food is acid enough to make it stronger. It is normal to have an acid stomach. Antacids promoted for the relief of "acid indigestion" may only cause the stomach to produce more acid to restore it's normal balance.

Antacids may be useful to protect bleeding ulcers from stomach acid, but they may not be appropriate for normal, healthy people. Antacids also tend to deplete iron intake from foods if taken before, during or shortly after food is eaten.

Sometimes the stomach acid backs up and burns the lining of the oesophagus that is not as well protected by mucus as the stomach. The cause of this may simply be overeating but may also be due to conditions such as hernia or an obstruction that requires medical attention.