Guide To Digestion Of Protein

Why Protein Digestion Is So Important

An essential macronutrient, protein is used by the body to build and repair cells, to regulate a huge number of body functions. For example, almost 50 percent of the dietary protein we consume each day goes into making enzymes, the specialized proteins that help to digest food, assemble or divide molecules to make new cells and other chemical substances. Protein is also used to make neurotransmitters, essential for sending nerve messages around the body. Protein is also used in the creation of DNA and RNA, the nucleic acids responsible for determining how our body cells are formed and how they behave.

Protein Digestion In The Stomach

Enzymes are crucial contributors to protein digestion. Protein-digesting enzymes are referred to as proteinases or proteases. Protein generally takes the form of very complex molecules arranged in chains of amino acids. So the bonds binding these complex molecules together must first be broken down. This digestive process begins in the stomach, where hydrochloric acid, secreted in the stomach’s gastric acid, attacks the protein molecules separating them and breaking them down into amino acids. Then the gastric enzyme pepsin – the only protease able to digest collagen (the fibrous protein found in animal connective tissue) – starts to digest the amino acids.

Protein Digestion In The Small Intestine

Digestion of proteins continues in the duodenum, the first segment of the small intestine. As in fat digestion, the pancreas helps the process by secreting the pancreatic protease enzymes trypsin and chymotrypsin. Like pepsin, trypsin breaks down a protein into single amino acid molecules, through a process called hydrolysis. During hydrolysis, a water molecule is inserted between the two amino acids which are bonded together. This breaks the bond between them. After breakdown, the amino acids are small enough to pass through the intestinal lining into tiny veins (capillaries) in the villi (the finger-like projections on the wall of the small intestine). Once in the bloodstream, the amino acids are distributed by both red blood cells and by the liquid blood plasma to tissues throughout the body where they are used in the creation and repair of cell structures. Such is the demand for protein, the body maintains a constant balance of amino acids in the blood.

Surplus Protein Calories In Diet

If protein requirements are exceeded by protein intake, the surplus amino acids may be converted to glucose for energy use, or converted to fatty acids and stored as adipose tissue.

Insufficient Protein In Diet

If we eat insufficient protein (not very likely for most people), the body may break down stored protein in the muscles and transport the amino acids to the more vital organs, as required. Alternatively, if our energy intake falls dangerously low, protein amino acids will be taken from the muscles and sent to the liver to be converted into glucose.