Digestion and absorption of proteins


Daily requirement

The daily protein requirement for adults is 0.5-0.7 gm/kg of body weight. And for children, 1-4 years of age is 3-4gm/kg of body weight.


Sources of protein

The proteins found in the intestine come from two sources:

  • Endogenous proteins: 30-40gm/day are secretary proteins as well as the protein components of deal cells.
  • Exogenous proteins: These are dietary proteins. 75-100gm/day is taken in the diet as plant and animal proteins.


Digestion of proteins

Proteins digestion starts in the stomach where proteins are fragmented by the action of pepsin into peptides. Pepsin is the most important enzyme of the stomach which is active at a low pH (2-3) and completely inactive at pH above 5. This means that the digestion of proteins mostly occurs in the upper small intestine, duodenum, and jejunum under the action of Proteolytic enzymes of pancreatic juice.

The enzyme found in pancreatic juice (trypsin), chymotrypsin, and carboxypeptidase causes the breakdown of proteins into peptides. The carboxypeptidase acts on peptides, breaking the peptide bond, which attached the terminal amino acid to the carboxyl end of the peptide. Protein digestion is finalized (completed) by peptidases.


Absorption of proteins

Proteins are mostly absorbed in the form of dipeptides, tripeptides, and amino acids. The rapid absorption of amino acids occurs in the duodenum and jejunum but is slow in the ileum. In the colon, they are digested by bacterial action. The proteins in stool are not dietary in origin but come from bacteria and cellular debris.

During the absorption of proteins, most peptide and amino acid molecules bind with a specific transport protein which requires Na+ binding before transport can occur. The Na+ ions then move by electrochemical gradients to the interior of the cell and pull the amino acids or peptides along with it.

From this discussion, it is noted that proteins are absorbed through co-transport or a secondary active transport process. However, some amino acids are transported by the process of facilitated diffusion.