Why can’t insulin be given orally to diabetic patients?

The correct option is: It will be digested in Gastro-Intestinal (GI) tract

To understand why insulin cannot be administered orally, let us break down its chemical nature and how the human digestive system functions step-by-step:

1. Chemical Nature of Insulin:
Insulin is a peptide hormone produced by the beta cells of the pancreatic islets. Chemically, it is a protein consisting of two polypeptide chains: Chain A (with 21 amino acids) and Chain B (with 30 amino acids), which are linked together by disulfide bonds. Because it is a protein, it is subject to the same digestive processes as the proteins we consume in our diet.

2. Digestion in the Gastro-Intestinal (GI) Tract:
When a protein or peptide is swallowed and enters the gastrointestinal tract, it encounters a highly acidic environment in the stomach and various proteolytic enzymes (proteases) throughout the stomach and small intestine:
- In the stomach, hydrochloric acid (HCl) denatures proteins, and the enzyme pepsin begins breaking peptide bonds.
- In the small intestine, pancreatic enzymes such as trypsin, chymotrypsin, and carboxypeptidases further cleave the polypeptide chains into smaller peptides and individual amino acids.
If insulin is taken orally, these proteolytic enzymes will digest and degrade the insulin molecule before it can reach the bloodstream, rendering it completely inactive and ineffective.

3. Absorption and Bioavailability Barriers:
In addition to enzymatic degradation, the intact insulin molecule is relatively large and hydrophilic, making it difficult to pass through the epithelial cells lining the intestinal wall. Thus, its oral bioavailability (the fraction of the administered dose that enters systemic circulation intact) is extremely low.

Conclusion:
Because insulin is a protein, oral administration leads to its complete digestion and inactivation by proteolytic enzymes in the gastrointestinal tract. Therefore, to ensure it enters the bloodstream intact and remains functional, insulin must be administered via subcutaneous injection or other non-oral routes.