Biology · Chemical Coordination and Integration · NEET
Insulin cannot be given by mouth because it is a peptide (protein) hormone. If you swallow it, the protein-digesting enzymes in your stomach and small intestine (pepsin, trypsin, chymotrypsin) break it down into amino acids, so it loses all activity before it can act. Memory hook: "Protein taken by mouth becomes food" — insulin gets digested like any protein in your meal, so it must be injected straight into the blood/tissue.
Top row: swallowed insulin is digested by GI proteases into inactive amino acids. Bottom row: injected insulin bypasses the gut, stays intact, and lowers blood glucose. This is why insulin must be injected, not taken as a pill.
Your doubts, answered
Is insulin a protein/peptide hormone or a steroid hormone?
Insulin is a peptide (protein) hormone. It is made of two polypeptide chains, A and B, joined by disulphide bridges, and it is secreted by the beta-cells of the Islets of Langerhans in the pancreas. Because it is a protein, it can be digested by protein-splitting enzymes. Steroid hormones (like testosterone or cortisol) are lipid-based and are not digested this way.
What exactly happens if a diabetic patient swallows insulin?
The insulin travels to the stomach and small intestine. There, proteolytic (protein-digesting) enzymes such as pepsin, trypsin and chymotrypsin cut it into its amino acids, exactly like they do to the protein in your food. Once broken into amino acids, insulin is no longer a working hormone, so blood glucose is not lowered. This is why the NEET-correct reason is 'it will be digested in the GI tract'.
Why must insulin be given by injection instead?
Injection (usually under the skin) puts insulin directly into the body tissue and blood, so it skips the stomach and intestine completely. This way the protein is not exposed to digestive enzymes and reaches its target cells (hepatocytes, adipocytes, muscle) still active, where it promotes GLUT-4 glucose uptake.
Why can some hormones like thyroxine be taken orally but not insulin?
Thyroxine (T3/T4) is an amino-acid-derived hormone that is small and not destroyed by digestive enzymes, so it can be given as a tablet. Insulin is a large polypeptide, so digestive proteases break it apart. The rule of thumb: small, non-protein hormones can survive the gut, but full protein/peptide hormones cannot.
Does this apply to all peptide hormones?
Yes. Any protein or peptide hormone (insulin, glucagon, growth hormone, ADH, oxytocin) would be digested if swallowed, so these are usually given by injection or special routes, never as a simple pill. This is a favourite NEET link: 'peptide = injected, steroid = can be oral'.
⚠️ The NEET trap ✗ Insulin cannot be given orally because the human body will make a strong immune response against it. ✓ Insulin cannot be given orally because it is a peptide hormone and will be digested by proteolytic enzymes in the GI tract into inactive amino acids. 🧠 NTA offers 'immune response' and 'structural variation' as traps. The real, NCERT-based reason is simple digestion — insulin is a protein, and the gut digests proteins. Pick 'digested in GI tract'.
Real NEET questions
NEET 2025
Why can't insulin be given orally to diabetic patients?
A · Because of structural variation
B · Its bioavailability will be increased
C · Human body will elicit strong immune response
D · It will be digested in Gastro-Intestinal (GI) tract ✓
Solution: Insulin is a peptide hormone. Peptides and proteins are broken down by proteolytic enzymes (pepsin, trypsin, chymotrypsin) in the gastro-intestinal tract. If swallowed, insulin is digested into its amino acids and loses biological activity, so it must be given by injection. Options A, B and C are distractors.
NEET 2016
Name a peptide hormone which acts mainly on hepatocytes, adipocytes and enhances cellular glucose uptake and utilization.
A · Insulin ✓
B · Glucagon
C · Secretin
D · Gastrin
Solution: Insulin is a peptide hormone from the beta-cells of the Islets of Langerhans. It acts mainly on hepatocytes and adipocytes and enhances cellular glucose uptake and utilisation, lowering blood glucose. Being a peptide is exactly why it cannot be taken orally.
NEET 2016
The two polypeptides of human insulin are linked together by:
A · Hydrogen bonds
B · Phosphodiester bond
C · Covalent bond
D · Disulphide bridges ✓
Solution: The A and B chains of mature insulin are held together by covalent disulphide (-S-S-) bridges between cysteine residues. This confirms insulin is a protein/peptide molecule, which is why proteolytic enzymes in the gut can digest it if swallowed.
Solved Chemical Coordination and Integration NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Insulin is a peptide (protein) hormone with two chains, A and B, joined by disulphide bridges, secreted by the beta-cells of the pancreatic Islets of Langerhans.
Which enzymes would digest insulin if taken by mouth?
Proteolytic enzymes of the digestive tract — pepsin in the stomach and trypsin and chymotrypsin in the small intestine — cut insulin into amino acids.
How is insulin given to diabetics instead?
It is given by injection (usually subcutaneous), which delivers the intact protein directly into the tissue and blood, bypassing the digestive enzymes.
Can other hormones be given as tablets?
Yes. Steroid hormones and small amino-acid-derived hormones like thyroxine survive the gut and can be taken orally, but protein/peptide hormones like insulin cannot.
Is 'immune response' the reason insulin is not oral?
No. The NEET/NCERT reason is digestion by GI enzymes, not an immune response. 'Immune response' is a common NTA distractor.
