What is the physiology of bilirubin metabolism in the newborn?

Bilirubin comes from the breakdown of haemoglobin (haem), producing unconjugated (fat-soluble) bilirubin. This is carried to the liver, where it is conjugated (made water-soluble) by glucuronyl transferase and excreted in bile/stool.

Newborns are prone to jaundice because of increased red cell breakdown, immature liver conjugation, and increased enterohepatic circulation.

Clinical relevance. Understanding bilirubin metabolism (production, conjugation, excretion) explains why newborns are prone to jaundice and distinguishes unconjugated from conjugated causes.

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Bilirubin metabolism in the newborn

Understanding bilirubin handling explains why newborns are prone to jaundice.

Production

• Bilirubin is produced from the breakdown of haemoglobin (haem) from old/haemolysed red cells, forming unconjugated (indirect, fat-soluble) bilirubin.

Transport and conjugation

• Unconjugated bilirubin is carried (bound to albumin) to the liver.
• In the liver, the enzyme glucuronyl transferase conjugates it (with glucuronic acid) into conjugated (direct, water-soluble) bilirubin.

Excretion

• Conjugated bilirubin is excreted in bile into the gut, and eliminated in stool (some converted by gut bacteria).

Why newborns are prone to jaundice

• Increased red cell breakdown (higher red cell mass, shorter red cell lifespan).
• Immature hepatic conjugation (low glucuronyl transferase activity).
• Increased enterohepatic circulation (reabsorption of bilirubin from the gut), worsened by poor feeding.