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The answers to the recent Medical Quiz on bilirubin are misleading
and not entirely correct. According to the answers, bilirubin "is toxic to
tissue and, therefore, is transported bound to albumin in the blood. It is
responsible for the emulsification of fats and facilitates their
absorption from the gastrointestinal tract."
(1) While it is true that bilirubin is toxic to central nervous
system tissue, it is not toxic to tissue in general. Substantial amounts
of bilirubin accumulate in tissues of infants with severe neonatal
jaundice and in patients with Crigler-Najjar syndrome with no evidence of
tissue toxicity provided that protein and lipid binding are adequate to
restrict its entry into the brain.
(2). The "therefore" in the answer creates a non-sequitur. Bilirubin
is not transported bound to albumin because it is toxic to tissue. It
binds to albumin because it is lipophilic and hydrophobic and this
restricts its CNS toxicity.
(3). Is there any evidence that bilirubin helps emulsify fats and
facilitates their gastrointestinal absorption? I think not. Steroids in
bile fulfil those functions. In any case, the concentration of
unconjugated bilirubin in bile and the gastrointestinal tract and the
water solubility of bilirubin are too low to have an effect on fat
absorption.
Bilirubin myths
The answers to the recent Medical Quiz on bilirubin are misleading
and not entirely correct. According to the answers, bilirubin "is toxic to
tissue and, therefore, is transported bound to albumin in the blood. It is
responsible for the emulsification of fats and facilitates their
absorption from the gastrointestinal tract."
(1) While it is true that bilirubin is toxic to central nervous
system tissue, it is not toxic to tissue in general. Substantial amounts
of bilirubin accumulate in tissues of infants with severe neonatal
jaundice and in patients with Crigler-Najjar syndrome with no evidence of
tissue toxicity provided that protein and lipid binding are adequate to
restrict its entry into the brain.
(2). The "therefore" in the answer creates a non-sequitur. Bilirubin
is not transported bound to albumin because it is toxic to tissue. It
binds to albumin because it is lipophilic and hydrophobic and this
restricts its CNS toxicity.
(3). Is there any evidence that bilirubin helps emulsify fats and
facilitates their gastrointestinal absorption? I think not. Steroids in
bile fulfil those functions. In any case, the concentration of
unconjugated bilirubin in bile and the gastrointestinal tract and the
water solubility of bilirubin are too low to have an effect on fat
absorption.
Competing interests: No competing interests