During an acute attack of porphyria, intravenous dextrose is administered even before specific therapy is available. What is the biochemical basis of this measure?
- A Carbohydrate loading represses hepatic ALA synthase, the so-called glucose effect ✓
- B Glucose directly complexes and neutralises circulating porphobilinogen
- C Glucose accelerates renal clearance of delta-aminolevulinic acid
- D Glucose provides substrate for alternative heme-independent electron transport
Correct answer: A. Carbohydrate loading represses hepatic ALA synthase, the so-called glucose effect
Explanation
High carbohydrate intake suppresses hepatic ALAS1 expression independently of heme, a phenomenon termed the glucose effect, thereby lowering production of neurotoxic ALA and porphobilinogen. Conversely, fasting or hypocaloric dieting induces ALAS1 and precipitates attacks, which is why patients are advised against crash diets. Glucose neither chelates porphobilinogen nor alters its renal handling, making options B and C mechanistically incorrect.
Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.
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