A 40-year-old man with chronic alcoholism presents with dermatitis on sun-exposed areas, diarrhoea, and confusion. Serum niacin levels are low. Which amino acid serves as a precursor for endogenous niacin synthesis, and what is the conversion ratio?

• A Tryptophan; 100 mg yields 1 mg niacin
• B Tryptophan; 60 mg yields 1 mg niacin ✓
• C Tyrosine; 60 mg yields 1 mg niacin
• D Methionine; 100 mg yields 1 mg niacin

Explanation

Tryptophan is the precursor for endogenous niacin synthesis. Approximately 60 mg of tryptophan yields 1 mg of niacin (the niacin equivalent). This ratio is used in calculating dietary niacin equivalents. Tyrosine and methionine are not niacin precursors. The 100:1 ratio is incorrect.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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