Pellagra due to niacin deficiency shows the '3 D' presentation. The underlying biochemical mechanism—specifically the relationship with tryptophan—is that:

• A Tryptophan is converted to niacin at a ratio of 60:1 (60 mg tryptophan yields 1 mg niacin equivalent)
• B Tryptophan conversion to niacin is blocked by high leucine in sorghum (jowar) diets
• C Both A and B are correct mechanisms relevant to pellagra pathogenesis ✓
• D Tryptophan cannot substitute for niacin because they are metabolised on separate pathways

Explanation

Both mechanisms are operationally relevant to pellagra. First, the body can synthesise niacin from tryptophan—60 mg of dietary tryptophan yields approximately 1 mg of niacin equivalent (niacin equivalent = dietary niacin + tryptophan/60). Second, in sorghum (jowar)-dependent populations, high dietary leucine competitively inhibits the kynurenine pathway, reducing tryptophan-to-niacin conversion, worsening niacin deficiency. Pellagra in India occurs classically in jowar-eating populations even when absolute tryptophan intake seems adequate, because leucine excess blocks the conversion pathway.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.