Niacin (vitamin B3) deficiency causes pellagra (3 Ds: dermatitis, diarrhea, dementia). Tryptophan can serve as a niacin precursor. Pellagra occurs secondary to carcinoid syndrome because:

• A Serotonin directly inhibits niacin absorption in the gut
• B Carcinoid tumors secrete excess histamine that degrades niacin
• C Carcinoid tumors overproduce serotonin by diverting tryptophan to 5-HTP → serotonin pathway ✓
• D Metastatic carcinoid causes hepatic damage reducing NAD+ synthesis

Explanation

In carcinoid syndrome, the tumor over-diverts tryptophan to the serotonin synthesis pathway (tryptophan → 5-hydroxytryptophan → serotonin, 5-HT). Since 60 mg tryptophan is required to generate 1 mg niacin, excessive tryptophan diversion to serotonin depletes the available tryptophan for endogenous NAD+ synthesis via the kynurenine pathway, precipitating niacin deficiency and pellagra. Hartnup disease (neutral amino acid transporter defect in intestine/kidney) causes pellagra by the same mechanism — impaired tryptophan absorption. Corn-predominant diets are deficient in tryptophan AND niacin, also causing pellagra.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.