- Constituent of NAD⁺/NADP⁺ - Derived from tryptophan; synthesis requires vitamins B₂ and B₆- Used to treat dyslipidemia; lowers VLDL and raises HDL

Deficiency:- Glossitis- Pellagra: diarrhea, dementia (also hallucinations), dermatitis ("broad collar" rash C3/C4 dermatome [Casal necklace], hyperpigmentation of sun-exposed limbs)- Can be caused by Hartnup disease (↓ tryptophan absorption), malignant carcinoid syndrome (↑ tryptophan metabolism), and isoniazid (↓ vitamin B₆)

Excess: Facial flushing (induced by prostaglandin, not histamine; can avoid by taking aspirin with niacin), hyperglycemia, hyperuricemia. (Deficiency → pellagra; excess → podagra).

Hartnup disease – autosomal recessive deficiency of neutral amino acid (eg, tryptophan) transporters in proximal renal tubular cells and on enterocytes → neutral aminoaciduria and ↓ absorption from the gut → ↓ tryptophan for conversion to niacin → pellagra-like symptoms. Treat with high-protein diet and nicotinic acid.