Vitamin B3

  • Also called niacin.
  • Function
    • Constituent of NAD+, NADP+ (used in dehydrogenase and reductase reactions).
      • NAD: Glycolysis, beta-oxidation, cell signaling, DNA repair, cofactor for isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, malate dehydrogenase in citric acid cycle
      • NADP: Anabolic reactions – fatty acid & cholesterol synthesis
    • Derived from tryptophan. Synthesis requires vitamins B2 and B6. Used to treat dyslipidemia; lowers levels of VLDL and raises levels of HDL.
    • NAD derived from Niacin (B3 ≈ 3 ATP).
  • Deficiency
    • Glossitis.
    • Severe deficiency leads to pellagra, which can also be caused by Hartnup disease, malignant carcinoid syndrome (↑ tryptophan metabolism), and isoniazid (↓ vitamin B6).
      • Symptoms of pellagra: Diarrhea, Dementia (also hallucinations), Dermatitis (C3/C4 dermatome circumferential “broad collar” rash [Casal necklace], hyperpigmentation of sunexposed limbs [A]).
      • The 3 D’s of B3.
    • 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.
    • Deficiency of vitamin B3 → pellagra.
  • Excess
    • Facial flushing (induced by prostaglandin, not histamine; can avoid by taking aspirin with niacin), hyperglycemia, hyperuricemia.
    • Excess of vitamin B3 → podagra.

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