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Vitamin B3 (niacin)

Vitamin B3 (niacin)

Niacin is a water-soluble B vitamin important in energy metabolism. It is a precursor of NAD+ and NADP+, so deficiency particularly affects high-turnover tissues such as skin, gut and the nervous system.

Function and metabolism
  • constituent of NAD+ and NADP+
    • essential coenzymes in redox reactions
  • can be synthesised from tryptophan
    • this pathway requires vitamins B2 and B6

Causes of deficiency
  • poor intake or malabsorption
  • Hartnup disease
    • autosomal recessive defect of neutral amino acid transport in enterocytes and proximal renal tubules
    • causes reduced tryptophan absorption and neutral aminoaciduria, leading to reduced niacin synthesis
  • carcinoid syndrome
    • increased tryptophan metabolism to serotonin reduces substrate for niacin synthesis
  • isoniazid therapy
    • may precipitate deficiency via functional vitamin B6 deficiency

Clinical features of deficiency
  • glossitis may occur
  • pellagra
    • the 3 Ds: dermatitis, diarrhoea and dementia
    • neuropsychiatric features may include hallucinations
    • photosensitive dermatitis classically affects sun-exposed skin
    • Casal necklace: circumferential pigmented rash around the neck/collar area

Treatment and excess
  • treat the cause and give nicotinic acid/niacin replacement
  • Hartnup disease is managed with a high-protein diet and nicotinic acid
  • niacin is also used for dyslipidaemia
    • lowers VLDL and raises HDL
  • adverse effects of excess niacin: facial flushing due to prostaglandin release, hyperglycaemia and hyperuricaemia
    • flushing can be reduced by aspirin
+PassMedicine Notes

Niacin is a water-soluble B vitamin important in energy metabolism. It is a precursor of NAD+ and NADP+, so deficiency particularly affects high-turnover tissues such as skin, gut and the nervous system.

Function and metabolism
  • constituent of NAD+ and NADP+
    • essential coenzymes in redox reactions
  • can be synthesised from tryptophan
    • this pathway requires vitamins B2 and B6
Causes of deficiency
  • poor intake or malabsorption
  • Hartnup disease
    • autosomal recessive defect of neutral amino acid transport in enterocytes and proximal renal tubules
    • causes reduced tryptophan absorption and neutral aminoaciduria, leading to reduced niacin synthesis
  • carcinoid syndrome
    • increased tryptophan metabolism to serotonin reduces substrate for niacin synthesis
  • isoniazid therapy
    • may precipitate deficiency via functional vitamin B6 deficiency
Clinical features of deficiency
  • glossitis may occur
  • pellagra
    • the 3 Ds: dermatitis, diarrhoea and dementia
    • neuropsychiatric features may include hallucinations
    • photosensitive dermatitis classically affects sun-exposed skin
    • Casal necklace: circumferential pigmented rash around the neck/collar area
Treatment and excess
  • treat the cause and give nicotinic acid/niacin replacement
  • Hartnup disease is managed with a high-protein diet and nicotinic acid
  • niacin is also used for dyslipidaemia
    • lowers VLDL and raises HDL
  • adverse effects of excess niacin: facial flushing due to prostaglandin release, hyperglycaemia and hyperuricaemia
    • flushing can be reduced by aspirin


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