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Vitamin B3 (Niacin, Nicotinic acid)

SI UNITS (recommended)

CONVENTIONAL UNITS

(Essential Water-Soluble Vitamin - Required for Energy Metabolism, NAD/NADP Synthesis & Cellular Redox Reactions)

Synonyms

  • Niacin
  • Nicotinic acid
  • Vitamin B3
  • Niacinamide (nicotinamide)
  • Vitamin PP (pellagra-preventive factor)
  • NAD precursor
  • Nicotinamide adenine dinucleotide (derived coenzyme)

Units of Measurement

  • µg/L
  • µg/dL
  • µg/100 mL
  • µg%
  • ng/mL
  • µg/mL

Unit Conversions

Molecular Weight

  • Nicotinic acid: 123.11 g/mol
  • Niacinamide: 122.12 g/mol

(Conversions identical for both.)

µg/dL → µg/L

1 µg/dL=10 µg/L1\ \text{µg/dL} = 10\ \text{µg/L}1 µg/dL=10 µg/L

ng/mL → µg/L

1 ng/mL=1 µg/L1\ \text{ng/mL} = 1\ \text{µg/L}1 ng/mL=1 µg/L

µg/mL → µg/L

1 µg/mL=1000 µg/L1\ \text{µg/mL} = 1000\ \text{µg/L}1 µg/mL=1000 µg/L

µg/100 mL = µg% = µg/dL

Description

Vitamin B3 refers to niacin (nicotinic acid) and niacinamide (nicotinamide), which are precursors for NAD and NADP, two essential cofactors involved in:

  • Cellular energy production (electron transport chain)
  • Redox reactions
  • Fatty acid and cholesterol metabolism
  • DNA repair and gene expression
  • Neurotransmitter synthesis

Niacin can be obtained from:

  • Diet (meat, fish, nuts, grains)
  • Conversion of tryptophan → niacin (requires B6, riboflavin & iron)

Niacin deficiency produces the classic triad Dermatitis – Diarrhea – Dementia (Pellagra).

Physiological Role

1. NAD / NADP Synthesis

Niacin is the primary precursor of:

  • NAD⁺ / NADH
  • NADP⁺ / NADPH

These control:

  • ATP generation
  • β-oxidation
  • Glycolysis
  • Pentose phosphate pathway
  • Antioxidant systems (via NADPH)

2. Neurological Function

Niacin plays a role in neuronal energy metabolism.

3. Skin Integrity

Essential in keratinocyte differentiation and UV protection.

4. Lipid Metabolism

Pharmacologic niacin:

  • ↓ LDL
  • ↓ Triglycerides
  • ↑ HDL (via inhibition of hepatic apoA-1 catabolism)

Clinical Significance

LOW VITAMIN B3

Pellagra - The Classic Triad

  1. Dermatitis (photosensitive, hyperpigmented)
  2. Diarrhea / GI disturbances
  3. Dementia / neuropsychiatric symptoms

Untreated pellagra → death (“the three Ds + death”).

Causes

  • Severe malnutrition
  • Alcohol use disorder
  • Chronic diarrhea
  • Carcinoid syndrome (↑ tryptophan use → serotonin)
  • Hartnup disease (tryptophan malabsorption)
  • Isoniazid therapy (interferes with B6-dependent tryptophan conversion)
  • HIV/AIDS
  • Elderly / institutionalized

Symptoms

  • Glossitis
  • Stomatitis
  • Weakness
  • Depression, irritability
  • Peripheral neuropathy

EXCESS VITAMIN B3

Usually due to pharmacologic high-dose niacin.

Nicotinic Acid Toxicity

  • Flushing (prostaglandin-mediated)
  • Pruritus
  • Hepatotoxicity (especially sustained-release forms)
  • Hyperglycemia
  • Hyperuricemia → gout
  • GI upset

Nicotinamide Toxicity

  • Less flushing
  • Can cause hepatotoxicity at very high doses

Reference Intervals

(Based on Tietz 8E, Mayo, ARUP, NIH)

Note: Vitamin B3 is usually measured via niacin or nicotinamide in plasma or urine, but ranges vary by lab.

Plasma Niacin

  • 0.50 – 8.45 µg/L

Plasma Nicotinamide

  • 5 – 48 µg/L

Urinary N-methyl nicotinamide

  • >1.0 mg/day = adequate
  • <0.5 mg/day = deficiency

(Functional excretion tests are more sensitive.)

Diagnostic Uses

1. Pellagra Diagnosis

Clinical triad + low niacin.

2. Nutritional Assessment

Useful in malnutrition and alcoholism.

3. Tryptophan Metabolism Disorders

  • Hartnup disease
  • Carcinoid syndrome

4. Monitoring Pharmacologic Niacin Therapy

Though levels are rarely used, symptoms & liver tests are monitored.

Analytical Notes

  • Fasting sample recommended.
  • Protect sample from light.
  • LC-MS/MS is gold standard.
  • Plasma levels fluctuate with diet-urinary excretion tests are more reliable for deficiency.

Clinical Pearls

  • Isoniazid therapy increases risk of niacin deficiency.
  • Carcinoid syndrome uses up tryptophan → isolated niacin deficiency.
  • Niacin-induced flushing can be reduced with aspirin.
  • B3 deficiency may coexist with other B-vitamin deficiencies in alcoholism.
  • In severe pellagra, treat immediately-do not wait for lab confirmation.

Interesting Fact

Vitamin B3 was called “vitamin PP” (pellagra-preventive factor) after its ability to cure pellagra was discovered in the early 20th century.

References

  1. Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Vitamins
  2. NIH Office of Dietary Supplements - Niacin
  3. WHO Micronutrient Guidelines
  4. Mayo Clinic Laboratories - Niacin / Nicotinamide
  5. ARUP Consult - Vitamin Testing
  6. MedlinePlus (NIH) - Niacin Testing

Last updated: January 27, 2026

Reviewed by : Medical Review Board

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