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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Niacin (vitamin B3)

Niacin forms NAD⁺ and NADP⁺, the coenzymes that move electrons in hundreds of reactions; its lack causes pellagra.

Updated 2026-11-01

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## Key facts

- Active forms: NAD⁺ (energy-yielding oxidations: GAPDH, PDH, Krebs cycle) and NADP⁺/NADPH (fat and steroid synthesis, antioxidant defence).
- The body can make it from tryptophan (about 60 mg of tryptophan gives 1 mg of niacin), with help from B6 and B2.
- Deficiency: pellagra, the four Ds: dermatitis on sun-exposed skin, diarrhoea, dementia and death.
- At pharmacological doses nicotinic acid lowers triglycerides and LDL and raises HDL, but it causes flushing and can harm the liver.

## In detail

- Sources: meat, fish, poultry, peanuts, legumes, fortified cereals. In maize it is bound (niacytin) and is not absorbed unless the maize is nixtamalised with lime.
- Synthesis: tryptophan → kynurenine → quinolinic acid → nicotinic acid mononucleotide → NAD⁺. Kynureninase depends on PLP (B6).
- NAD⁺ accepts a hydride and is reduced to NADH; NADH carries those electrons to the respiratory chain (complex I).
- NADPH: made by G6PD and malic enzyme; used by fatty acid synthase, HMG-CoA reductase, glutathione reductase and neutrophil NADPH oxidase.
- NAD⁺ is also a substrate (not only a coenzyme) for sirtuins and for PARP, which repairs DNA.
- Causes of deficiency: a diet based on non-nixtamalised maize, alcoholism, isoniazid (uses up B6), carcinoid syndrome (diverts tryptophan to serotonin), Hartnup disease (tryptophan is not absorbed).
- Excess (pharmacological nicotinic acid): prostaglandin flushing, itch, hyperglycaemia, hyperuricaemia, hepatotoxicity.
- Lab: urinary niacin metabolites (N1-methylnicotinamide); pellagra is mainly a clinical diagnosis.

**Also called**
Vitamin B3, Nicotinic acid, Nicotinamide

**Formula**
C₆H₅NO₂

**PubChem**
[CID 938](https://pubchem.ncbi.nlm.nih.gov/compound/938)

## Structure

- A pyridine with a carboxyl group at position 3 (nicotinic acid); nicotinamide has an amide instead.
- In NAD⁺ the nicotinamide is the reactive part: its carbon 4 accepts the hydride.

## Clinical notes

- Pellagra persists in populations eating mostly non-nixtamalised maize, in people with alcoholism and in refugees.
- Carcinoid syndrome can cause pellagra because the tumour spends tryptophan on making serotonin.
- Nicotinic acid flushing is caused by prostaglandins and is reduced by aspirin taken beforehand.

## Pathways

- [Glycolysis](https://durgical.com/en/biochemistry/pathways/glycolysis/)
- [Krebs cycle (citric acid cycle)](https://durgical.com/en/biochemistry/pathways/krebs-cycle/)

## Related conditions

- [Pellagra (niacin deficiency)](https://durgical.com/en/conditions/pellagra/)
- [Atherosclerosis and dyslipidaemia](https://durgical.com/en/conditions/atherosclerosis-and-dyslipidaemia/)

## Related

- [NAD⁺ (nicotinamide adenine dinucleotide)](https://durgical.com/en/biochemistry/molecules/nad/)
- [Glyceraldehyde-3-phosphate dehydrogenase (GAPDH)](https://durgical.com/en/biochemistry/enzymes/glyceraldehyde-3-phosphate-dehydrogenase/)
- [Isocitrate dehydrogenase](https://durgical.com/en/biochemistry/enzymes/isocitrate-dehydrogenase/)
- [Vitamin B6 (pyridoxal phosphate)](https://durgical.com/en/biochemistry/molecules/vitamin-b6/)
- [Riboflavin (vitamin B2)](https://durgical.com/en/biochemistry/molecules/riboflavin/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which coenzymes are made from niacin?

NAD⁺ and NADP⁺, and their reduced forms NADH and NADPH.

## Which amino acid is a precursor of niacin?

Tryptophan, via the kynurenine pathway, which needs vitamin B6 and riboflavin.

## Why can carcinoid syndrome cause pellagra?

The tumour diverts tryptophan to serotonin, leaving little to make niacin.

## What are the four Ds of pellagra?

Dermatitis on sun-exposed skin, diarrhoea, dementia and death if untreated.

## Why does nixtamalised maize protect against pellagra?

Cooking with lime releases maize's bound niacin and makes it absorbable.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/biochemistry/molecules/niacin/)

Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Nicotinic Acid (CID 938)](https://pubchem.ncbi.nlm.nih.gov/compound/938) (PubChem, NCBI, 2026) · Public domain; [Niacin: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002409.htm) (MedlinePlus, US National Library of Medicine, 2026); [Pellagra: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000342.htm) (MedlinePlus, US National Library of Medicine, 2026); [Hartnup disease: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/hartnup-disease/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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