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

Loading the 3D model…

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.

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.

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.

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.

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.

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) (PubChem, NCBI, 2026) · Public domain; Niacin: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Pellagra: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Hartnup disease: MedlinePlus Genetics (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).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.