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Vitamins — coenzymes, deficiency and toxicity
Water- and fat-soluble vitamins with their active coenzyme form, deficiency disease with one classic sign, and the toxicity traps of A, D and B6.
7 min read · updated 2026-08-24
Water-soluble vitamins (B and C) — coenzymes, not stored
They wash out, so deficiency appears fast and toxicity is rare (except B6).
| Vitamin | Active form / coenzyme | Deficiency | Classic sign |
|---|---|---|---|
| B1 Thiamine | TPP (thiamine pyrophosphate) | Beri-beri; Wernicke–Korsakoff | Wet (cardiac) & dry (neuro); ophthalmoplegia + ataxia + confusion |
| B2 Riboflavin | FMN, FAD | Ariboflavinosis | Angular stomatitis, cheilosis, magenta tongue |
| B3 Niacin | NAD⁺, NADP⁺ | Pellagra | 3 D's — dermatitis, diarrhoea, dementia (Casal's necklace) |
| B5 Pantothenate | Coenzyme A | Rare | Burning-feet syndrome |
| B6 Pyridoxine | PLP (pyridoxal phosphate) | (INH-induced) | Peripheral neuropathy, sideroblastic anaemia, seizures |
| B7 Biotin | Carboxylase prosthetic group | Raw-egg (avidin) | Dermatitis, alopecia |
| B9 Folate | THF (tetrahydrofolate) | Megaloblastic anaemia | No neuro signs; neural-tube defects |
| B12 Cobalamin | Methylcobalamin, adenosylcobalamin | Pernicious/megaloblastic | Subacute combined degeneration (posterior + lateral columns) |
| C Ascorbate | Reducing agent, prolyl/lysyl hydroxylase cofactor | Scurvy | Bleeding gums, corkscrew hairs, perifollicular haemorrhage |
B9 vs B12: both cause megaloblastic anaemia, but only B12 gives neurological damage. Giving folate alone masks the anaemia while the neuropathy worsens — the classic trap.
Fat-soluble vitamins (A, D, E, K) — stored, so toxicity matters
| Vitamin | Active form | Deficiency | Classic sign |
|---|---|---|---|
| A Retinol | Retinal, retinoic acid | Xerophthalmia | Night blindness, Bitot's spots |
| D Calciferol | 1,25-(OH)₂-cholecalciferol (calcitriol) | Rickets / osteomalacia | Bow legs, rachitic rosary |
| E Tocopherol | Antioxidant | Haemolysis, neuropathy | RBC fragility, ataxia |
| K Phylloquinone | γ-carboxylation cofactor (II, VII, IX, X) | Bleeding | Prolonged PT/INR, haemorrhagic disease of newborn |
Toxicity traps
- Vitamin A — teratogenic (retinoids in pregnancy); chronic excess → pseudotumour cerebri, hepatotoxicity, hair loss.
- Vitamin D — hypercalcaemia, nephrocalcinosis, "stones, bones, groans".
- Vitamin B6 — the one water-soluble vitamin that is toxic: high-dose pyridoxine causes a sensory peripheral neuropathy.
Anchor: Water-soluble = washed out, deficiency fast, only B6 is toxic. Fat-soluble = stored, so A and D are the overdose villains and K guards the clotting factors 2-7-9-10.
Sources
- Harper's Illustrated Biochemistry, 32e
- Textbook of Biochemistry for Medical Students (Vasudevan), 9e