The B-Complex Vitamins: Riboflavin, Niacin and Pyridoxine

One family, one job: they are coenzymes

A vitamin is not fuel and not a building block — it is a tool an enzyme borrows to do its work. The B-complex is a loose group of water-soluble vitamins that the body cannot store in any quantity, so a steady dietary supply matters and excess is simply flushed out in urine. Their unifying feature is chemical, not anatomical: each is converted into a coenzyme — a small molecule that clamps onto an enzyme and makes a specific reaction possible. Take the coenzyme away and the enzyme is a lock without a key. This is why B-vitamin deficiencies hit the busiest, most metabolically demanding tissues first: skin, gut lining, blood, and nerves. And it is why replacing the vitamin can reverse the picture so completely. B12 and folate, the two B vitamins central to DNA synthesis and the megaloblastic anaemias, are important enough to get their own chapter; here we take the remaining players, each with its own coenzyme, its own deficiency, and — the pharmacology — its own link to a drug.

Riboflavin (B2): the electron shuttle

Riboflavin becomes two closely related coenzymes, FAD and FMN, the flavins that shuttle electrons in and out of countless redox reactions — most visibly in the electron transport chain that makes ATP. Deficiency, ariboflavinosis, rarely comes alone (a poor diet short on one B vitamin is usually short on several), and it announces itself around the mouth and skin: angular stomatitis and cheilitis (cracked, inflamed corners and lips), a raw magenta glossitis, a sore throat, and a greasy seborrhoeic dermatitis around the nose and eyes. It is not a dangerous deficiency and it corrects quickly with replacement. Riboflavin's other claim to fame is cosmetic and harmless: take a supplement and the excess turns the urine a startling bright fluorescent yellow — a useful reminder that water-soluble vitamins in excess are simply excreted. Pharmacologically, high-dose riboflavin has a modest evidence-based role in migraine prophylaxis, which is why it reappears in the Central Nervous System section alongside the other preventive agents.

Niacin (B3): NAD, NADP, and the three Ds

Niacin is unusual: the body can partly make its own, from the amino acid tryptophan. Niacin becomes NAD and NADP, the workhorse coenzymes of hundreds of oxidation-reduction reactions — NAD driving catabolism and energy release, NADP feeding biosynthesis. Crucially, the body can synthesize niacin from dietary tryptophan, so deficiency needs both a diet low in niacin and one low in tryptophan, or something that diverts the pathway. Its deficiency disease is pellagra, remembered by the three (really four) Ds: dermatitis, diarrhoea, dementia, and, untreated, death. The dermatitis is the giveaway — a symmetrical, sharply demarcated rash on sun-exposed skin (classically a collar-like band around the neck), because the photosensitive skin flags where the deficiency bites. Pellagra has a rogues' gallery of causes: chronically poor maize-based diets (maize niacin is bound and poorly absorbed), alcoholism, carcinoid syndrome (the tumour diverts tryptophan into serotonin, starving the niacin pathway), the inherited Hartnup disease (defective tryptophan absorption), and — the pharmacology to remember — the anti-TB drug isoniazid, which interferes with the tryptophan-to-niacin conversion.

Niacin has a second life as a drug in its own right. In gram doses — far above any nutritional need — nicotinic acid is a lipid-lowering agent: it raises HDL more than any other agent and lowers triglycerides and LDL. For decades it was a mainstay of dyslipidaemia treatment, but it has fallen out of routine use, and the reason is instructive. First, large outcome trials failed to show that adding niacin to a statin actually reduced cardiovascular events — a reminder that moving a number on a lipid panel is not the same as preventing a heart attack. Second, it is poorly tolerated: the signature side effect is intense cutaneous flushing, a hot, red, prickling wave across the face and upper body. That flush is prostaglandin-mediated, which is the elegant part — because it is driven by prostaglandins, taking aspirin beforehand blunts it, a small mechanistic trick that ties this vitamin straight to the Cardiovascular / lipid-lowering chapter and to the prostaglandin pharmacology behind NSAIDs.

Pyridoxine (B6): the amino-acid and neurotransmitter cofactor

If riboflavin and niacin move electrons, pyridoxine moves nitrogen — and that puts it at the heart of the nervous system. Pyridoxine's active coenzyme is pyridoxal phosphate (PLP), the essential cofactor for transamination (shuffling amino groups between amino acids), for a key step in haem synthesis, and — the reason it dominates the clinical stories — for the enzymes that build neurotransmitters: GABA, dopamine, and serotonin. Because PLP is needed to make GABA, the brain's principal inhibitory transmitter and its main chemical brake, a B6 deficiency lowers the seizure threshold. And because PLP maintains peripheral nerves, deficiency causes a peripheral neuropathy — the burning, tingling feet of the man in the clinic. The dominant cause in practice is not diet but a drug. Isoniazid, again, is the villain: it reacts with pyridoxine and accelerates its excretion, producing a functional B6 deficiency. So pyridoxine is co-prescribed with isoniazid as a matter of routine, especially in those already at risk (the malnourished, alcoholics, diabetics, pregnant women), specifically to prevent the neuropathy.

That same GABA logic makes pyridoxine a genuine antidote. In a severe isoniazid overdose, the drug so depletes GABA that the patient develops refractory seizures that ordinary anticonvulsants barely touch; high-dose intravenous pyridoxine restores GABA synthesis and is the specific antidote — a fact that lands this vitamin squarely in the Toxicology and Antimicrobials chapters. Beyond isoniazid, pyridoxine has a scatter of other uses: it treats some sideroblastic anaemias (via its role in haem synthesis — see the Haematology section), and, combined with the antihistamine doxylamine, it is a first-line treatment for nausea and vomiting of pregnancy. But B6 carries a genuine sting in its tail, and it is a favourite exam trap: taken chronically in high doses as a supplement, pyridoxine paradoxically causes the very thing its deficiency causes — a sensory peripheral neuropathy. Too little and too much both damage the nerves, which is why "more vitamin is always safer" is simply false.

The quiet ones: biotin (B7) and pantothenate (B5)

Two more B vitamins round out the family, and both are notable mainly for how rarely they cause trouble. Pantothenic acid (B5) is a component of coenzyme A, the universal carrier of acyl groups at the crossroads of fat and carbohydrate metabolism; it is so widespread in food (the name comes from the Greek for "everywhere") that dietary deficiency is essentially unheard of. Biotin (B7) is the coenzyme for the carboxylase enzymes; deficiency is rare too, seen mainly with prolonged raw-egg-white consumption (avidin in raw egg white binds and traps biotin) or certain inherited enzyme defects, and shows up as a rash, alopecia, and neurological changes. Biotin earns its keep in the exam for one modern pitfall worth flagging: high-dose biotin supplements, sold widely for hair and nails, interfere with many laboratory immunoassays — falsely distorting thyroid function tests, troponin, and hormone panels. A patient's over-the-counter supplement can therefore manufacture a lab result that looks like disease, a trap that connects to the Endocrine section.