Water-Soluble Vitamins (B Complex and C)
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Direct answer
Nine B-complex members plus vitamin C make up the water-soluble group: absorbed by specific transporters, stored minimally (except cobalamin in liver, enough for three to five years), and excreted when in excess, so toxicity is rare and deficiency develops in weeks to months. Each B vitamin is a coenzyme precursor — thiamine (TPP), riboflavin (FAD, FMN), niacin (NAD, NADP), pantothenate (CoA), pyridoxine (PLP), biotin, folate (THF) and cobalamin — and matching the coenzyme to the deficiency syndrome is the entire exam: beriberi and Wernicke encephalopathy for thiamine, pellagra for niacin, angular stomatitis for riboflavin, megaloblastic anaemia for folate and cobalamin, and scurvy for ascorbate. Indian diets make folate, cobalamin (in vegetarians) and thiamine (polished rice) the realistic deficiencies.
What you must remember
- Thiamine (B1): TPP for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, branched-chain ketoacid dehydrogenase and transketolase; dry beriberi (neuropathy), wet beriberi (high-output failure) and Wernicke-Korsakoff syndrome.
- Riboflavin (B2): FMN and FAD prosthetic groups; deficiency gives angular stomatitis, cheilosis, glossitis and magenta tongue; milk, liver and green leafy vegetables are sources.
- Niacin (B3): NAD and NADP; about 60 milligrams tryptophan can substitute for 1 milligram niacin, linking pellagra to Hartnup disease, carcinoid and isoniazid therapy; the classic three Ds — dermatitis (Casal necklace), diarrhoea, dementia.
- Pyridoxine (B6): PLP for transamination, decarboxylation and the first haem-synthesis step; deficiency from isoniazid or penicillamine causes peripheral neuropathy and sideroblastic anaemia; excess causes sensory neuropathy, a rare vitamin toxicity in this group.
- Folate (B9): THF for one-carbon transfer; deficiency is the commonest cause of megaloblastic anaemia in India and periconceptional 400 micrograms daily prevents most neural tube defects; inhibited by methotrexate, phenytoin and co-trimoxazole.
- Cobalamin (B12): methylcobalamin (methionine synthase) and adenosylcobalamin (methylmalonyl-CoA mutase); deficiency in strict vegetarians, post-gastrectomy, pernicious anaemia and ileal disease, giving megaloblastic anaemia plus subacute combined degeneration and a positive methylmalonic acid test that folate deficiency lacks.
- Biotin and pantothenate: biotin serves the carboxylases (pyruvate, acetyl-CoA, propionyl-CoA), with deficiency from raw-egg avidin or inadequate total parenteral nutrition (alopecia, rash, acidosis); pantothenate deficiency is essentially never seen alone.
- Ascorbate (C): cofactor for prolyl and lysyl hydroxylases of collagen (Fe2+ and alpha-ketoglutarate dependent), dopamine beta-hydroxylase; scurvy shows perifollicular haemorrhage, bleeding gums, woody oedema; it also enhances non-haem iron absorption at meals.
How to work through a megaloblastic-anaemia case
A 30-year-old lactating vegetarian presents with pallor, paraesthesiae and unsteady gait; the smear shows macro-ovalocytes and hypersegmented neutrophils. The fork in the pathway is folate versus cobalamin, and the biochemistry decides: cobalamin deficiency raises both serum methylmalonic acid and homocysteine, whereas folate deficiency raises homocysteine alone. Treating the wrong one is the classic error — folate given alone to a cobalamin-deficient patient corrects the anaemia while neurological damage progresses, so cobalamin must be ruled out first, a point Indian postgraduate exams phrase as "folate is a loaded gun in B12 deficiency."
Then connect to public health: periconceptional folic acid, 400 micrograms daily from before conception through the first trimester, prevents the majority of open neural tube defects, which is why food fortification and antenatal iron-folate programmes exist; the dose rises to 4-5 milligrams in a mother with a previously affected child. The water-soluble logic explains why antenatal iron with folic acid tablets are given together — different deficiency, same diet.
Where students slip
Students treat neuropathy in a megaloblastic patient as always cobalamin; folate deficiency can cause irritability and cognitive changes, but combined degeneration of the posterior columns and lateral corticospinal tracts belongs to B12 — the localising distinction. Second, pellagra from niacin deficiency should trigger a search for secondary causes (Hartnup transport failure, carcinoid diverting tryptophan to serotonin, isoniazid competing with PLP) rather than diet alone. Third, remember thiamine before glucose: intravenous dextrose in a depleted patient consumes remaining TPP and precipitates Wernicke encephalopathy — ophthalmoplegia, ataxia, confusion — an ordering error with direct consequences.
Frequently asked questions
Why must thiamine precede glucose in suspected Wernicke encephalopathy?
Glucose oxidation consumes thiamine pyrophosphate, so dextrose given first can precipitate or worsen the encephalopathy in a depleted patient.
How is cobalamin deficiency distinguished biochemically from folate deficiency?
Both raise homocysteine, but only cobalamin deficiency raises methylmalonic acid, because adenosylcobalamin is needed by methylmalonyl-CoA mutase.
Which water-soluble vitamin prevents neural tube defects?
Folic acid, 400 micrograms daily periconceptionally, supplying one-carbon units for DNA synthesis in the rapidly closing neural tube.
What is the coenzyme role of vitamin C in collagen synthesis?
Ascorbate keeps iron in the ferrous state for prolyl and lysyl hydroxylases; without hydroxylation, collagen triple helices are unstable, and scurvy follows.
Why does isoniazid therapy cause both pellagra-like and neuropathic features?
Isoniazid raises pyridoxine turnover (causing PLP-deficiency neuropathy) and, through PLP depletion, impairs the kynureninase step converting tryptophan to niacin, mimicking pellagra.