Step by Step
BWS
B vitamins are coenzymes
The water-soluble vitamins are vitamin C and the eight B vitamins: B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin), B9 (folate), B12 (cobalamin). Most B vitamins function as coenzymes — they are chemically modified forms that assist enzymes in metabolic reactions.
Memory trick: 'The Rhythm Nightly Pays Big Favors' = B1 B2 B3 B5 B6 B7 B9 B12.
Coenz
Key coenzyme forms
B1 (TPP): pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, transketolase. B2 (FAD/FMN): ETC, fatty acid oxidation. B3 (NAD⁺/NADP⁺): glycolysis, Krebs, PPP. B5 (CoA): pyruvate oxidation, fatty acid synthesis. B6 (PLP): amino acid metabolism, transamination. B7 (biotin): carboxylation reactions (ACC, pyruvate carboxylase). B9 (THF): one-carbon transfers, DNA synthesis. B12 (adenosylcobalamin/methylcobalamin): odd-chain FA metabolism, B9 recycling.
Exc
Excreted in urine — faster deficiency
Because water-soluble vitamins are not stored (except B12 which has a 3-5 year liver store), deficiency develops faster than fat-soluble vitamin deficiencies. Excess is excreted in urine — toxicity is rare. Exception: very high-dose B6 can cause peripheral neuropathy. B3 (niacin) in high doses causes flushing and can be hepatotoxic.
Syn
Intestinal synthesis
Gut bacteria synthesize some B vitamins (especially B7 and B12). Prolonged antibiotic use can deplete gut flora → relative vitamin deficiency. B12 requires intrinsic factor (from gastric parietal cells) for absorption in the terminal ileum — unique among vitamins.
Applied Walkthrough
1
A patient on a strict diet of polished white rice develops beriberi (B1 deficiency) — polishing removes the rice bran where thiamine is concentrated.
2
A patient with pernicious anemia (autoimmune destruction of parietal cells) cannot make intrinsic factor → cannot absorb B12 → megaloblastic anemia + neurological symptoms.
3
A patient on isoniazid (TB drug) develops peripheral neuropathy — isoniazid antagonizes B6 (PLP) → B6 supplementation is given routinely with isoniazid.
4
An alcoholic presents with confusion, ophthalmoplegia, ataxia (Wernicke's encephalopathy) — alcohol impairs thiamine absorption and utilization.
Exam Application
Exams test which vitamins are water-soluble vs fat-soluble, the coenzyme forms and their metabolic roles, deficiency diseases for each B vitamin, and why B12 is unique (requires IF, stored in liver, deficiency causes neurological symptoms). The connection between B vitamins and specific metabolic pathways is very high yield.
⚠ Common Trap
Students think all water-soluble vitamins are completely harmless in excess — B6 in high doses causes sensory peripheral neuropathy, and high-dose niacin causes flushing and hepatotoxicity. Also: B12 deficiency takes YEARS to develop (large liver stores) while other B vitamin deficiencies develop in weeks to months.
✓ Quick Self-Check
1. Name the eight B vitamins in order.
B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin), B9 (folate), B12 (cobalamin).
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2. What coenzyme does vitamin B1 form?
Thiamine pyrophosphate (TPP) — cofactor for pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, and transketolase.
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3. Why does B12 deficiency take years to develop?
B12 is stored in the liver with a 3-5 year reserve — unlike most other water-soluble vitamins which have minimal storage.
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4. What is unique about B12 absorption?
B12 requires intrinsic factor (secreted by gastric parietal cells) for absorption in the terminal ileum.
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5. Which B vitamin deficiency is caused by isoniazid?
Vitamin B6 (pyridoxine) — isoniazid antagonizes B6, causing peripheral neuropathy. B6 supplementation is given prophylactically.
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