Step by Step
Fol
Folate (B9) — one-carbon metabolism
Folate (as THF — tetrahydrofolate) carries one-carbon units for: DNA synthesis (thymidylate synthesis — dTMP from dUMP, requires DHFR). Purine synthesis. Amino acid interconversion (serine ↔ glycine, homocysteine → methionine). Deficiency: megaloblastic anemia (large, immature RBCs — impaired DNA synthesis), neural tube defects (NTDs — spina bifida) in embryos. Sources: leafy greens (foliage → folate). Fortified in flour/cereal.
Memory trick: Folate = 'FOliage' (leafy greens). Folate for Fetal neural tubes.
B12
Cobalamin (B12) — two unique roles
B12 (as methylcobalamin or adenosylcobalamin) has two key enzymatic roles: (1) Methionine synthase: homocysteine + methyl-THF → methionine + THF. Requires B12. (2) Methylmalonyl-CoA mutase: methylmalonyl-CoA → succinyl-CoA. Requires B12. Deficiency: megaloblastic anemia + NEUROLOGICAL DAMAGE (subacute combined degeneration — demyelination of dorsal and lateral spinal columns).
Memory trick: B12 = Brain and Blood. Both are affected. Folate = only Blood.
Trap
The folate trap — why B12 deficiency depletes active folate
When B12 is deficient, methionine synthase is inactive → methyl-THF cannot donate its methyl group → it gets 'trapped' as methyl-THF (inactive) → no free THF available for DNA synthesis → megaloblastic anemia even when folate levels are normal. Giving folate corrects the anemia but NOT the neurological damage → dangerous masking of B12 deficiency.
Memory trick: The methyl trap = folate is locked up and useless without B12 to free it.
Abs
B12 absorption — unique requirements
B12 requires intrinsic factor (IF, secreted by gastric parietal cells) for absorption in the terminal ileum. Causes of deficiency: pernicious anemia (autoimmune → anti-parietal cell or anti-IF antibodies), gastrectomy, terminal ileum resection (Crohn's), veganism (no animal products). Serum methylmalonic acid (MMA) elevated = B12 deficiency (not folate).
Applied Walkthrough
1
A vegan presents with megaloblastic anemia after 5 years on a plant-based diet — B12 deficiency (no animal products) after liver stores depleted.
2
Neurological exam reveals loss of vibration sense and proprioception (dorsal column damage) + spastic paraparesis (lateral column damage) — subacute combined degeneration.
3
Giving folate alone corrects the anemia but the neurological damage continues to worsen — dangerous. B12 injection is required.
4
A pregnant woman with low folate in the first trimester: neural tube fails to close properly → spina bifida. Folic acid supplementation BEFORE conception prevents 70% of NTDs — why flour is fortified.
Exam Application
Exams test the distinction between folate and B12 deficiency (both = megaloblastic anemia; only B12 = neurological damage), the folate trap mechanism, B12's two enzymatic roles (methionine synthase + methylmalonyl-CoA mutase), B12 absorption (IF required), and causes of each deficiency. The danger of treating B12 deficiency with folate is extremely high yield.
⚠ Common Trap
Students give folate to all megaloblastic anemia — this MASKS B12 deficiency while allowing neurological damage to progress. Always check B12 levels. Also: serum folate and B12 can be normal in tissue deficiency — methylmalonic acid (elevated in B12 deficiency) and homocysteine (elevated in both) are better functional markers.
✓ Quick Self-Check
1. What two conditions cause megaloblastic anemia?
Folate deficiency and vitamin B12 deficiency — both impair DNA synthesis in rapidly dividing cells.
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2. What is the key distinction between folate and B12 deficiency?
B12 deficiency causes neurological damage (subacute combined degeneration) in addition to megaloblastic anemia. Folate deficiency causes only anemia (no neurological effects).
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3. What is the methyl-folate trap?
In B12 deficiency, methyl-THF accumulates (trapped) because B12 is needed to remove the methyl group. Free THF is depleted → DNA synthesis fails → megaloblastic anemia even when dietary folate is adequate.
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4. Why is it dangerous to give folate to a B12-deficient patient?
Folate corrects the anemia (masking the diagnosis) but does nothing for the neurological damage — which continues to progress.
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5. What lab test specifically indicates B12 deficiency (not folate)?
Elevated serum methylmalonic acid (MMA) — B12 is required for methylmalonyl-CoA mutase. MMA rises in B12 deficiency but NOT in folate deficiency.
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