Vitamin B12 and Folate: Megaloblastic Anaemia and the Trap of Treating Blindly
Two vitamins, one job: building the DNA that lets a cell divide. Starve a marrow of either and it produces huge, clumsy, immature red cells — megaloblastic anaemia. But the two are not interchangeable, and one deadly shortcut can hide behind the other. Give folate alone to a patient who is actually short of B12 and you will polish the blood count while the spinal cord quietly rots. This is the story of how to tell them apart — and why you never treat blind.
A 68-year-old woman, a lifelong vegetarian, shuffles into the clinic complaining that her feet feel like they are wrapped in cotton wool. She has been dropping things; on the stairs she must look at her feet or she loses her balance in the dark. Her family thinks she is just getting old and forgetful. Her blood count is only mildly abnormal — the haemoglobin barely low, a few large cells. A junior doctor, seeing "anaemia", reaches for folic acid. Stop his hand. The emergency in this room is not the anaemia. It is the numb, unsteady legs — a cord that is degenerating — and the wrong prescription could make it permanent.
One shared job: building DNA
B12 and folate are partners in a single chemical assembly line. Both vitamins feed the synthesis of DNA — the blueprint a cell must copy before it can divide. When either runs short, the cell can still make cytoplasm, haemoglobin and RNA, but it cannot finish maturing its nucleus in time. The result is a cell that grows large while its nucleus lags behind: a megaloblast. In the blood this shows up as macrocytosis (large red cells, a high MCV) and, the classic fingerprint, hypersegmented neutrophils — white cells whose nucleus has six or more lobes instead of the usual three to five.
Because the block is in nuclear maturation, megaloblastic change is not confined to red cells. The whole marrow feels it: white cells and platelets can fall too, so a severe deficiency may look like a pancytopenia. The single most useful early clue on a blood film is not the size of the red cells but those hypersegmented neutrophils — they appear before the anaemia is obvious.
- Both B12 and folate are required for DNA synthesis, so deficiency impairs nuclear maturation.
- The cytoplasm keeps growing while the nucleus lags → large, immature megaloblasts.
- Blood picture: macrocytosis (high MCV) and hypersegmented neutrophils.
- Severe cases hit all three lines — a megaloblastic pancytopenia.
Vitamin B12: the vitamin that needs a passport
B12 (cobalamin) comes almost entirely from animal foods. Meat, fish, eggs and dairy carry it; plants essentially do not. But diet is only half the problem — absorbing B12 is an unusually fussy, multi-step journey. The vitamin must first bind a protein called intrinsic factor (Intrinsic factor), secreted by the parietal cells of the stomach. Only the B12–intrinsic-factor complex is recognised and absorbed, and only at one specific place: the terminal ileum, the very last stretch of the small intestine. Break any link in that chain — the food, the stomach, or the ileum — and B12 fails to get in.
That fragile pathway explains every classic cause. Pernicious anaemia (Pernicious anaemia) is the most famous: an autoimmune attack in which antibodies target either intrinsic factor itself or the parietal cells that make it, so the passport is never issued. Gastrectomy or atrophic gastritis removes the parietal cells; disease or surgical resection of the ileum (Crohn's disease, for example) removes the arrival gate. Strict vegans lack the dietary source. Body stores are large — several years' worth in the liver — so dietary B12 deficiency builds slowly, but once absorption fails, the clock is running.
Two extremely common prescriptions quietly lower B12. Metformin, first-line in type 2 diabetes, interferes with the calcium-dependent uptake of the B12–intrinsic-factor complex in the ileum — check B12 in any long-term metformin user with neuropathy or a rising MCV. Long-term proton pump inhibitors (PPIs) suppress the gastric acid needed to release B12 from food, reducing absorption over years. Neither is exotic; both sit in millions of medicine cabinets.
The feature that sets B12 apart from folate — and the reason this whole article exists — is neurology. B12 deficiency can cause subacute combined degeneration of the cord: demyelination of the dorsal columns and corticospinal tracts, giving symmetrical paraesthesiae, loss of vibration and proprioception, an unsteady (ataxic) gait, and even dementia. Crucially, these signs can appear WITHOUT anaemia. A normal haemoglobin does not exclude dangerous B12 deficiency — the nerves may fall before the blood does.
- B12 sources are animal foods; absorption needs intrinsic factor and the terminal ileum.
- Causes: pernicious anaemia (autoimmune), gastrectomy, ileal disease/resection, veganism.
- Drug causes: metformin and long-term PPIs both lower B12.
- Neurological signs (subacute combined degeneration) can occur before or without anaemia.
- Liver stores last years, so absorptive failure — not diet — dominates the clinic.
Folate: fast in, fast out
Folate is the mirror image of B12 in almost every way. Its main sources are green leafy vegetables (the name comes from foliage), and it is absorbed high up, in the jejunum, needing no intrinsic factor. The critical difference is storage: the body holds only a few months' — really only weeks' — worth of folate. So while B12 deficiency takes years to appear, folate deficiency can develop within weeks of poor intake or heavy demand. That short fuse makes folate the classic deficiency of the malnourished, the pregnant and the acutely stressed marrow.
The causes follow from that. Poor diet and alcoholism (alcohol both worsens intake and impairs folate metabolism) are leading culprits. Increased demand strips stores fast: pregnancy and chronic haemolysis, where the marrow works overtime, both raise the folate requirement. Malabsorption — coeliac disease damaging the jejunum — cuts supply. And a distinct pharmacological group, the folate antagonists, blocks folate at the enzyme level: methotrexate, trimethoprim and phenytoin are the names to know.
These drugs reappear across the whole curriculum. Methotrexate inhibits dihydrofolate reductase and is a workhorse in the Inflammation section (rheumatoid arthritis, psoriasis) as well as oncology — patients often take folic acid alongside it to blunt the toxicity. Trimethoprim, met in the Antimicrobials section, inhibits the bacterial version of the same enzyme, but can nudge human folate too. Phenytoin, an antiepileptic, lowers folate over long-term use. Recognising a drug as a folate antagonist tells you both its side-effect and, sometimes, its mechanism.
- Folate comes from green vegetables and is absorbed in the jejunum — no intrinsic factor needed.
- Body stores are tiny, so deficiency appears in weeks, not years.
- Causes: poor diet, alcoholism, pregnancy/haemolysis (demand), coeliac (malabsorption).
- Folate antagonists: methotrexate, trimethoprim, phenytoin.
- Folate deficiency causes megaloblastic anaemia but NO neurological cord damage.
Treating them — and the pregnancy angle
Replacement is simple once you know which vitamin is missing. For B12, the classic route in true deficiency is intramuscular hydroxocobalamin — a series of loading injections followed by maintenance doses every few months, which reliably bypasses any absorption defect. High-dose oral B12 can also work, even in some pernicious anaemia, because a small fraction is absorbed by passive diffusion independent of intrinsic factor. For folate, the treatment is oral folic acid. Both correct the anaemia within weeks; watch for a brisk reticulocyte response as the marrow springs back to life.
Folate has one more starring role: pregnancy. Adequate folate around conception and in early pregnancy sharply reduces the risk of neural tube defects (Neural tube defects) such as spina bifida in the developing fetus. This is why folic acid is recommended before conception and through the first trimester — the neural tube closes so early that waiting for a positive test is already too late. It is one of the clearest wins in preventive pharmacology.
The one pearl to carry out of this entire article: NEVER give folate alone when B12 deficiency is possible. Folic acid will correct the megaloblastic anaemia and normalise the blood count — but it does nothing for the nerves, and by removing the anaemia it removes your warning flag while the subacute combined degeneration of the cord marches on, often to irreversible damage. Always check and replace B12 first, or give the two together. Treat the blood only after you have protected the spinal cord.
- B12: intramuscular hydroxocobalamin (loading then maintenance), or high-dose oral.
- Folate: oral folic acid.
- A brisk reticulocyte rise confirms the marrow is responding.
- Pre-conception folic acid prevents neural tube defects — start before pregnancy.
- Golden rule: exclude/replace B12 before or with folate — never folate alone.
- Giving folate alone in unrecognised B12 deficiency: it masks the anaemia while the neuropathy worsens toward irreversible cord damage.
- Forgetting drug causes — metformin and long-term PPIs for B12, methotrexate/trimethoprim/phenytoin for folate.
- Assuming a normal haemoglobin rules out B12 deficiency; the neurological signs can come first.
- Treating an isolated high MCV as harmless without checking B12/folate, thyroid and alcohol.
A vegetarian patient has numb, tingling feet and an unsteady gait, with only mild macrocytic anaemia. What is the single most dangerous next step?
- B12 and folate build DNA; deficiency of either → megaloblastic anaemia (macrocytosis, hypersegmented neutrophils).
- B12: animal foods, needs intrinsic factor + terminal ileum; causes include pernicious anaemia, ileal disease, veganism, metformin and PPIs.
- Only B12 deficiency causes neurological cord damage — and it can precede or occur without anaemia.
- Folate: green vegetables, jejunum, tiny stores; causes include alcohol, pregnancy, coeliac and antagonists (methotrexate, trimethoprim, phenytoin).
- Never give folate alone in possible B12 deficiency — replace B12 first; folic acid pre-conception prevents neural tube defects.
- Hoffbrand AV, Moss PAH. Hoffbrand's Essential Haematology — Megaloblastic anaemias: vitamin B12 and folate.
- Katzung BG. Basic & Clinical Pharmacology — Agents used in anemias: vitamin B12, folic acid & haematopoietic growth factors.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Haematopoietic agents: cobalamin & folate.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Haemopoietic system & vitamin deficiencies.
- Provan D, et al. Oxford Handbook of Clinical Haematology — Investigation and management of megaloblastic anaemia.
