Medicine chapters
Vitamin B12 and folate deficiency
How vitamin B12 and folate deficiency cause megaloblastic anaemia, their causes and risk factors, the blood film and confirmatory tests, treatment, and the role of folic acid in pregnancy.
- B12 and folate are both needed for DNA synthesis; lack of either gives megaloblastic anaemia with oval macrocytes and hypersegmented neutrophils.
- Pernicious anaemia (autoimmune loss of intrinsic factor) is the commonest cause of B12 deficiency; only B12 lack damages the nervous system (subacute combined degeneration) and raises methylmalonic acid.
- Never give folic acid alone to a patient who may be B12 deficient; give 400 μg folic acid before conception to prevent neural tube defects.
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Overview
Macrocytic anaemia means anaemia with a raised MCV (normal 80–96 fL). It is split into:
- Megaloblastic: due to faulty DNA synthesis, most often vitamin B12 or folate deficiency.
- Non-megaloblastic: alcohol, liver disease, hypothyroidism, reticulocytosis, myelodysplasia, drugs and pregnancy.
B12 and folate deficiency are common in Pakistan because of vegetarian diets, poor nutrition, repeated pregnancies and gut disease. The exam tests the blood film, the causes, the difference between B12 and folate deficiency, and folic acid in pregnancy.
Why the cells become large: role of B12 and folate
- Folate (as methylene tetrahydrofolate) donates a carbon unit to make thymidine for DNA.
- B12 is needed to recycle folate: it removes the methyl group from methyl-tetrahydrofolate while converting homocysteine to methionine.
- Without B12, folate is "trapped" as methyl-THF and DNA synthesis stalls (methyl-folate trap).
- RNA and cytoplasm keep growing while the nucleus lags behind (nuclear–cytoplasmic asynchrony), so cells are large with immature nuclei: megaloblasts.
- B12 also converts methylmalonyl CoA to succinyl CoA. When B12 is lacking, methylmalonic acid (MMA) builds up and myelin is damaged.
Homocysteine is raised in both B12 and folate deficiency. Methylmalonic acid is raised only in B12 deficiency.
Microscopic findings (megaloblastic anaemia)
Peripheral blood:
- Oval macrocytes (MCV usually above 100 fL; Kumar and Clark use above 96 fL).
- Hypersegmented neutrophils (more than five lobes): often the earliest sign.
- Anisocytosis and poikilocytosis; in severe cases pancytopenia.
- Reticulocytes low for the degree of anaemia.
Bone marrow (not always needed):
- Hypercellular with megaloblasts: large erythroblasts with open, finely stippled ("sieve-like") chromatin.
- Giant metamyelocytes.
Biochemistry: raised LDH (often very high) and mildly raised unconjugated bilirubin, because many megaloblasts die in the marrow (ineffective erythropoiesis).
Pancytopenia with a very high LDH and mild jaundice can mimic haemolysis or leukaemia. Always check B12 and folate in pancytopenia.
Vitamin B12 (cobalamin)
Normal handling
- Found only in animal foods (meat, fish, eggs, milk); not in plants.
- Freed from food protein in the stomach, carried by R binder (haptocorrin), released by pancreatic enzymes, then bound to intrinsic factor (from gastric parietal cells).
- The B12–intrinsic factor complex is absorbed in the terminal ileum (cubilin receptor).
- In plasma, transcobalamin II delivers B12 to cells ("active B12", holotranscobalamin).
- Body stores (mainly liver) last 2 years or more, so deficiency develops slowly.
Aetiology and risk factors
| Site | Causes |
|---|---|
| Diet | Strict vegans and lacto-vegetarians with poor intake |
| Stomach | Pernicious anaemia (commonest); total or partial gastrectomy; congenital intrinsic factor deficiency |
| Small bowel | Terminal ileal resection or Crohn's disease; bacterial overgrowth (blind loop); tropical sprue; fish tapeworm (Diphyllobothrium latum) |
| Pancreas | Pancreatic insufficiency (minor effect) |
| Drugs | Metformin, proton pump inhibitors (long-term), nitrous oxide (inactivates B12) |
| Transport | Congenital transcobalamin II deficiency |
Pernicious anaemia
- Autoimmune atrophic gastritis of the gastric body and fundus with loss of parietal cells → no intrinsic factor and no acid (achlorhydria).
- Commoner in the elderly, in women, and with other autoimmune diseases: thyroid disease, vitiligo, Addison's disease, type 1 diabetes.
- Parietal cell antibodies: present in about 90% but also in some healthy people (sensitive, not specific).
- Intrinsic factor antibodies: present in only about 50%, but specific for pernicious anaemia.
- Raised risk of gastric carcinoma (and gastric carcinoid).
Intrinsic factor antibody is the most specific test for pernicious anaemia; parietal cell antibody is more sensitive but less specific.
Clinical features
- Anaemia developing slowly; often well tolerated.
- Lemon-yellow tint (pallor plus mild jaundice).
- Glossitis (red, smooth, sore tongue) and angular stomatitis.
- Neurological features (can occur without anaemia):
- Symmetrical paraesthesiae in the feet and hands (peripheral neuropathy).
- Subacute combined degeneration of the cord: dorsal columns (early loss of vibration and joint position sense, sensory ataxia, positive Romberg's) plus lateral corticospinal tracts (weakness, spasticity, extensor plantars). Ankle jerks may be absent because of the neuropathy: the classic "absent ankle jerks with extensor plantars".
- Dementia, depression, psychosis ("megaloblastic madness"), optic atrophy.
Neurological damage is a feature of B12 deficiency only; folate deficiency does not cause subacute combined degeneration.
Lab investigations and confirmatory tests
- FBC and film: macrocytosis, hypersegmented neutrophils.
- Serum B12 low (usually well under 160 ng/L). Pregnancy and the oral contraceptive can lower B12 without true deficiency.
- Holotranscobalamin ("active B12"): may be a better marker.
- Methylmalonic acid and homocysteine raised: useful when B12 is borderline and suspicion is high.
- Confirming the cause: intrinsic factor antibodies (and parietal cell antibodies).
- Old Schilling test (radioactive B12 absorption with and without intrinsic factor) is no longer done, but may still appear in MCQs as the classic test that located the cause.
- Endoscopy if gastric symptoms (gastric cancer risk).
Treatment
- Hydroxocobalamin 1000 μg intramuscularly, several doses over the first 2 weeks (a total of 5–6 mg) to refill stores, then 1000 μg every 3 months for life in pernicious anaemia.
- Give more frequent injections while neurological signs are improving.
- Response: patient feels better within 48 hours; reticulocytes rise by day 2–3 and peak at 5–7 days; Hb normalises over about 2 months. Neuropathy improves over 6–12 months, but established cord damage may be permanent.
- Watch for: hypokalaemia (potassium moves into new cells), iron deficiency as production surges (add iron), raised urate.
- Oral B12 can be used for dietary deficiency and in adherent patients.
- Prophylactic B12 after total gastrectomy or ileal resection.
- Avoid transfusion in chronic anaemia unless essential; it can cause heart failure in the elderly.
Never treat megaloblastic anaemia with folic acid alone unless B12 is known to be normal. Folic acid can correct the blood count but lets the neuropathy progress. If the patient is very ill, give both after taking blood for levels.
Folate deficiency
Role of folic acid
- Needed for DNA synthesis (thymidine and purines) and amino acid metabolism.
- Found in green leafy vegetables and liver; cooking destroys much of it.
- Absorbed in the jejunum (upper small bowel).
- Body stores are small (about 10 mg) and run out in about 4 months on a poor diet; faster when needs are high.
Aetiology and risk factors
| Mechanism | Examples |
|---|---|
| Poor intake (main cause) | Elderly, poverty, alcohol excess, anorexia |
| Increased needs | Pregnancy and lactation, prematurity, chronic haemolysis (sickle cell, thalassaemia, spherocytosis), malignancy, exfoliative skin disease, haemodialysis |
| Malabsorption | Coeliac disease, tropical sprue, Crohn's disease |
| Drugs (antifolates) | Phenytoin, primidone, methotrexate, trimethoprim, pyrimethamine |
Features and tests
- Anaemia and glossitis; no neuropathy.
- Serum folate low (reflects recent intake). Red cell folate reflects tissue stores over the red cell lifespan and is a better guide to chronic deficiency.
- Homocysteine raised; MMA normal.
- Look for a cause: coeliac serology, diet and alcohol history, drugs.
Management
- Folic acid 5 mg daily by mouth for about 4 months, and longer if the cause persists.
- Check B12 first (see above).
- Prophylactic folic acid in chronic haemolytic anaemias and dialysis patients.
- Folinic acid (not folic acid) is used to rescue cells after high-dose methotrexate.
Folic acid in pregnancy
- Needs rise sharply in pregnancy; deficiency causes megaloblastic anaemia of pregnancy.
- Folic acid before conception and for the first 12 weeks lowers the risk of neural tube defects (spina bifida, anencephaly).
- Low-risk women: 400 μg daily.
- High-risk women: 5 mg daily: a previous baby with a neural tube defect, women on antiepileptic drugs, diabetes, obesity, sickle cell disease or thalassaemia.
- Some countries fortify flour; check current national guidance for antenatal supplementation in Pakistan.
"Four hundred for all, five for the few." 400 μg for every woman planning pregnancy; 5 mg for the high-risk few.
B12 versus folate deficiency
| Feature | Vitamin B12 deficiency | Folate deficiency |
|---|---|---|
| Stores last | 2 years or more | About 4 months |
| Main cause | Pernicious anaemia | Poor diet, pregnancy, alcohol |
| Absorbed in | Terminal ileum (needs intrinsic factor) | Jejunum |
| Neurological signs | Yes (SACD, neuropathy, dementia) | No |
| Homocysteine | Raised | Raised |
| Methylmalonic acid | Raised | Normal |
| Treatment | IM hydroxocobalamin | Oral folic acid 5 mg |
Non-megaloblastic macrocytosis
- Alcohol (commonest cause of a raised MCV without anaemia), liver disease, hypothyroidism.
- Reticulocytosis (haemolysis, bleeding).
- Myelodysplasia, aplastic anaemia, myeloma.
- Drugs: hydroxycarbamide, azathioprine, zidovudine.
- Pregnancy and the newborn (physiological).
- Cold agglutinins (falsely high MCV; normalises when the sample is warmed).
Expect: hypersegmented neutrophils and oval macrocytes; the commonest cause of B12 deficiency; intrinsic factor versus parietal cell antibodies; fish tapeworm, ileal resection, metformin and vegans as causes; subacute combined degeneration with absent ankle jerks and extensor plantars; MMA raised only in B12 deficiency; the danger of giving folic acid alone; hypokalaemia and reticulocyte peak after B12; drugs causing folate deficiency (phenytoin, methotrexate, trimethoprim); and 400 μg versus 5 mg folic acid in pregnancy.
Quick revision
- Megaloblastic: oval macrocytes, hypersegmented neutrophils, high LDH, megaloblasts in marrow.
- B12 needs intrinsic factor; absorbed in terminal ileum; stores last years.
- Pernicious anaemia: IF antibody specific, parietal cell antibody sensitive; gastric cancer risk.
- Only B12 deficiency causes neurological damage and a raised MMA.
- Treat B12 with IM hydroxocobalamin, then 3-monthly for life; watch potassium.
- Folate: 5 mg daily for 4 months; check B12 first.
- Pregnancy: 400 μg (all) or 5 mg (high risk, e.g. previous NTD, antiepileptics).
- Alcohol is a common cause of macrocytosis without megaloblasts.
Written by The House Job from standard references (Kumar and Clark's Clinical Medicine; Pathoma: Fundamentals of Pathology).
FAQ
What is the earliest blood film sign of megaloblastic anaemia?
Hypersegmented neutrophils, with more than five nuclear lobes, often appear before the anaemia is marked.
How do you distinguish B12 from folate deficiency biochemically?
Both raise homocysteine, but only B12 deficiency raises methylmalonic acid.
Why does B12 deficiency take years to appear after gastrectomy?
The liver holds enough B12 for two years or more, and daily losses are small.
Why can potassium fall after starting B12?
The burst of new red cell production pulls potassium into the cells.
Who should take 5 mg rather than 400 μg of folic acid before pregnancy?
Women at high risk, such as those with a previous neural tube defect pregnancy or those taking antiepileptic drugs.