Describe the absorption, functions and deficiency of vitamin B12 and folic acid. Explain the methyl trap hypothesis and differentiate the two deficiencies.

Vitamin B₁₂ — Chemistry and Sources

• Sources — animal foods only: liver, meat, fish, egg, milk. Absent from plants
• Requirement 1–2 µg/day; body stores 2–5 mg, mostly in liver
• Stores last 3–5 years — deficiency takes years to appear
• Active coenzymes — methylcobalamin and deoxyadenosylcobalamin

Absorption

Dietary B12 bound to protein → Released by gastric acid and pepsin → Binds haptocorrin (R-binder) from saliva → Pancreatic proteases release it in the duodenum → Binds intrinsic factor (from gastric parietal cells) → IF–B12 complex absorbed in the terminal ileum (cubilin receptor) → Transported in blood on transcobalamin II

• A defect at any of these steps causes deficiency — which is why the causes are so varied
• There is an enterohepatic circulation of B12, which is why malabsorption causes deficiency faster than dietary lack

The Two B₁₂-dependent Reactions

Folic Acid

• Structure — pteridine + PABA + glutamate
• Active form — tetrahydrofolate (THF), formed by dihydrofolate reductase
• Function — one-carbon transfer at various oxidation levels
• Sources — green leafy vegetables (Latin folium, leaf), liver, pulses, citrus. Destroyed by prolonged cooking
• Requirement 200 µg/day; 400–500 µg in pregnancy
• Stores last only 3–4 months

One-carbon reactions requiring folate

• Thymidylate synthesis (dUMP → dTMP) — the rate-limiting step of DNA synthesis
• Purine synthesis — carbons 2 and 8
• Methionine regeneration from homocysteine
• Glycine ↔ serine interconversion; histidine catabolism

Megaloblastic Anaemia — WHY It Occurs

↓ B12 or folate → ↓ Thymidylate synthesis → DNA synthesis impaired, but RNA and protein synthesis continue → Nuclear–cytoplasmic asynchrony — the nucleus lags behind → Large cells with immature nuclei → Ineffective erythropoiesis → intramedullary haemolysis

Distinguishing B₁₂ from Folate Deficiency

• Serum methylmalonic acid is the discriminating test — it is raised only in B12 deficiency

Applied Aspects

• Pernicious anaemia — autoimmune destruction of parietal cells; anti-IF and anti-parietal cell antibodies; needs parenteral B12 for life, since oral is useless without intrinsic factor
• Subacute combined degeneration — posterior column and corticospinal tract damage → loss of vibration and joint sense with brisk reflexes and extensor plantars; may be irreversible if treatment is delayed
• Never give folate alone in an unexplained megaloblastic anaemia — always exclude B12 first
• Periconceptional folic acid prevents neural tube defects — 400 µg daily started before conception, since the neural tube closes by day 28, often before pregnancy is recognised
• Strict vegans and their breast-fed infants need B12 supplementation — an important issue in India
• Metformin, proton pump inhibitors and nitrous oxide all impair B12 status
• Folic acid fortification of flour has reduced neural tube defects substantially where introduced, though it may mask B12 deficiency
• Tropical sprue and coeliac disease cause deficiency of both, since folate is absorbed in the jejunum and B12 in the ileum
• Fish tapeworm (Diphyllobothrium latum) competes for B12 and is a classical cause of deficiency
• The Schilling test is now largely obsolete, replaced by antibody testing and serum methylmalonic acid