Folate Deficiency

• Folate deficiency is one of the most common vitamin deficiencies worldwide, especially in low-income settings without mandatory food fortification.
• In Hong Kong, the prevalence is lower than in Sub-Saharan Africa or South/Southeast Asia, but still encountered in:
  • Elderly patients with poor dietary intake or institutional living
  • Chronic alcoholics — a major at-risk group in HK
  • Pregnant women — folate requirements roughly double in pregnancy
  • Patients on haemodialysis — folate is dialysable and lost with each session
  • Patients with chronic haemolytic anaemia (e.g. thalassaemia intermedia, sickle cell disease) — high red cell turnover demands more folate for erythropoiesis
• Body store sufficient for 3 months of use ^[1] — this means folate deficiency develops relatively quickly (weeks to a few months) compared to B12 deficiency (years), which is clinically important.

High Yield — GC Lecture Point: Body store sufficient for 3 months of use — contrast this with B12 where liver store sufficient for 3 years of use + enterohepatic circulation → YEARS to manifest S/S ^[1][2]. This is a favourite exam comparison.

• Folate source: diet only (green vegetables, liver, nuts) ^[1]
• Unlike B12, there is no significant enterohepatic circulation for folate, so body stores deplete quickly.
• Destroyed by cooking ^[1] — folate is heat-labile; prolonged cooking (especially boiling) can destroy up to 90% of folate content. This is why overcooking vegetables is a classic risk factor.
• Dietary folate exists mainly as polyglutamate forms → must be hydrolysed to monoglutamate by folate conjugase (γ-glutamyl hydrolase) at the jejunal brush border before absorption.
• Converted to methyl-THF → absorbed in the upper small intestine ^[1] — specifically the proximal jejunum via the proton-coupled folate transporter (PCFT).
• Folate binds weakly to albumin in plasma ^[1].

Folate's biochemical role: DNA synthesis ^[1] — specifically:

1. Thymidylate synthesis (the rate-limiting step for why deficiency causes megaloblastic anaemia):

   • 5,10-methylene-THF donates a one-carbon unit to deoxyuridylate monophosphate (dUMP) → thymidylate (dTMP), catalysed by thymidylate synthase
   • Without sufficient folate → dUMP accumulates → gets misincorporated into DNA as uracil → DNA strand breaks → ineffective erythropoiesis and megaloblastic change

2. Purine synthesis:

   • 10-formyl-THF donates carbon units for the de novo synthesis of purine rings (positions C2 and C8)
   • Impaired purine synthesis further compounds the DNA replication defect

3. Homocysteine metabolism:

   • 5-methyl-THF donates its methyl group to homocysteine → methionine, catalysed by methionine synthase (which requires vitamin B12 as a cofactor)
   • This is the critical intersection of B12 and folate metabolism (the "methyl trap" hypothesis — see below)

This is conceptually important and frequently examined:

• B12 traps folate into cells ^[1] — here is how:
  • In B12 deficiency, methionine synthase cannot function → 5-methyl-THF cannot be demethylated back to THF
  • 5-methyl-THF accumulates ("trapped") → functional folate deficiency occurs even if serum folate is normal or high
  • This is why B12 deficiency can cause elevated serum folate (folate is trapped intracellularly in the methyl form) but reduced RBC folate (the functional intracellular pool is unavailable)
  • This is also why giving folate to a B12-deficient patient can partially correct the anaemia (by flooding the system with enough folate to bypass the trap) but will NOT correct the neurological damage — and may actually worsen it by diverting remaining B12 away from neuronal methylation reactions

The two key drug targets are shown in red: Methionine synthase (requires B12) and DHFR (inhibited by methotrexate and trimethoprim).

• Nearly impossible to develop folate deficiency if consuming normal diet while residing in country with routine folic acid fortification of food ^[5]
• Hong Kong does not have mandatory folic acid fortification of flour/rice (unlike the USA, Canada, Australia), making dietary deficiency more plausible here
• Only individuals who are unable to consume a nutrient-rich diet will develop folate deficiency including chronic alcoholism, anorexia nervosa and reduced oral intake in the setting of systemic illness ^[5]
• Elderly patients in residential care homes in HK — a commonly tested scenario
• Alcoholism impairs folate status through multiple mechanisms:
  • Poor dietary intake (calories from alcohol replace nutrient-rich food)
  • Direct toxic effect on intestinal absorption of folate
  • Impaired hepatic metabolism and storage of folate
  • Increased renal excretion of folate

• Surgery (gastric bypass) ^[5] — roux-en-Y gastric bypass diverts food past the duodenum and proximal jejunum
• Inflammatory bowel diseases (IBD) ^[5] — particularly Crohn's disease affecting the proximal small bowel; also sulfasalazine used in IBD inhibits folate absorption
• Coeliac disease — villous atrophy in the jejunum directly impairs folate absorption (rare in HK but important to know)
• Tropical sprue — more relevant in Southeast Asian travellers returning to HK

• Pregnancy ^[5] — folate requirements increase from ~400 μg/day to ~600 μg/day; the rapidly dividing fetal cells and expanding maternal blood volume drive this
• Chronic haemolytic anaemia ^[5] — compensatory erythroid hyperplasia in the marrow consumes more folate (e.g. thalassaemia intermedia, sickle cell disease, hereditary spherocytosis). In Hong Kong, thalassaemia is the most relevant chronic haemolytic condition.
• Exfoliative skin diseases ^[5] — severe psoriasis with rapid keratinocyte turnover
• Haemodialysis ^[5] — folate is a small water-soluble molecule that is dialysed out during each session

• Methotrexate (folate antagonist — inhibits dihydrofolate reductase (DHFR)) ^[5]

  • DHFR converts dihydrofolate (DHF) back to tetrahydrofolate (THF) — if blocked, THF cannot be regenerated, and the one-carbon cycle grinds to a halt
  • This is why folinic acid (leucovorin) rescue works — leucovorin is a reduced folate (5-formyl-THF) that bypasses the DHFR block
  • Note: folic acid supplementation alone does NOT work as rescue because folic acid still needs DHFR to be activated

• Trimethoprim (inhibits DHFR) ^[5] — same mechanism as methotrexate but trimethoprim has higher affinity for bacterial DHFR than human DHFR, so clinical folate deficiency is uncommon unless used long-term (e.g. prophylactic TMP-SMX in immunocompromised patients)

• Anticonvulsants (Valproate / Carbamazepine / Phenytoin) ^[5]

  • Phenytoin: multiple mechanisms — inhibits intestinal folate conjugase (impairs absorption), induces hepatic enzymes that increase folate catabolism, may directly interfere with folate metabolism
  • Valproate: mechanism less clear, possibly through interference with folate-dependent one-carbon metabolism; also the most important teratogen in this group (neural tube defects)
  • Carbamazepine: similar hepatic enzyme induction

• Other drugs worth knowing:

  • Sulfasalazine — inhibits PCFT (the jejunal folate transporter) and folate conjugase
  • Pyrimethamine — DHFR inhibitor used for toxoplasmosis/malaria
  • Oral contraceptive pills — older formulations; modern low-dose pills have minimal effect

The core pathophysiology is nuclear-cytoplasmic maturation asynchrony ^[4]:

1. Folate deficiency → insufficient 5,10-methylene-THF → impaired thymidylate synthesis → cannot make dTMP → cannot replicate DNA properly
2. Cells that are actively dividing are most affected:
   • Erythroid precursors in the bone marrow: DNA synthesis slows, but RNA and protein synthesis (including haemoglobin) continue normally → the cytoplasm matures but the nucleus remains immature and large → produces megaloblasts (large erythroblasts with open, immature chromatin)
   • Many megaloblasts die before leaving the marrow → ineffective erythropoiesis → intramedullary haemolysis → mild ↑unconjugated bilirubin, ↑LDH, ↓haptoglobin (features mimicking haemolysis)
   • Those that do survive are released as macro-ovalocytes (large, oval red cells, MCV often > 100 fL)
3. Granulocyte precursors are also affected → the nucleus has difficulty lobulating properly → hypersegmented neutrophils (≥5 lobes in ≥5% of neutrophils, or any neutrophil with ≥6 lobes) — this is an early and diagnostic finding ^[7]
4. All rapidly dividing cells are affected → GI epithelium (glossitis, mouth ulcers, diarrhoea), potentially other tissues

This is a critical distinction from B12 deficiency:

• B12 is required as a cofactor for methylmalonyl-CoA mutase (converts methylmalonyl-CoA → succinyl-CoA in the Krebs cycle) — this reaction is B12-dependent but folate-independent

  • In B12 deficiency, methylmalonyl-CoA and its precursor methylmalonic acid (MMA) accumulate → toxic to myelin → subacute combined degeneration of the spinal cord (SACD)
  • In folate deficiency, MMA levels are normal → no myelin toxicity → no SACD

• B12 is also required for proper methylation reactions in the nervous system (via methionine synthase → methionine → S-adenosylmethionine [SAM], the universal methyl donor) — SAM is needed for myelin maintenance

  • While folate deficiency also impairs this pathway (less 5-methyl-THF available), the effect is less severe because the primary bottleneck is B12

• Neuropsychiatric symptoms: present in both vitamin B12 and folate deficiency but most of the findings are commonly ascribed to vitamin B12 deficiency ^[5]

  • Folate deficiency can cause depression, irritability, and cognitive impairment through impaired SAM-dependent methylation in the brain
  • Subacute combined degeneration of cord: classical picture confined to B12 def → does NOT occur in folate def ^[8]

• Both B12 and folate deficiency → impaired conversion of homocysteine to methionine → hyperhomocysteinaemia
• Elevated homocysteine is a risk factor for:
  • Arterial thrombosis (stroke, MI)
  • Venous thromboembolism
  • Atherosclerosis
• In folate deficiency: homocysteine is elevated, but MMA is normal (because MMA metabolism is B12-dependent, not folate-dependent)
• In B12 deficiency: both homocysteine and MMA are elevated

This table is a very commonly examined comparison.

Folate deficiency presents as macrocytic anaemia (MCV > 100 fL). The key initial branch point is whether the macrocytosis is megaloblastic or non-megaloblastic — because this determines your entire investigative pathway.

High Yield GC Lecture Point: The peripheral blood smear (PBS) is king for this differentiation. Megaloblastic anaemia: oval macrocytes + hypersegmented neutrophils (> 5 lobes). Non-megaloblastic: round macrocytes + normal neutrophils. ^[1][4]

These conditions cause macrocytosis but do not produce hypersegmented neutrophils or oval macrocytes. The PBS shows round macrocytes with normal neutrophils ^[4]:

In severe folate deficiency, all three cell lines may be affected because DNA synthesis is impaired across all haematopoietic lineages → pancytopenia ^[11]. The differential for pancytopenia includes:

• Megaloblastic anaemia (B12 or folate deficiency) — the key diagnosis
• MDS — particularly important in elderly patients; dysplastic features on PBS and bone marrow
• Aplastic anaemia — hypocellular marrow; drug exposure history (chloramphenicol, carbamazepine, gold)
• Acute leukaemia — blasts on PBS
• Bone marrow infiltration — myelofibrosis, metastatic carcinoma; leukoerythroblastic picture on PBS
• Hypersplenism — splenomegaly with sequestration
• HIV — direct marrow suppression and/or opportunistic infections

Bone marrow examination: not routinely needed, except when laboratory findings incompatible with pernicious anaemia / folate deficiency → particularly when you are thinking of MDS (can also cause pancytopenia with macrocytosis), or when patient does not improve when you replace with B12/folate ^[11]

A logical approach:

1. Drug history first — the easiest and most reversible cause to identify
2. Alcohol and dietary history — ask specifically about daily intake, food variety, vegetable consumption; look for signs of malnutrition (muscle wasting, subcutaneous fat depletion, nail and mucosal changes) ^[16]
3. GI history — chronic diarrhoea, steatorrhoea, abdominal pain (thinking of coeliac, Crohn's, post-surgical); malabsorption presents as weight loss, osmotic diarrhoea and nutrient deficiency (B12, zinc, selenium etc.) ^[17]
4. Obstetric / gynaecological history — pregnancy, multiple pregnancies
5. Haematological history — known chronic haemolytic condition (thalassaemia, sickle cell, hereditary spherocytosis); look for pallor, jaundice, splenomegaly ^[18] and check reticulocyte count
6. Renal history — haemodialysis (folate is dialysable)

• CBC with differentials ^[5][21]:

  • Macrocytic anaemia with MCV > 100 fL
  • Low reticulocyte count — this is important because it tells you the marrow is NOT responding appropriately (i.e., the problem is impaired production, not destruction or loss)
  • Mild leukopenia or thrombocytopenia — all rapidly dividing cell lines are affected by impaired DNA synthesis → some degree of pancytopenia

• Peripheral blood smear ^[5][21]:

  • Macroovalocytes — large, oval-shaped red cells (not round — round macrocytes suggest non-megaloblastic causes)
  • Hypersegmented neutrophils — diagnostic, defined as > 5% with ≥ 5 lobes or ≥ 1% with ≥ 6 lobes ^[8]

High Yield GC Lecture Point: Hypersegmented neutrophil → diagnostic, more than 6 segments ^[7]. This is an early finding — it may appear before the MCV rises significantly. It reflects the same nuclear maturation failure in granulocyte precursors as in erythroid cells.

• Serum folate level ^[5][21]:

  • Low serum folate ( < 3–4 ng/mL or < 7 nmol/L) confirms deficiency
  • Normal level of serum folate ( > 4 ng/mL) suggests folate is not deficient unless individual has recently consumed a folate-containing meal or supplement ^[5][21]
  • Serum folate reflects recent intake (last few days) — it can fluctuate with a single meal. A patient who fasted overnight may have a transiently low serum folate without true deficiency, and a deficient patient who just ate a salad may have a transiently normal level.

• RBC folate level ^[5][21]:

  • Reflects longer-term folate status (over the lifespan of the red cell, ~120 days) — more reliable than serum folate
  • RBC folate < 150 ng/mL indicates prolonged deficiency ^[8]
  • RBC folate can be obtained in the above situation but is not routinely needed because it does not provide additional information and is more costly to obtain ^[5][21]
  • Key caveat: RBC folate is also low in B12 deficiency (due to the methyl trap) — so a low RBC folate does NOT distinguish between B12 and folate deficiency by itself

High Yield: Serum and red cell folate level are part of the standard megaloblastic workup ^[22]. However, in real life folate deficiency anaemia is rare in HK — folate deficiency occurs in alcoholic patient, rapid cell turnover in psoriasis — but it will not be so low to cause folate deficiency anaemia ^[11]. This is an important HK-specific clinical pearl from Professor Kwong's data.

You cannot diagnose folate deficiency without simultaneously excluding B12 deficiency. This is not optional — it is a patient safety issue.

• Serum vitamin B12 level ^[5][22]:

  • Normal > 300 pg/mL, borderline 200–300 pg/mL, deficient < 200 pg/mL ^[8]
  • Serum holotranscobalamin (active B12, i.e. B12 bound to transcobalamin II) is the better test ^[5][22] — it measures only the biologically active fraction (10–30% of total serum B12), avoiding the confounding effect of haptocorrin-bound inactive B12

• Methylmalonic acid (MMA) and homocysteine ^[5][21][1]:

  • NOT routinely needed but reserved for cases in which initial results for vitamin B12 and folate levels are borderline or inconclusive or if clinical findings are discordant ^[5][21]
  • The metabolite pattern is the definitive differentiator:

Further testing of metabolites if cannot tell B12 vs folate deficiency: MMA normal + homocysteine ↑ = folate deficiency; MMA ↑ + homocysteine ↑ = B12 deficiency ± concomitant folate deficiency; both normal = no B12/folate deficiency ^[1]

The PBS is the single most important initial investigation — it provides morphological evidence before biochemical results return.

Limitations:

• Fluctuates with dietary intake — a single measurement may be misleading
• Serum folate may be influenced by dietary intake, alcohol, AEDs, anorexia nervosa ^[8]
• In B12 deficiency, serum folate may be falsely elevated (methyl trap) ^[4]

RBC folate is not routinely needed because it does not provide additional information and is more costly to obtain ^[5][21]. In practice, it is most useful when serum folate is borderline or when you need to distinguish a transient dietary fluctuation from true deficiency.

This is mandatory — not to diagnose folate deficiency per se, but to exclude the more dangerous B12 deficiency.

High Yield GC Lecture Point: Serum holotranscobalamin — the TCII bound to Vit B12, active fraction, what we use nowadays ^[11][22]. This is the modern replacement for total serum B12, which was the "old days" method.

NOT routinely needed but reserved for cases in which initial results for vitamin B12 and folate levels are borderline or inconclusive or if clinical findings are discordant ^[5][21]

In megaloblastic anaemia, the marrow precursors die before leaving the marrow → this is essentially intramedullary red cell destruction. The laboratory footprint mimics haemolysis:

Clinical pearl: A very high LDH with only mild anaemia in a patient with macrocytosis is a strong clue to megaloblastic anaemia. The LDH can be so high that it mimics lymphoma or metastatic cancer — always think of megaloblastic anaemia in the differential of a markedly elevated LDH.

These are NOT used to diagnose folate deficiency but are essential when B12 deficiency is being investigated:

High Yield GC Lecture Point: Anti-parietal cell antibody – Sensitive but less specific, also detectable in small proportion of normal individuals. Anti-intrinsic factor antibody – Specific but less sensitive, some patients with PA may be negative ^[22]

Once folate deficiency is confirmed, you need to identify WHY:

Bone marrow examination — not routinely needed, except when laboratory findings incompatible with PA ^[22] / folate deficiency, particularly when you are thinking of MDS (can also cause pancytopenia with macrocytosis), or when patient does not improve when you replace with parenteral B12 / folate ^[11]

Indications for bone marrow biopsy:

1. Unexplained pancytopenia that does not respond to B12/folate replacement
2. Suspicion of MDS (e.g. elderly patient with refractory cytopenias and dysplastic PBS features)
3. Clinical and laboratory findings discordant with megaloblastic anaemia
4. Co-existing features suggesting marrow infiltration or aplasia

Bone marrow findings in megaloblastic anaemia (if performed):

• Hypercellular marrow — erythroid hyperplasia (the marrow is trying to compensate but failing)
• Megaloblastic erythroid precursors — large cells with open, immature ("lacy") chromatin despite mature cytoplasm
• Giant metamyelocytes and band forms — granulocyte precursors also affected
• Increased iron stores — iron is available but cannot be incorporated into haemoglobin efficiently (the problem is DNA synthesis, not iron supply)

This is the single most important principle and bears repeating:

Use of folate alone in presence of vitamin B12 deficiency may cause worsening of neurological deficit despite partially masking its haematological deficits (reason unclear) ^[8]

Why does this happen? The likely explanation:

• Folate supplementation floods the one-carbon cycle with substrate → partially restores thymidylate synthesis → haematological parameters improve (MCV normalises, Hb rises)
• However, the B12-dependent methionine synthase reaction remains blocked → S-adenosylmethionine (SAM) production stays low → myelin methylation remains impaired → neurological damage continues or worsens
• The improving blood counts may give false reassurance, delaying B12 diagnosis while SACD progresses irreversibly

• Route of administration: parenteral if urgent or impaired absorption, oral if dietary deficiency ^[8]
• For folate specifically, oral replacement is almost always sufficient because even in malabsorptive states, pharmacological doses (1–5 mg) far exceed the daily requirement (~0.2 mg), so enough gets absorbed even through a damaged gut

• Duration: lifelong if underlying condition irreversible, otherwise until deficiency corrected ^[8]
• Reversible causes: dietary deficiency (once diet improves), pregnancy (once delivered), drug-induced (once drug stopped or folinic acid rescue given)
• Potentially lifelong: chronic haemolytic anaemia (thalassaemia, sickle cell, hereditary spherocytosis), inflammatory bowel disease with ongoing malabsorption, post-surgical short bowel

Usually replace over weeks but urgent replacement indicated in ^[8]:

• Symptomatic: severe anaemia (consider transfusion) or neuropsychiatric symptoms due to risk of adverse events and irreversible neurological deficits
• Pregnancy and neonate/infant as development may be affected

• Indication: Severe symptomatic anaemia (e.g. Hb < 7 g/dL with cardiovascular compromise, angina, heart failure)
• Caution: Transfuse slowly and in small volumes in elderly or those with cardiac disease — rapid transfusion of a severely anaemic patient can precipitate pulmonary oedema (the heart is already in a high-output state to compensate for anaemia, and suddenly increasing preload can tip it over)
• Not a definitive treatment — transfusion buys time while folate replacement takes effect (days to weeks for reticulocyte response)

Important monitoring considerations:

• Associated with ↓serum potassium and rapid iron depletion ^[8] — as the marrow suddenly kicks into high gear with folate replacement, it consumes potassium (for new cell membrane Na+/K+-ATPase) and iron (for new haemoglobin). Monitor K+ and iron studies, and supplement if necessary
• May develop iron deficiency (as evidenced by dimorphic red cells on PBS) → require iron replacement if necessary ^[8]
• If there is no reticulocyte response by day 10, reconsider the diagnosis — is it really folate deficiency? Could it be MDS, B12 deficiency, or a drug effect?

Folate with B12 if B12 status unknown, e.g. urgent treatment needed while awaiting B12/folate result ^[8]

• Give both IM hydroxocobalamin (B12) 1000 μg AND oral folic acid 5 mg/day
• Do NOT wait for results if the patient is severely symptomatic
• Once results return, adjust treatment accordingly

• Folic acid supplementation: 1–2 mg/day for moderate/severe haemolysis, 5 mg/day if pregnant ^[24]
• Folic acid → as in any cause of haemolysis ^[25]
• Folate demands are chronically increased due to erythroid hyperplasia
• If folate stores become depleted on top of chronic haemolysis, the patient can develop a megaloblastic crisis — sudden worsening of anaemia when the marrow can no longer compensate

• Folic acid 5 mg/day (10× the standard dose) starting ≥3 months before conception
• Continue through first trimester (until neural tube closure is complete at ~28 days post-conception)
• This is one of the most impactful preventive interventions in medicine

Two distinct clinical situations:

1. Prophylaxis during chronic low-dose MTX (e.g. RA, psoriasis):

   • Folic acid 5 mg weekly, given 24–48 hours after the weekly MTX dose
   • Reduces mucositis, nausea, hepatotoxicity, and cytopenias by ~30–80%
   • Does NOT reduce MTX efficacy in RA

2. Leucovorin rescue after high-dose MTX (e.g. osteosarcoma, ALL, lymphoma):

   • Folinic acid (leucovorin) 15 mg Q6h × 10 doses, starting 24 hours after MTX
   • Dose may need to be increased based on serum MTX levels
   • Folic acid will NOT work in this scenario — it cannot bypass the DHFR block

• Mechanism: Progressive folate depletion → worsening ineffective erythropoiesis → steadily falling haemoglobin. Because body stores last only ~3 months ^[1], the decline can be relatively rapid compared to B12 deficiency.
• The anaemia is often insidious enough for cardiovascular compensation (↑heart rate, ↑stroke volume, ↑cardiac output) to maintain tissue oxygen delivery for a while.
• When compensation fails, the patient develops high-output cardiac failure:
  • The heart has been working harder with reduced blood viscosity and increased flow rates → eventually myocardial oxygen demand exceeds supply → ventricular dilatation and failure
  • Presents with dyspnoea at rest, peripheral oedema, raised JVP, pulmonary congestion
  • Particularly dangerous in elderly patients with pre-existing coronary artery disease — the anaemia can unmask or exacerbate ischaemic heart disease, triggering angina or acute coronary syndromes (demand ischaemia / type 2 MI)

Nuclear-cytoplasmic maturation asynchrony ^[4] affects all rapidly dividing haematopoietic lineages, not just the erythroid line:

• Leukopenia (especially neutropenia):

  • Impaired granulopoiesis → reduced neutrophil numbers → increased susceptibility to bacterial infections
  • Usually mild (WBC 2–4 × 10⁹/L), but can be clinically significant in patients with co-existing immunosuppression (e.g. alcoholics, HIV, post-chemotherapy)

• Thrombocytopenia:

  • Impaired thrombopoiesis → reduced platelet production
  • Usually mild and rarely causes clinically significant bleeding
  • However, in severe or prolonged folate deficiency, platelets can drop enough to produce mucocutaneous bleeding (petechiae, easy bruising, gum bleeding)

• Megaloblastic erythroid precursors in the marrow are fragile and many undergo apoptosis before reaching maturity → intramedullary haemolysis (destruction within the marrow, not in the peripheral circulation)
• This produces a biochemical picture that mimics haemolytic anaemia:
  • ↑ LDH (often markedly elevated, > 1000 U/L) ^[5]
  • ↑ Unconjugated (indirect) bilirubin → the classic lemon-yellow tint ^[8]
  • ↓ Haptoglobin ^[5]
• The clinical consequence: mild jaundice — combined with pallor, this gives the characteristic lemon-yellow complexion
• The LDH can be so high that it may trigger unnecessary investigation for lymphoma or metastatic malignancy if the clinician is unaware

• Patients with underlying chronic haemolytic anaemia (thalassaemia intermedia, hereditary spherocytosis, sickle cell disease, pyruvate kinase deficiency) already have increased folate demands due to compensatory erythroid hyperplasia
• If folate stores become depleted in these patients, the marrow suddenly cannot compensate → acute, dramatic worsening of anaemia (sometimes called a megaloblastic crisis or aplastic crisis secondary to folate depletion)
• This is distinct from a parvovirus B19-induced aplastic crisis but can have a similar clinical presentation
• This is why folic acid supplementation is generally recommended for any form of haemolysis ^[25] — it prevents this complication

• Neural tube closure occurs between days 21–28 of embryogenesis — often before the woman even knows she is pregnant
• Folate is required for the rapid cell division during neural tube closure → insufficient folate at this critical window → incomplete closure → neural tube defect
• The exact molecular mechanism involves impaired thymidylate synthesis → increased uracil misincorporation into DNA → DNA strand breaks in rapidly dividing neural crest cells → apoptosis and failure of neural fold fusion

• Maternal folate deficiency (dietary, drug-induced)
• Teratogens, e.g. sodium valproate ^[10] — valproate is both a folate antagonist and an independent teratogen
• Previous NTD-affected pregnancy
• Maternal diabetes, obesity
• Family history of NTDs

• ↓Incidence due to ↑antenatal screening and ↑folate supplement ^[10]
• Periconceptional folic acid supplementation (0.4–0.5 mg/day for all; 5 mg/day for high-risk) has reduced the incidence of NTDs by 50–70% worldwide
• Countries with mandatory folic acid food fortification have seen further reductions

• Folate deficiency → impaired conversion of homocysteine to methionine (via methionine synthase, which requires both 5-methyl-THF and B12) → elevated plasma homocysteine
• Hyperhomocysteinaemia is a recognised independent risk factor for:
  • Atherosclerosis (coronary artery disease, stroke, peripheral vascular disease) — homocysteine damages vascular endothelium → endothelial dysfunction → promotes plaque formation
  • Venous thromboembolism (deep vein thrombosis, pulmonary embolism) — homocysteine activates factor V, inhibits protein C activation, impairs endothelial thrombomodulin expression → prothrombotic state
  • Cerebrovascular disease — both ischaemic stroke and vascular dementia

• The association between homocysteine and cardiovascular disease is well-established epidemiologically
• However, randomised controlled trials of folate supplementation to reduce cardiovascular events have shown mixed results — lowering homocysteine does not consistently reduce cardiovascular events (the HOPE-2, VITATOPS, and SEARCH trials)
• Therefore, while hyperhomocysteinaemia is a complication of folate deficiency and a marker of cardiovascular risk, it is debated whether treating folate deficiency specifically to reduce cardiovascular risk is effective
• Still, correcting folate deficiency normalises homocysteine levels, and this is considered good practice even if the cardiovascular benefit is uncertain

• Cells in the GI tract divide rapidly and thus sensitive to lack of nucleic acid that occurs when vitamin B12 and folate are deficient ^[5]
• Mouth ulcers occurs in folate deficiency ^[5]
• Glossitis / angular cheilitis ^[5] — atrophic glossitis (smooth, painful tongue with loss of papillae), angular stomatitis (cracking at mouth corners)
• These are direct consequences of impaired epithelial cell turnover — the oral mucosa has one of the highest cell turnover rates in the body (~3–5 day lifespan for buccal epithelial cells)

• Folate deficiency can cause villous atrophy in the small intestine (similar to coeliac disease) → this creates a vicious cycle: folate deficiency impairs enterocyte renewal → mucosal atrophy → worsened malabsorption → further folate depletion
• Presents as diarrhoea, malabsorption, weight loss
• This malabsorptive picture can compound deficiencies of other nutrients (iron, calcium, fat-soluble vitamins)

• Mechanism: Folate is required for SAM (S-adenosylmethionine) synthesis via the methionine cycle → SAM is the universal methyl donor → needed for synthesis of monoamine neurotransmitters (serotonin, dopamine, noradrenaline) and for methylation reactions in the brain
• Folate deficiency → ↓SAM → ↓monoamine synthesis → depressive symptoms
• Epidemiological studies consistently show low serum folate associated with ↑risk of depression
• L-methylfolate (the bioactive form) is used as an adjunctive treatment for major depressive disorder in some guidelines

• Particularly in the elderly — folate deficiency is a reversible cause of cognitive decline
• This is why CBC, serum B12 and B9 (folate) are included as minimum investigations in the dementia workup ^[14][15] (NICE guidelines)
• The mechanism involves both impaired neurotransmitter synthesis and possibly accelerated brain atrophy through hyperhomocysteinaemia-mediated vascular damage

• Subacute combined degeneration of the spinal cord: classical picture confined to B12 deficiency → does NOT occur in folate deficiency ^[8]
• No peripheral neuropathy, no posterior column signs, no corticospinal tract signs from folate deficiency alone
• No optic atrophy

• Associated with ↓serum potassium and rapid iron depletion ^[8]
• Mechanism: Brisk reticulocyte response → rapidly proliferating erythroid precursors take up K⁺ for new cell membrane Na⁺/K⁺-ATPase → serum K⁺ drops
• Can be dangerous, especially in patients who are already K⁺-depleted (chronic alcoholics, malnourished patients, those on diuretics)
• Must monitor K⁺ in the first 1–2 weeks of replacement and supplement if necessary

• May develop iron deficiency (as evidenced by dimorphic red cells on PBS) → require iron replacement if necessary ^[8]
• Mechanism: The suddenly active marrow consumes iron at a high rate for new haemoglobin synthesis → iron stores become depleted → the patient transitions from macrocytic to microcytic anaemia (dimorphic picture on PBS)
• Monitor ferritin and transferrin saturation; provide iron supplementation if iron stores are low

• If folate is given without checking B12, the haematological parameters improve (MCV falls, Hb rises) → the clinician may think the problem is solved
• Meanwhile, B12 deficiency continues unchecked → neurological deficit worsens ^[8]
• This is the most dangerous iatrogenic complication and is entirely preventable by always checking B12 before or alongside folate replacement

• Previously reported association of high folic acid intake with ↑cancer risk ^[8]
• The concern (mostly from observational studies) is that high-dose, long-term folic acid supplementation may promote growth of pre-existing neoplastic cells by providing the nucleotide building blocks needed for tumour DNA replication
• Most robust data come from studies of colorectal adenoma recurrence — some showed increased recurrence with folic acid supplementation
• Current clinical guidance: Do not exceed recommended doses chronically without indication. Use folic acid at therapeutic doses (1–5 mg/day) for the duration needed, then reassess