Overview & Pathophysiology

Heat-labile, water-soluble vitamin.
Supplied by green vegetables, fruits, animal organs; easily destroyed by boiling or heating.
Absorbed primarily in proximal small intestine via carrier-mediated system; undergoes active enterohepatic circulation.
Essential coenzyme for DNA synthesis, single-carbon transfer reactions, and cellular proliferation.
Body stores extremely limited; severe megaloblastic anemia develops rapidly (2-3 months) on folate-free diet.
Pathophysiology characterized by defective DNA replication, delayed nuclear maturation, and resultant ineffective erythropoiesis (apoptosis of megaloblastic cells during maturation).

Etiologic Classification

Category	Specific Causes
Inadequate Intake	Unfortified goat milk feeding, severe malnutrition (marasmus, kwashiorkor), delayed weaning, prolonged heating/boiling of food.
Increased Demand	Rapid growth (prematurity, infancy, pregnancy), chronic hemolytic anemia (sickle cell, thalassemia, pyruvate kinase deficiency), hypermetabolic states (infections, hyperthyroidism), extensive skin disease (psoriasis).
Defective Absorption	Celiac disease, tropical sprue, inflammatory bowel disease, chronic diarrheal states, jejunal resection, enteroenteric fistulas.
Drug-Induced	Folate antagonists (methotrexate, pyrimethamine, trimethoprim, pentamidine), anticonvulsants (phenytoin, phenobarbital), oral contraceptives, sulfasalazine.
Genetic/Metabolic	Hereditary folate malabsorption (mutations in proton-coupled folate transporter SLC46A1), methylenetetrahydrofolate reductase (MTHFR) deficiency, glutamate formiminotransferase deficiency.

Insidious onset.
Pallor, lethargy, weakness, easy fatigability.
Irritability, anorexia, poor weight gain, failure to thrive.
Gastrointestinal: Chronic diarrhea, sore red tongue, glossitis, angular stomatitis.
Hemorrhagic manifestations (petechiae) secondary to severe thrombocytopenia in advanced cases.
Maternal deficiency associated with fetal neural tube defects, prematurity, and low birth weight.

Anemia with inappropriately low absolute reticulocyte count.
Macrocytosis: Elevated Mean Corpuscular Volume (MCV >100 fL).
Elevated Red Cell Distribution Width (RDW).
Advanced stages: Leukopenia, neutropenia, moderate thrombocytopenia (pancytopenia).

Erythrocytes: Macro-ovalocytes, marked anisocytosis, poikilocytosis, teardrop cells.
Erythrocyte inclusions: Howell-Jolly bodies, Cabot rings, punctate basophilia.
Leukocytes: Pathognomonic hypersegmented neutrophils (>5 lobed nuclei).

Hypercellular marrow with profound erythroid hyperplasia (reversed myeloid:erythroid ratio).
Megaloblastic changes: Giant pronormoblasts, giant metamyelocytes, cytoplasmic vacuolation.
Nuclear-cytoplasmic asynchrony (immature, lacy, open chromatin pattern alongside mature cytoplasm).

Folate Levels: Serum folate <3 ng/mL; RBC folate <160 ng/mL (superior indicator of chronic tissue deficiency).
Metabolites: Elevated serum homocysteine; strictly normal methylmalonic acid (differentiates from Vitamin B12 deficiency).
Hemolysis Markers: Elevated lactate dehydrogenase (LDH), elevated unconjugated bilirubin, decreased haptoglobin (reflecting intramedullary destruction/ineffective erythropoiesis).

Mandatory exclusion of Vitamin B12 deficiency prior to folate administration.
Rationale: Folic acid therapy effectively corrects hematologic parameters but fails to halt, and may inadvertently precipitate or accelerate, irreversible neurological degeneration characteristic of B12 deficiency.

Standard Therapy: Oral folic acid 1-5 mg/day (or 0.5-1.0 mg/day).
Duration: 3-4 weeks minimum, continuing until complete hematologic recovery and formation of new erythrocyte population.
Chronic Conditions: Lifelong supplementation required for chronic hemolytic anemias or permanent malabsorption states.
Toxicity/Antagonist Rescue: Folinic acid (5-formyltetrahydrofolate / leucovorin) specifically indicated for treating toxic effects of dihydrofolate reductase inhibitors (e.g., methotrexate).