Folic Acid - Sources, Forms, and Deficiency Symptoms

What is Folic Acid?

• Yamada's Textbook of Gastroenterology, 7th ed.
• Lippincott Illustrated Reviews: Pharmacology

Forms of Folate

Dietary Sources

• Green leafy vegetables (spinach, kale, broccoli)
• Citrus fruits (oranges, lemons)
• Legumes (beans, lentils, chickpeas)
• Fortified grain products (bread, cereals, pasta - mandatory in the US since 1998: 1.4 mg folic acid per kg of grain)
• Liver and organ meats

Important: Folate is heat-sensitive - 10-15 minutes of cooking can destroy a significant amount of it. - Robbins & Kumar Basic Pathology

Causes of Folic Acid Deficiency

1. Inadequate dietary intake - poor diet, food insecurity, elderly
2. Increased demand - pregnancy, lactation, chronic hemolytic anemias (e.g. sickle cell disease)
3. Malabsorption - celiac disease, inflammatory bowel disease, environmental enteropathy (folate is absorbed in the upper third of the small intestine)
4. Alcoholism - decreases both intake and intestinal absorption
5. Drugs:
   • Dihydrofolate reductase inhibitors: methotrexate, trimethoprim
   • Direct DNA synthesis inhibitors: azathioprine, zidovudine
   • Folate absorption reducers: phenytoin, phenobarbital
6. Acidic foods and substances in legumes - inhibit conversion of polyglutamates to absorbable monoglutamates
7. Rare genetic cause: hereditary folate malabsorption (mutations in the folate transporter gene)

• Lippincott Illustrated Reviews: Pharmacology; Robbins & Kumar Basic Pathology

Symptoms of Folic Acid Deficiency

Hematologic (most prominent)

• Megaloblastic anemia - the hallmark finding; caused by impaired DNA synthesis (reduced dTMP production), leading to large, abnormal RBCs with elevated MCV
• Weakness and easy fatigability - onset is insidious
• Pallor, fatigue, palpitations, shortness of breath, dizziness
• Hypersegmented neutrophils on blood smear (classic lab finding)

Gastrointestinal

• Sore tongue (glossitis)
• GI symptoms are common because the cells lining the gut also turn over rapidly and require folate for DNA synthesis

Neurologic - KEY DISTINCTION

• Folate deficiency does NOT cause neurologic symptoms - this is a critical difference from vitamin B12 deficiency.
• In contrast, B12 deficiency causes subacute combined degeneration of the spinal cord (tingling, ataxia, dementia).

⚠️ Important clinical warning: Giving folic acid alone to a patient who is actually B12-deficient will correct the anemia (masking the B12 deficiency) but allow the neurologic damage from B12 deficiency to silently progress. Always rule out B12 deficiency before treating with folic acid alone. - Robbins & Kumar Basic Pathology; The Washington Manual of Medical Therapeutics

Recommended Daily Allowance (RDA)

• Yamada's Textbook of Gastroenterology

Treatment

• Symptom to Diagnosis: An Evidence Based Guide, 4th ed.; Lippincott Illustrated Reviews: Pharmacology

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Folic Acid (Vitamin B9) - Biochemistry

1. Forms of Folic Acid

Folic acid and folate differ in glutamate residue count. Food folates have up to 7 glutamate residues linked by γ-peptide bonds and must be hydrolyzed to monoglutamates before absorption.

• Lippincott Illustrated Reviews: Biochemistry, 8th ed.; Harper's Illustrated Biochemistry, 32nd ed.

2. Sources

• Leafy, dark-green vegetables (spinach, broccoli, kale)
• Citrus fruits
• Legumes (beans, lentils)
• Liver
• Fortified grain/cereal products (mandatory in the US since 1998: 1.4 mg/kg grain)

Note: Folate is heat-labile - significant loss occurs during cooking.

3. Biochemical Function - One-Carbon Metabolism

• Serine, Glycine, Choline → Methylene-THF
• Histidine → Formimino-THF
• Formate → Formyl-THF

• Purine synthesis (Formyl-THF)
• TMP synthesis (Methylene-THF → TMP, for DNA)
• Methionine synthesis from homocysteine (Methyl-THF)
• Amino acid metabolism (serine, glycine interconversion)
• Harper's Illustrated Biochemistry, 32nd ed.

4. Deficiency - Causes

1. Inadequate dietary intake - poor diet, food insecurity, elderly
2. Increased demand - pregnancy, lactation, hemolytic anemias
3. Alcoholism - reduces intake and absorption
4. Malabsorption - celiac disease, IBD (upper small intestine pathology)
5. Drug-induced:
   • Methotrexate, trimethoprim - inhibit dihydrofolate reductase (DHFR)
   • Azathioprine, zidovudine - directly inhibit DNA synthesis
   • Phenytoin, phenobarbital - reduce folate absorption
6. Vitamin B12 deficiency - causes "Folate Trap" (see below)

5. Deficiency - Symptoms / Clinical Features

Megaloblastic (Macrocytic) Anemia

• Folic acid deficiency → reduced purine and TMP synthesis → cells cannot replicate DNA → large, immature RBC precursors (megaloblasts) accumulate in bone marrow
• MCV >100 μm³
• Hypersegmented neutrophils on blood smear
• Symptoms: fatigue, weakness, pallor, dyspnea, palpitations

GI Symptoms

• Sore tongue (glossitis)
• GI mucosal changes (rapidly dividing cells affected)

Neural Tube Defects (in pregnancy)

• Spina bifida and anencephaly are most common
• Folate supplementation before conception and during the first trimester significantly reduces risk

NO Neurologic Symptoms

• Unlike B12 deficiency, folate deficiency does not cause subacute combined degeneration (no peripheral neuropathy, no dementia)

Hyperhomocysteinemia

• Folate deficiency → failure to remethylate homocysteine → elevated plasma homocysteine → risk factor for atherosclerosis, thrombosis, and hypertension
• Lippincott Illustrated Reviews: Biochemistry, 8th ed.; Harper's Illustrated Biochemistry, 32nd ed.

6. The "Folate Trap" (Biochemistry Concept)

• Methyl-THF (the form circulating in blood) can only be converted back to THF via methionine synthase, which requires Vitamin B12
• In B12 deficiency, folate gets "trapped" as methyl-THF and cannot re-enter the one-carbon pool
• Result: functional folate deficiency even when dietary folate is adequate
• This is why both B12 and folate deficiency produce identical megaloblastic anemia

7. Key Biochemistry Points (DHFR & Drug Targets)

• Dihydrofolate reductase (DHFR) converts dihydrofolate → THF (essential step)
• Methotrexate inhibits human DHFR → used in cancer chemotherapy
• Trimethoprim inhibits bacterial DHFR → antibiotic
• Pyrimethamine inhibits parasite DHFR → antimalarial

8. RDA

• Lippincott Illustrated Reviews: Biochemistry, 8th ed.