Folic Acid

**Folic acid** (Vitamin B9) is a vital water-soluble vitamin essential for **nucleoprotein synthesis** and **normal erythropoiesis** (red blood cell production). In veterinary medicine, it is primarily used to treat folate deficiencies, which frequently arise secondary to gastrointestinal disorders. **Clinical Pearls & Diagnostic Utility:** - **Absorption Site:** Folate is absorbed exclusively in the **proximal small intestine**, whereas cobalamin (Vitamin B12) is absorbed in the distal small intestine. - **Diagnostic Indicator:** Measuring serum folate and cobalamin helps localize intestinal disease. Low folate suggests proximal small intestinal disease. - **Bacterial Overgrowth:** Dogs with Exocrine Pancreatic Insufficiency (EPI) or Small Intestinal Bacterial Overgrowth (SIBO) often have *increased* serum folate because enteric bacteria synthesize folate. - **Antifolate Rescue:** It is also used to prevent or treat deficiencies caused by the chronic administration of dihydrofolate reductase inhibitors (e.g., trimethoprim, pyrimethamine).

กลไกการออกฤทธิ์: Folic acid is a precursor that must be enzymatically reduced to become biologically active. **Mechanism Pathway:** **Folic Acid** → reduced by **dihydrofolate reductase (DHFR)** → **Tetrahydrofolate (THF / 5-methyltetrahydrofolate)**. - **THF** acts as a crucial coenzyme and methyl donor in the synthesis of purine and pyrimidine nucleotides. - This pathway is absolutely necessary for **DNA synthesis**, cellular division, and the maintenance of normal **erythropoiesis**. - Drugs like trimethoprim and methotrexate competitively inhibit DHFR, blocking this pathway and potentially causing megaloblastic anemia or leukopenia, which is why folic acid supplementation is used as a rescue therapy.

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