Active Forms of Folic Acid (Vitamin B9)
The biologically active forms of folic acid are tetrahydrofolate (THF), 5-methyltetrahydrofolate (5-MTHF), 5,10-methylenetetrahydrofolate (5,10-MTHF), and folinic acid (5-formyltetrahydrofolate). 1
Primary Active Forms
Tetrahydrofolate (THF) is the fundamental active cofactor form that functions in one-carbon metabolism as a methyl group donor 1. Folic acid itself is synthetic and inactive, requiring enzymatic conversion by dihydrofolate reductase in the liver to become THF 1, 2.
5-methyltetrahydrofolate (5-MTHF) is the predominant circulating form in human blood and the primary species transported into peripheral tissues for cellular metabolism 1, 3. This is the form that:
- Crosses cell membranes via folate receptors 1
- Serves as the main methyl donor for converting homocysteine to methionine 4
- Does not require hepatic activation, making it immediately bioavailable 5
5,10-methylenetetrahydrofolate (5,10-MTHF) is an intermediate active form produced when THF is transformed during the methylation cycle, catalyzed by the vitamin B6-dependent enzyme serine hydroxymethyltransferase 1.
Folinic acid (5-formyltetrahydrofolate) is another biologically active form that bypasses the MTHFR enzyme entirely 4. This form is particularly important because:
- It is mandatory (not folic acid) for patients receiving pyrimethamine therapy 4
- It is used for methotrexate rescue therapy 4, 2
- It should never be substituted with folic acid when specifically indicated 4
Clinical Significance of Active Forms
The distinction between synthetic folic acid and active folates matters clinically because folic acid requires hepatic conversion that can be impaired by:
- Genetic variants (MTHFR polymorphisms, particularly the 677TT genotype affecting 23.6% of some populations) 1
- A common 19-base pair deletion of dihydrofolate reductase (present in 29.5% as homozygous deletion) 1
- Limited activity of dihydrofolate reductase enzyme 1
- Certain medications that inhibit dihydrofolate reductase 2
For patients with MTHFR 677TT genotype who cannot efficiently convert folic acid to active forms, 5-MTHF supplementation is particularly beneficial 4. In clinical trials, patients with the MTHFR 677 CC genotype showed dramatic stroke risk reduction (HR 0.24) when receiving adequate folate and B12, while TT homozygotes responded better when both folate and B12 levels were above the median 1.
Bioavailability Considerations
Folic acid has nearly twice the bioavailability of naturally occurring food folate 4. The conversion hierarchy is:
- 1 mcg food folate = 0.6 mcg folic acid from fortified food = 0.5 mcg folic acid supplement on empty stomach 4
However, 5-MTHF (levomefolic acid) provides 9 mg dietary folate equivalents (DFE) per dose and does not accumulate in blood like unconverted folic acid does in cases of reduced hepatic transformation 6, 5.
Critical Pitfall to Avoid
Never substitute folic acid for folinic acid when folinic acid is specifically indicated 4. This is particularly critical in toxoplasmosis treatment with pyrimethamine and methotrexate rescue therapy, where only the active folinic acid form will prevent megaloblastic anemia 4, 2.
The functional interdependence with vitamin B12 is crucial: B12 deficiency leads to functional folate deficiency through accumulation of 5-MTHF (the "methyl trap"), as reduced methionine synthase activity cannot utilize the 5-MTHF 1. Therefore, assessment of folate status should always include B12 evaluation 1.