Folate
Folate carries one-carbon units for thymidine and purine synthesis and for the remethylation of homocysteine to methionine. Periconceptional folic acid prevents most neural tube defects, which makes it the strongest outcome evidence for any vitamin supplement. Folic acid is the synthetic, fully oxidized form; it must be reduced by dihydrofolate reductase before it can enter the folate cycle.
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What It Does
Dividing cells need folate to make DNA, which is why deficiency shows up first as megaloblastic anemia, indistinguishable on a blood smear from B12 deficiency, and in the developing neural tube. 5-methyltetrahydrofolate (5-MTHF), the main circulating form, donates its methyl group to homocysteine through the B12-dependent methionine synthase. Low folate raises homocysteine.
The neural tube evidence is decisive. In the MRC Vitamin Study, folic acid reduced recurrent neural tube defects by 72% in women with a previously affected pregnancy (MRC 1991). US mandatory fortification of enriched grain in 1998 prevents an estimated 1,300 affected births each year (Williams 2015). The neural tube closes by about four weeks after conception, often before a pregnancy is known, which is why the recommendation covers everyone who could become pregnant. Large trials that lowered homocysteine with folic acid and other B vitamins did not reduce heart attacks.
MTHFR is the enzyme that makes 5-MTHF. The C677T variant reduces its activity, and homozygotes (about 10-15% of people of European ancestry, more in Hispanic populations) have higher homocysteine mainly when folate intake is low. The A1298C variant has a smaller effect. Folic acid still raises folate status in these people, and the US Preventive Services Task Force reaffirmed 400-800 mcg of folic acid for everyone who could become pregnant without regard to MTHFR genotype (USPSTF 2023).
Unmetabolized folic acid appears in the blood when intake outpaces reduction by dihydrofolate reductase. It was detectable in more than 95% of US serum samples in NHANES (Pfeiffer 2015). Whether it causes harm is unknown; the concerns about immune function and cancer come from small or observational studies. The real risk of high folic acid intake is masking B12 deficiency: it corrects the anemia while the neuropathy progresses.
Groups at risk of low folate include people with heavy alcohol use or malabsorption, and those taking methotrexate, sulfasalazine, or anticonvulsants such as phenytoin and valproate.
Forms and Bioavailability
Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.
| Form | Absorption | Typical dose | Cost | Labels |
|---|---|---|---|---|
| Folic Acidpteroylmonoglutamic acid | HighNearly 100% absorbed on an empty stomach and about 85% with food. Must be reduced by dihydrofolate reductase (DHFR), which is slow in humans, before entering the folate cycle. | 400 mcg daily (all who can become pregnant); 400-800 mcg in prenatals | Low | 396 |
| L-Methylfolate5-MTHF, L-5-methyltetrahydrofolate | HighThe main circulating form of folate. Bypasses both DHFR and MTHFR, so reduced MTHFR activity (as with the TT genotype) does not limit its use. | 400-1,000 mcg daily | Moderate | 140 |
| Folinic Acidleucovorin, calcium folinate | HighA reduced folate that bypasses DHFR. It still passes through MTHFR to become methylfolate. | 400-800 mcg daily as a supplement; higher prescription doses | Moderate | 11 |
Labels column: number of labels named as folate products that list this form. A product can list more than one form, and some labels do not state a form at all.
Folic acid is well absorbed and stable, and it is the form behind all of the neural tube prevention evidence. L-methylfolate, sold as a calcium salt (Metafolin) or glucosamine salt (Quatrefolic), skips both reduction and the MTHFR step and does not produce unmetabolized folic acid. It raises red cell folate effectively. It has not been tested for neural tube defect prevention, so for pregnancy planning it is a reasonable alternative rather than an established equivalent.
Folinic acid (5-formyl-THF, leucovorin) bypasses dihydrofolate reductase and is the drug used for methotrexate rescue; it has little reason to be in an everyday supplement.
Typical Dosing
| Group | RDA |
|---|---|
| Adults 19+ | 400 mcg DFE |
| Pregnancy | 600 mcg DFE |
| Lactation | 500 mcg DFE |
| Upper limit (UL) | 1,000 mcg per day of folic acid from supplements and fortified foods; does not apply to natural food folate |
Everyone who could become pregnant is advised to take 400 mcg of folic acid daily in addition to food folate. 1 mcg DFE equals 1 mcg food folate, 0.6 mcg folic acid with food, or 0.5 mcg folic acid on an empty stomach.
Food sources: beef liver, spinach, black-eyed peas, fortified breakfast cereals, enriched rice, asparagus, Brussels sprouts.
What to Look For
Since 2020, labels state folate in mcg DFE (dietary folate equivalents), with folic acid in parentheses. 400 mcg of folic acid taken with food equals 680 mcg DFE. Methylfolate labels sometimes state compound weight, sometimes DFE; check which.
For pregnancy planning, start 400-800 mcg folic acid daily at least one month before conception. Women with a prior neural tube defect pregnancy or on valproate need a higher, prescribed dose. Anyone taking more than 1,000 mcg of folic acid daily should have B12 status checked.
Red flags: "MTHFR support" formulas with 5-15 mg of methylfolate taken without a medical indication, and genetic testing panels that recommend supplements based on MTHFR alone. The American College of Medical Genetics recommends against routine MTHFR testing.
Folic Acid
The form with the outcome data. For neural tube defect prevention, nothing else has been tested.
What to look for: 400-800 mcg folic acid (680-1,360 mcg DFE) daily, starting at least a month before conception.
Find on iHerb Affiliate linkL-Methylfolate
A reasonable alternative that avoids unmetabolized folic acid. Its advantage for MTHFR variant carriers is theoretical.
What to look for: A named salt (Metafolin or Quatrefolic) with mcg DFE stated, at doses near 400-1,000 mcg rather than the milligram range.
Find on iHerb Affiliate linkProducts on Titrate
22,380 of the 214,745 labels in the NIH Dietary Supplement Label Database list folate as an active ingredient (11,171 on market). 769 are sold as folate products by name.
Top on-market labels, ranked by label completeness and simplicity:
- Fully Active Folate 800 with Quatrefolic 800 mcg
- Fully Active Folate 400 with Quatrefolic 400 mcg
- Active Folate 1000
- Active Folate Liquid Alcohol Free
- Folate 1,000
Supplement Search loads one category at a time. Choose the B-Complex tab, then type a form such as "folic acid" under Must contain, or a form to avoid under Must not contain.
Related Tools
References
NIH Office of Dietary Supplements. Folate: Fact Sheet for Health Professionals.
MRC Vitamin Study Research Group. Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. Lancet. 1991;338(8760):131-137.
Williams J, et al. Updated estimates of neural tube defects prevented by mandatory folic acid fortification: United States, 1995-2011. MMWR Morb Mortal Wkly Rep. 2015;64(1):1-5.
Pfeiffer CM, et al. Unmetabolized folic acid is detected in nearly all serum samples from US children, adolescents, and adults. J Nutr. 2015;145(3):520-531.
US Preventive Services Task Force. Folic Acid Supplementation to Prevent Neural Tube Defects: US Preventive Services Task Force Reaffirmation Recommendation Statement. JAMA. 2023;330(5):454-459.
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