Benefits
Supports Healthy Neural Tube Development in Pregnancy
Adequate folate before and in early pregnancy may reduce the risk of a neural tube defect such as spina bifida. In a Cochrane review of 5 trials (6,708 births), daily folic acid around conception lowered neural tube defects (risk ratio 0.31), with similar effects at 400 mcg or more a day. Health authorities advise 400 mcg a day for anyone who could become pregnant.
Supports Preconception Health
Because the neural tube forms in the first weeks of pregnancy, often before a pregnancy is known, folic acid is advised from at least a month before conception. Folate is not shown to help people conceive: in a trial of 2,370 couples seeking infertility treatment, folic acid plus zinc for the male partner did not improve semen quality or live births.
Supports Healthy Red Blood Cell Formation
Folate is needed to make DNA in developing red blood cells. Deficiency can cause megaloblastic anemia, with large, immature red blood cells, fatigue and shortness of breath. Adequate folate intake supports normal red blood cell formation.
Supports DNA Synthesis and Cell Division
Folate supplies one-carbon units used to build DNA and RNA, which makes it important for fast-dividing cells such as those in bone marrow and in a developing baby.
Helps Maintain Healthy Homocysteine Levels
Folic acid lowers blood homocysteine by about a quarter, with a further 7% from added vitamin B12. However, randomized trials including HOPE-2, NORVIT and VISP found this did not reduce the combined rate of heart attacks, strokes and cardiovascular deaths, although HOPE-2 reported fewer strokes. Folate should not be taken to prevent heart disease.
Folate and Mood
Low folate status has been linked to depression in some, but not all, studies. As an add-on to antidepressants, results are mixed: in a 12-week trial in 475 adults with moderate to severe depression who were not folate deficient, 5 mg a day of folic acid did not improve depression scores, while some smaller trials, including high-dose methylfolate taken under medical care, were more positive.
High-Dose Folic Acid and Cancer Questions
Some observational studies link higher dietary folate with lower colorectal cancer risk, but a pooled analysis of 13 randomized trials found no effect of folic acid supplements on overall or site-specific cancer, and some studies raise concern that high doses taken after precancerous lesions form might promote their growth. Intakes above the upper limit are not advised.
Mechanism of action
DNA and RNA Synthesis
Folate, as THF, donates one-carbon units in the synthesis of purines and pyrimidines, the building blocks of DNA and RNA. Specifically, it supports the conversion of deoxyuridine monophosphate (dUMP) to thymidine monophosphate (TMP), a key step in DNA synthesis, catalyzed by the enzyme thymidylate synthase. This is crucial for cell division and growth, particularly in rapidly dividing cells like those in bone marrow, skin, or the developing fetus.
Methylation Reactions
Folate is integral to the methionine cycle, where 5-methyltetrahydrofolate (5-MTHF) donates a methyl group to homocysteine, converting it to methionine via the enzyme methionine synthase, with vitamin B12 as a cofactor. Methionine is then converted to S-adenosylmethionine (SAM), the primary methyl donor for DNA, RNA, proteins, and lipid methylation, influencing gene expression and epigenetic regulation.
Homocysteine Metabolism
By helping convert homocysteine to methionine, folate keeps homocysteine from building up. High homocysteine is linked with cardiovascular disease, although lowering it with B vitamins did not reduce cardiovascular events in large trials.
Red Blood Cell Formation
Folate supports erythropoiesis (red blood cell production) by enabling DNA synthesis in developing red blood cells. Deficiency can lead to megaloblastic anemia, characterized by large, immature red blood cells.
Methylation and Cell Maintenance
Through the methionine cycle, folate supports the methylation reactions that help regulate genes and maintain cells. This is a biochemical role; it has not been shown to prevent disease in the trials cited here.
Clinical trials
Randomized, double-blind prevention trial with a factorial design at 33 centers in seven countries, comparing folic acid, seven other vitamins, both, or neither, started before pregnancy. (MRC Vitamin Study Research Group 1991, Lancet)
1,817 women who had a previous pregnancy affected by a neural tube defect; 1,195 had a completed pregnancy with a known outcome.
Neural tube defects occurred in 6 pregnancies in the folic acid groups vs 21 in the other groups, a 72% lower risk (relative risk 0.28, 95% CI 0.12 to 0.71). The other vitamins showed no significant effect, and no harm from folic acid was found.
Randomized controlled trial of a multivitamin containing 0.8 mg folic acid vs a trace-element supplement, taken from at least one month before conception until at least the second missed period. (Czeizel et al. 1992, N Engl J Med)
Women planning a pregnancy, mostly a first one; pregnancy outcome known in 2,104 vitamin and 2,052 trace-element users.
There were no neural tube defects in the vitamin group vs 6 in the trace-element group, and congenital malformations overall were less common with the vitamin (13.3 vs 22.9 per 1,000).
Multicenter randomized, placebo-controlled trial of 5 mg folic acid plus 30 mg zinc a day for 6 months in the male partners of couples planning infertility treatment. (Schisterman et al. 2020, JAMA)
2,370 couples (men mean age 33).
Live birth did not differ (34% vs 35%), and most semen measures did not change compared with placebo. Sperm DNA fragmentation was slightly higher with the supplement (29.7% vs 27.2%), and abdominal discomfort, nausea and vomiting were more common.
Folic Acid and Carotid Intima-media Thickness (FACIT) trial: 3-year randomized, double-blind, placebo-controlled trial of 800 mcg/day folic acid vs placebo. Outcomes included cognitive function as a secondary end point. (Durga et al. 2007, Lancet)
818 Dutch adults aged 50 to 70 with raised homocysteine and normal vitamin B12. 3-year intervention.
Over 3 years, folic acid improved memory and information processing speed vs placebo, with a borderline effect on sensorimotor speed; plasma homocysteine fell about 26%. These were secondary end points in people selected for raised homocysteine and normal vitamin B12, so the results may not apply to people with normal homocysteine or adequate folate status. A later pooled analysis of 11 B-vitamin trials with cognitive data on about 22,000 people, which included FACIT, found that lowering homocysteine had no significant effect on memory, speed, executive function or global cognition, and FACIT was the main source of disagreement among the domain trials.
Folic Acid Clinical Trial (FACT): randomized, double-blind, placebo-controlled, Phase III trial of folic acid (4 mg/day) vs placebo for prevention of pre-eclampsia in pregnant women at increased risk. (Wen et al. 2018, BMJ)
2,464 high-risk pregnant women.
Primary end point negative: 4 mg/day folic acid did not reduce pre-eclampsia in high-risk women (14.8% vs 13.5% with placebo; RR 1.10, 95% CI 0.90 to 1.34), and no other maternal or neonatal outcome differed. FACT tested high-dose folic acid started at 8 to 16 weeks of gestation; it does not address periconceptional folic acid for neural tube defect prevention.
FolATED trial: double-blind, placebo-controlled randomized trial in three centers in Wales of folic acid 5 mg/day added to antidepressants for 12 weeks. (Bedson et al. 2014, Health Technol Assess)
475 adults with moderate to severe depression taking or starting antidepressants; people with folate or B12 deficiency were excluded.
Adding 5 mg/day folic acid did not significantly improve depression scores or other measured outcomes vs placebo and was not cost-effective; mental health scores on the SF-12 questionnaire were 3.0% lower with folic acid. The result applies to standard folic acid in people who were not folate deficient.
Randomized controlled trial in Tianjin, China, in elderly participants with mild cognitive impairment assigned to folic acid (400 µg/day) or conventional treatment for 12 months; 152 completed the trial (77 vs 75). Outcomes: cognitive function, inflammatory cytokines. (Ma et al. 2016, Sci Rep)
152 elderly adults with MCI in Tianjin, China, who completed the trial. 12-month intervention.
Folic acid improved some cognitive scores (Full Scale IQ, Information, Digit Span; small effect sizes) and lowered IL-6 and TNF-alpha vs conventional treatment. The comparison group received no placebo, and results may differ where grain is fortified with folic acid.
Open-label randomized trial in 593 patients with stable coronary artery disease already on statin therapy: folic acid 0.5 mg/day (n=300) vs no folic acid (n=293), mean follow-up 24 months. Primary end point: all-cause mortality plus a composite of vascular events. (Liem et al. 2003, J Am Coll Cardiol)
593 patients with stable coronary artery disease on statins (Netherlands).
Folic acid lowered plasma homocysteine by 18% but did not reduce the primary end point (10.3% vs 9.6%; RR 1.05, 95% CI 0.63 to 1.75) within two years. The authors suggested homocysteine may merely be a modifiable marker of disease. This matches the larger HOPE-2, NORVIT and VISP trials, which found no reduction in their primary cardiovascular end points.
Double-blind, multicenter randomized trial in patients after successful coronary stenting: an initial IV dose (folic acid 1 mg, B6 5 mg, B12 1 mg), then daily oral folic acid 1.2 mg, vitamin B6 48 mg and vitamin B12 60 mcg for six months, vs placebo. Outcome: angiographic restenosis at 6 months. (Lange et al. 2004, N Engl J Med)
636 post-coronary-stent patients.
Restenosis was more common with the B-vitamin combination than with placebo (34.5% vs 26.5%, P = 0.05), late luminal loss was greater, and more patients needed repeat target-vessel revascularization (15.8% vs 10.6%, P = 0.05). Because folic acid was given with B6 and B12, the effect cannot be attributed to folic acid alone.
NORVIT trial: randomized 2x2 factorial trial comparing four daily regimens (folic acid 0.8 mg plus vitamin B12 0.4 mg; the same plus vitamin B6 40 mg; vitamin B6 40 mg alone; placebo) after acute myocardial infarction. (Bønaa et al. 2006, N Engl J Med)
3,749 post-MI patients.
Folic acid plus B12, with or without B6, lowered homocysteine 27% but did not reduce recurrent heart attack, stroke or sudden coronary death (relative risk 1.08, 95% CI 0.93 to 1.25). The arm taking folic acid, B12 and B6 showed a trend toward more events (relative risk 1.22, 95% CI 1.00 to 1.50), and the authors advised against this treatment. This does not affect folate's role in neural tube development or in correcting deficiency anemia.