Folic acid

Serious harms

Folic acid is prescribed for folate-deficiency anaemia, to prevent a recurrent neural tube defect in pregnancy and to reduce methotrexate side effects ¹²³.

Condition	How often	Who is at risk	Source
B12-deficiency nerve and spinal cord damage, progressing while anaemia is corrected	32 of 70 with pernicious anaemia on 5 mg daily relapsed neurologically over 3.5 years	Untreated B12 deficiency, including lifelong vegetarians	²⁴
Anaphylaxis	1 in 10,000 to 1 in 1000	Not stated	²⁵
Loss of seizure control	Not quantified	Phenytoin, phenobarbital or primidone	¹²
Faster loss of kidney function; vascular events or death	GFR fell 5.8 mL/min/1.73 m² more; events 23.5% against 14.4% of 238 at 36 months (HR 2.0), 1 per 11	Diabetic nephropathy; with B6 and B12	⁶
Prostate cancer	10-year probability 9.7% against 3.3% of 643 men (HR 2.63), 1 per 16	1 mg daily after colorectal adenoma; secondary analysis	⁷

Folic acid enters the cell’s folate pool through dihydrofolate reductase (DHFR), independently of vitamin B12 ⁴⁸. In B12 deficiency, it restarts DNA synthesis in bone marrow but cannot restore the methionine cycle ⁴. The neuropathy, which can occur without anaemia, is attributed to lost methionine synthase activity ⁴⁸.

Folic acid reduction and the folate one-carbon cycleCytosol of hepatocytes and dividing cells

Folic acid
Dihydrofolate reductase (DHFR)NADPHMethotrexateSlow and variable in human liver; reduced food folates bypass this step
7,8-Dihydrofolate
Dihydrofolate reductase (DHFR)NADPHMethotrexateAlso reduces the dihydrofolate released by thymidylate synthase (TYMS)
Tetrahydrofolate
Serine hydroxymethyltransferase (SHMT1 cytosolic, SHMT2 mitochondrial)Serine to glycine, Pyridoxal 5′-phosphateBranch: TYMS transfers this carbon to dUMP, forming dTMP for DNA
5,10-Methylenetetrahydrofolate
Methylenetetrahydrofolate reductase (MTHFR)NADPH, FADIrreversible in vivo; inhibited by SAM; the 677C>T variant lowers activity
5-Methyltetrahydrofolate
Methionine synthase (MTR)Methylcobalamin, Homocysteine to methionineVitamin B12 deficiency slows this step and traps folate as 5-methyl-THF
Tetrahydrofolate

Neurological relapses typically occurred within 2 years; signs often resolved with cobalamin but became irreversible in several cases ⁴. Under the US label, doses above 0.1 mg daily may obscure pernicious anaemia ¹. EFSA rated the certainty that folic acid itself worsens the neuropathy as low (15 to 50% probability) ⁴. B12 deficiency was associated with 2 years or more of proton pump inhibitor use (OR 1.65; 25,956 cases, 184,199 controls) and may affect up to 1 in 10 metformin users ⁹¹⁰.

Across 13 trials, first cancers occurred in 7.7% of 24,799 given folic acid and 7.3% of 24,822 controls over a mean 5.5 years (RR 1.06, 0.99 to 1.13); only cancers during treatment were counted ¹¹.

Other pathways it affects

Under US fortification, unmetabolised folic acid was detectable in the blood of over 95% of people, with or without supplements ¹⁵.

The reduced folate carrier, the main route into tissues including brain, has about 100 times lower affinity for folic acid than for reduced folates ⁴.

In people, cord-blood methylation at LINE-1, IGF2 and BDNF was lower when 400 µg was continued in later pregnancy (86 samples), without a shown health effect ¹⁶⁴. In one rat study, folic acid at one dietary level increased colon aberrant crypt foci when started before a carcinogen and reduced them when started after ⁴.

What it is for and how it works

Folate is needed to make DNA; dietary deficiency causes anaemia with large red cells ¹⁹.

DHFR reduces folic acid to tetrahydrofolate, which binds the one-carbon units used in DNA synthesis ⁴¹⁹.

In the body

Human gut and liver convert folic acid slowly; above 1 mg daily, most of the excess is excreted unchanged in urine ²⁰¹.

In 5 adults, 80 ± 12% of labelled portal-vein folate was unchanged folic acid 15 minutes after 220 µg ²⁰. DHFR activity per gram of 6 human livers was under 2% of rat values ²¹. Fluorouracil toxicity may occur with folic acid; trimethoprim or sulphonamides may reduce its effect, potentially seriously in megaloblastic anaemia ². Methotrexate and trimethoprim inhibit DHFR; fluorouracil inhibits thymidylate synthase ¹⁹.

Thymidylate cycleCytosol, nucleus and mitochondria

Tetrahydrofolate (THF)5,10-Methylene-THFSerine hydroxymethyltransferase (SHMT1, SHMT2)L-Serine → glycine, Pyridoxal 5′-phosphate
5,10-Methylene-THFDihydrofolate (DHF)Thymidylate synthase (TYMS)dUMP → dTMPFluorouracilPemetrexedFolate supplies the carbon and the two electrons that reduce it to methyl
Dihydrofolate (DHF)Tetrahydrofolate (THF)Dihydrofolate reductase (DHFR)NADPHMethotrexateAlso reduces folic acid to DHF, slowly in human liver

Genetic variation

A common gene variant is associated with lower blood folate; it alters MTHFR, an enzyme other than DHFR ²²¹⁹.

About 10 to 15% of people of European or Japanese origin are homozygous for 677C>T (TT) ²³. TT women had 16% lower red cell folate than CC women ²².

Benefit against harm

Folic acid before conception lowers the risk of neural tube defects, malformations of brain and spinal cord ²⁵.

After a previous affected pregnancy, with 4 mg daily to 12 weeks, defects occurred in 6 of 593 (1.0%) against 21 of 602 (3.5%) pregnancies (RR 0.28), 1 prevented per 40 women ²⁶. Across 5 trials (6708 births), RR was 0.31 (0.17 to 0.58); only one trial was at low risk of bias, and a single trial of 261 tested 0.4 mg or less ²⁵.

Group	Benefit	Serious harm
Methotrexate for rheumatoid arthritis; 624 adults, 8 to 52 weeks	Transaminase rise 5% against 21% (RR 0.23; NNT 6)	No significant loss of methotrexate effect ³
Homocysteine-lowering B vitamins; 15 trials, 71,422 participants, 1 to 7.3 years	Stroke 4.3% against 5.1% (RR 0.90); myocardial infarction and death unchanged	Serious adverse events 8.3% against 8.5% (RR 1.07, 1.00 to 1.14) ²⁷

Without the medicine

Folate occurs in dark green vegetables, legumes, nuts, liver and yeast, with considerable cooking losses ⁴.

How the recommendations were set

The European upper level of 1000 µg a day, retained in 2023, is based on the risk that neurological symptoms progress in B12 deficiency ⁴.

Stopping

Blood folate falls when folic acid is stopped in pregnancy ³⁶.

In 60 women randomised to stop 400 µg at 14 weeks, serum folate fell from 45.7 to 19.5 nmol/L by week 36 and homocysteine rose from 6.6 to 7.6 µmol/L ³⁶.

A prescription is changed together with the prescriber.

References

Regulator
Amneal Pharmaceuticals of New York LLC; US Food and Drug Administration. Folic acid tablets, USP (1 mg), Rx only: prescribing information DailyMed, US National Library of Medicine; ANDA 040625, label effective 15 April 2026, 2026.
https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=fd85395e-6a0b-4072-a80b-36095c0c6dd6
Funding and finding
Funding
US Food and Drug Administration
Finding
Indicated for megaloblastic anaemias due to folate deficiency and anaemias of nutritional origin, pregnancy, infancy or childhood. Warning: folic acid alone is improper therapy for pernicious anaemia and other megaloblastic anaemias in which vitamin B12 is deficient. Precautions: doses above 0.1 mg daily may obscure pernicious anaemia, haematologic remission occurring while neurologic manifestations remain progressive, which may result in severe nervous system damage before the correct diagnosis is made. Interactions: folic acid antagonises the anticonvulsant action of phenytoin. Adverse reactions: allergic sensitisation after oral and parenteral use, including erythema, rash, itching, malaise and bronchospasm, and one case suggesting anaphylaxis after injection; at 15 mg daily for 1 month, gastrointestinal effects and altered sleep, difficulty concentrating, irritability, overactivity, excitement, depression, confusion and impaired judgment; decreased serum vitamin B12 with prolonged therapy; in an uncontrolled study, more seizures in some epileptic patients taking phenobarbital, primidone or phenytoin. Overdosage section: except in pregnancy and lactation, therapeutic doses above 0.4 mg daily should not be given until pernicious anaemia has been ruled out. Dosage: doses above 0.1 mg should not be used unless B12-deficiency anaemia has been ruled out or is adequately treated; daily doses above 1 mg do not enhance the haematologic effect, and most of the excess is excreted unchanged in urine. Long-term carcinogenicity, mutagenicity and fertility studies in animals have not been conducted.
Regulator
Ennogen Pharma Limited; Medicines and Healthcare products Regulatory Agency. Folic Acid Tablets 5mg: summary of product characteristics MHRA Products, summary of product characteristics, PL 40147/0037, text revised 2 June 2025, 2025.
https://mhraproducts4853.blob.core.windows.net/docs/d74429fde1f9dd1cee5029719d9200a92869acea
Funding and finding
Funding
Medicines and Healthcare products Regulatory Agency
Finding
Contraindications: not to be given alone in Addisonian anaemia or other vitamin B12 deficiency states because it may precipitate subacute combined degeneration of the spinal cord; long-term folate therapy is contraindicated in untreated cobalamin deficiency, including lifelong vegetarians; in elderly people a cobalamin absorption test should precede long-term therapy; folate given to such patients for 3 months or longer has precipitated cobalamin neuropathy; not for use in malignant disease unless megaloblastic anaemia from folate deficiency is an important complication. Warnings: folic acid can mask B12 deficiency while subacute irreversible damage to the nervous system continues; folate should not be routinely used in patients receiving coronary stents; caution in patients who may have folate-dependent tumours; the 5 mg tablet is not intended for healthy pregnant women, for whom lower doses are recommended, but for folate deficiency or risk of recurrent neural tube defect. Interactions: serum phenytoin, phenobarbital and primidone may fall, requiring monitoring and dose adjustment; fluorouracil toxicity may occur and the combination should be avoided; folate supplements enhance lithium efficacy; folate deficiency from methotrexate or trimethoprim cannot be treated with folic acid, folinic acid being required; folic acid may reduce zinc absorption. Undesirable effects: allergic reactions including erythema, rash, pruritus, urticaria, dyspnoea and anaphylactic reaction including shock, rare (1 in 10,000 to 1 in 1000); worsening of co-existing B12 deficiency, frequency not known. Pharmacokinetics: therapeutic folic acid enters the portal circulation largely unchanged, being a poor substrate for dihydrofolate reductase; dietary folates have about half the bioavailability of crystalline folic acid.
Independent research
Shea B, Swinden MV, Tanjong Ghogomu E, Ortiz Z, Katchamart W, Rader T, et al. Folic acid and folinic acid for reducing side effects in patients receiving methotrexate for rheumatoid arthritis Cochrane Database of Systematic Reviews, 2013.
doi:10.1002/14651858.CD000951.pub2 PubMed 23728635
Funding and finding
Participants
624
Follow-up
8 to 52 weeks
Funding
Internal sources: University of Ottawa, Clinical Epidemiology Unit; University of Ottawa, Institute of Population Health. External sources: no sources of support supplied.
Finding
Folic or folinic acid against placebo: gastrointestinal side effects 256 against 346 per 1000 (RR 0.74, 95% CI 0.59 to 0.92; absolute reduction 9.0%; number needed to treat 11, 7 to 35; follow-up 24 to 52 weeks); transaminase elevation 48 against 208 per 1000 (RR 0.23, 0.15 to 0.34; absolute reduction 16.0%; NNT 6, 6 to 7; 8 to 52 weeks); withdrawal for any reason 98 against 250 per 1000 (RR 0.39, 0.28 to 0.53; absolute reduction 15.2%; NNT 7, 6 to 9; 8 to 52 weeks); stomatitis RR 0.72 (0.49 to 1.06). No statistically significant effect on the efficacy of methotrexate measured by joint counts and global assessments.
Regulator
EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA Panel), Turck D, Bohn T, Castenmiller J, de Henauw S, Hirsch-Ernst KI, et al. Scientific opinion on the tolerable upper intake level for folate EFSA Journal, 2023.
doi:10.2903/j.efsa.2023.8353 PubMed 37965303
Funding and finding
Funding
European Food Safety Authority, on request from the European Commission (question EFSA-Q-2021-00366). Preparatory systematic reviews were conducted by a contractor, Karolinska Institutet in collaboration with the University of Oslo, under an EFSA procurement procedure.
Finding
Folates consist of a pterin moiety linked to p-aminobenzoic acid and one or more glutamates, substituted at N5 or N10 by one-carbon units; natural folates are reduced polyglutamates usually with five to seven glutamates. A jejunal hydrolase (EC 3.4.17.21) converts polyglutamates to monoglutamates, which PCFT (SLC46A1), the most important intestinal transporter, and RFC (SLC19A1) move into enterocytes; 5-methyl-THF is the main circulating form; RFC, the main transporter from blood to tissues, has about 100 times lower affinity for folic acid than for reduced folates. Folylpolyglutamate synthase retains folates as THF polyglutamates; 5-methyl-THF monoglutamate and folic acid are poor substrates, and 5-methyl-THF is lost from cells unless methionine synthase converts it to THF. Folic acid, a fully oxidised monoglutamate, is reduced by DHFR via dihydrofolate to THF. The MTHFR reaction is essentially irreversible under physiological conditions and SAM is its allosteric inhibitor; in cobalamin deficiency 5-methyl-THF is metabolically trapped, SAM falls, MTHFR becomes fully active, and a functional folate deficiency develops with raised plasma 5-methyl-THF and homocysteine, depleted erythrocyte folate and fewer methyl groups for transmethylation. The megaloblastic anaemia of cobalamin deficiency is identical to that of folate deficiency; neurological manifestations (myeloneuropathy from demyelination) can occur with or without anaemia, are almost never seen in folate deficiency, and have an unknown mechanism, with congenital defects of MTHFR and methionine synthase pointing to methionine synthase rather than methylmalonyl-CoA mutase. Folic acid can initiate DNA synthesis in cobalamin deficiency through DHFR but cannot correct the methylation defect. MTHFR 677TT is associated with lower serum folate and higher homocysteine; in a genome-wide study of 2232 Irish adults aged 18 to 28 years MTHFR 677C>T was the only significant modifier of red cell and serum folate. ULs for folic acid retained: 1000 ug/day for adults including pregnant and lactating women, with progression of neurological symptoms in cobalamin-deficient patients as critical effect.
Regulator
European Medicines Agency, Pharmacovigilance Risk Assessment Committee and Coordination Group for Mutual Recognition and Decentralised Procedures (human). Folic acid: CMDh scientific conclusions and grounds for variation, amendments to the product information and timetable for the implementation (PSUSA/00001459/201706) European Medicines Agency, periodic safety update single assessment PSUSA/00001459/201706; CMDh position adopted January 2018, 2018.
https://www.ema.europa.eu/en/documents/psusa/folic-acid-cmdh-scientific-conclusions-and-grounds-variation-amendments-product-information-and-timetable-implementation-psusa00001459201706_en.pdf
Funding and finding
Funding
European Medicines Agency
Finding
From a review of cases of anaphylactic reaction in the database of one marketing authorisation holder, based on temporality, possible alternative aetiologies and sensitivity testing, the PRAC concluded that there was reasonable evidence of a causal association between anaphylactic reaction and use of folic acid. Anaphylactic reaction was added to section 4.8 of the summary of product characteristics of folic acid-containing medicines under immune system disorders with frequency not known, because no exposure data from clinical trials were available; the package leaflet was updated with severe allergic reaction. Benefit-risk balance judged unchanged; implementation by 9 May 2018.
Independent research
House AA, Eliasziw M, Cattran DC, Churchill DN, Oliver MJ, Fine A, et al. Effect of B-vitamin therapy on progression of diabetic nephropathy: a randomized controlled trial JAMA, 2010.
doi:10.1001/jama.2010.490 PubMed 20424250
Funding and finding
Registration
ISRCTN41332305
Participants
238
Follow-up
mean 31.9 months; primary outcome at 36 months
Funding
This work was funded by grant MCT-41551 from the Canadian Institutes of Health Research and by the Kidney Foundation of Canada. The sponsors had no role in the design and conduct of the study, in the collection, management, analysis, and interpretation of the data, or in the preparation, review, or approval of the manuscript.
In-kind support
Pan American Laboratories (Covington, Louisiana) provided in-kind the B vitamins and matching placebos.
Finding
One tablet of folic acid 2.5 mg, vitamin B6 25 mg and vitamin B12 1 mg daily against placebo. Radionuclide GFR fell by 16.5 against 10.7 mL/min/1.73 m2 at 36 months: difference -5.8 (95% CI -10.6 to -1.1). Composite of myocardial infarction, stroke, revascularisation and death from any cause: 36-month risk 23.5% (15.0% to 32.0%) against 14.4% (6.9% to 21.8%), HR 2.0 (1.0 to 4.0), P = 0.04. Dialysis HR 1.1 (0.4 to 2.6); death from any cause did not differ significantly. Homocysteine fell by 2.2 µmol/L against a rise of 2.6 µmol/L.
Independent research
Figueiredo JC, Grau MV, Haile RW, Sandler RS, Summers RW, Bresalier RS, et al. Folic acid and risk of prostate cancer: results from a randomized clinical trial Journal of the National Cancer Institute, 2009.
doi:10.1093/jnci/djp019 PubMed 19276452
Funding and finding
Registration
NCT00272324
Participants
643
Follow-up
Up to 10.8 years, median 7.0 years
Funding
Supported in part by grants (R01-CA-059005 and U54-CA-100971) from the National Cancer Institute, National Institutes of Health; supported in part by a post-PhD Research Fellowship from the National Cancer Institute of Canada (#017602 to J.C.F.). The study sponsors had no role in the study design, data collection, analysis and interpretation of the data, the decision to submit the manuscript for publication, or the writing of the manuscript.
In-kind support
Wyeth, which markets folate supplements, provided study agents for the trial.
Finding
34 prostate cancers after randomisation. Estimated 10-year probability of prostate cancer 9.7% (95% CI 6.5% to 14.5%) with folic acid against 3.3% (1.7% to 6.4%) with placebo: age-adjusted HR 2.63 (1.23 to 5.65); multiadjusted HR 2.58 (1.14 to 5.86). Baseline dietary folate (HR per SD 0.65, 0.35 to 1.20) and plasma folate in non-users of multivitamins (0.42, 0.17 to 1.04) tended in the opposite direction. Screening was not protocol-driven, and the authors call the estimates imprecise.
Independent research
Gherasim C, Lofgren M, Banerjee R. Navigating the B12 Road: Assimilation, Delivery, and Disorders of Cobalamin Journal of Biological Chemistry, 2013.
doi:10.1074/jbc.R113.458810 PubMed 23539619
Funding and finding
Funding
This work was supported, in whole or in part, by National Institutes of Health Grant DK45776.
Finding
Review. Mammals have two B12-dependent enzymes: methionine synthase, which uses methylcobalamin, and methylmalonyl-CoA mutase, which uses adenosylcobalamin to convert methylmalonyl-CoA to succinyl-CoA. Methionine synthase works in two half-reactions: methylcobalamin methylates homocysteine to methionine, leaving cob(I)alamin, which removes the methyl group from 5-methyl-THF to give THF and methylcobalamin; occasional oxidation to cob(II)alamin is repaired by reductive methylation needing NADPH, methionine synthase reductase and S-adenosylmethionine. Food cobalamin binds haptocorrin; in the duodenum pancreatic proteases release it, intrinsic factor binds it, and intestinal cells take it up by cubilin/AMN receptor-mediated endocytosis. About 20% of circulating cobalamin is bound to transcobalamin, which delivers it to cells through the transcobalamin receptor. Mutations in the cblG (methionine synthase) and cblE (reductase) loci cause isolated hyperhomocysteinaemia without methylmalonic aciduria.
Independent research
Lam JR, Schneider JL, Zhao W, Corley DA. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency JAMA, 2013.
doi:10.1001/jama.2013.280490 PubMed 24327038
Funding and finding
Participants
210,155
Follow-up
January 1997 to June 2011
Funding
This project was supported by a Kaiser Permanente Community Benefit grant. Kaiser Permanente had no role in the design and conduct of the study; the collection, management, analysis, and interpretation of the data; the preparation, review, or approval of the manuscript; or the decision to submit the manuscript for publication.
Finding
A 2-year or longer supply of PPIs was dispensed to 3120 of 25,956 cases (12.0%) and 13,210 of 184,199 controls (7.2%): OR 1.65 (95% CI 1.58 to 1.73); for H2-receptor antagonists without PPI OR 1.25 (1.17 to 1.34). More than 1.5 PPI pills a day OR 1.95 (1.77 to 2.15) against less than 0.75 pills a day OR 1.63 (1.48 to 1.78).
Regulator
Medicines and Healthcare products Regulatory Agency. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk Drug Safety Update volume 15, issue 11, June 2022: 1, published 20 June 2022, 2022.
https://www.gov.uk/drug-safety-update/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk
Funding and finding
Funding
Medicines and Healthcare products Regulatory Agency
Finding
Decreased vitamin B12 levels, or vitamin B12 deficiency, is now considered a common adverse drug reaction of metformin. After a European review of the brand leader Glucophage with MHRA input, the product information was updated to state that vitamin B12 deficiency is common and may affect up to 1 in 10 people who take it, and that the risk increases with higher dose, longer treatment duration and pre-existing risk factors. The agency advises testing serum vitamin B12 when deficiency is suspected, for example in patients presenting with megaloblastic anaemia or new-onset neuropathy, considering periodic monitoring in patients with risk factors, and continuing metformin while the deficiency is corrected. The mechanism is described as multifactorial, comprising altered intestinal motility, bacterial overgrowth and reduced uptake of vitamin B12 in the small intestine. Patients with deficiency can be asymptomatic or present with megaloblastic anaemia, neuropathy or both; other features include depression, irritability, cognitive impairment, glossitis, mouth ulcers and visual and motor disturbances, and the statement notes that anaemia or neuropathy caused by deficiency should be diagnosed and treated as soon as possible to avoid permanent symptoms. Listed risk factors: baseline B12 at the lower end of the normal range; conditions reducing absorption, including older age, total or partial gastrectomy, Crohn's disease and other inflammatory bowel disorders and autoimmune conditions; diets low in B12 such as strict vegan and some vegetarian diets; concomitant medicines impairing B12 absorption, including proton pump inhibitors and colchicine; and genetic predisposition such as Imerslund-Grasbeck syndrome and transcobalamin II deficiency. The four cited references are Aroda 2016, Beulens 2015, de Jager 2010 and Miller 2018.
Independent research
Vollset SE, Clarke R, Lewington S, Ebbing M, Halsey J, Lonn E, et al. Effects of folic acid supplementation on overall and site-specific cancer incidence during the randomised trials: meta-analyses of data on 50,000 individuals Lancet, 2013.
doi:10.1016/S0140-6736(12)62001-7 PubMed 23352552
Funding and finding
Participants
49,621
Follow-up
Scheduled treatment period only, 2.0 to 7.7 years, weighted mean 5.5 years; post-trial follow-up not included
Funding
British Heart Foundation, Medical Research Council, Cancer Research UK, Food Standards Agency. CTSU receives its core funding from the British Heart Foundation, UK Medical Research Council and Cancer Research UK; support for this project was also provided by a grant from the UK Food Standards Agency (N05072). Sources of funding for individual trials are described in their separate publications. The funders of the individual trials had no role in study design, data collection, data analysis, data interpretation, or writing or submission of the report.
Finding
Folic acid quadrupled median plasma folate (57.3 against 13.5 nmol/L). First cancers 1904 of 24,799 (7.7%) on folic acid against 1809 of 24,822 (7.3%) on control: RR 1.06 (95% CI 0.99 to 1.13), P = 0.10; adenoma trials 96 against 72 cancers (RR 1.33, 0.98 to 1.80), vascular trials RR 1.04 (0.98 to 1.11). Colorectal RR 1.07 (99% CI 0.83 to 1.37), lung 1.08 (0.86 to 1.35), breast 0.89 (0.66 to 1.20), prostate 1.15 (0.94 to 1.41). The authors state that they do not address effects emerging years or decades after the trials ended.
Industry-funded
Ebbing M, Bønaa KH, Nygård O, Arnesen E, Ueland PM, Nordrehaug JE, et al. Cancer incidence and mortality after treatment with folic acid and vitamin B12 JAMA, 2009.
doi:10.1001/jama.2009.1622 PubMed 19920236
Funding and finding
Registration
NCT00671346
Participants
6,837
Follow-up
Median 39 months of treatment and 78 months in all, including post-trial follow-up through 31 December 2007
Funding
The NORVIT and WENBIT trials were funded by the Advanced Research Program and Research Council of Norway; the Norwegian Foundation for Health and Rehabilitation; The Norwegian Council on Cardiovascular Disease; the Norwegian Heart and Lung Patient Organization; the Norwegian Red Cross; the Northern Norway Regional Health Authority; the Western Norway Regional Health Authority; the Norwegian Ministry of Health and Care Services; the University of Tromsø; the University of Bergen; Department of Heart Disease at Haukeland University Hospital; the Foundation to Promote Research Into Functional Vitamin B12 Deficiency; by an unrestricted private donation; and by Alpharma Inc, Copenhagen, Denmark, who provided study medication. Alpharma Inc provided the study capsules, generated the randomization sequence and concealed the randomization code, free of charge, and rendered a limited grant to finance the initial phase of the trials; the company had no role in the design and conduct of the trials or the current study, in data collection, management, analysis and interpretation, or in preparation, review or approval of the manuscript.
In-kind support
Study capsules, generation of the randomisation sequence and concealment of the randomisation code by Alpharma Inc
Finding
Folic acid 0.8 mg plus vitamin B12 0.4 mg daily, with or without vitamin B6 40 mg (n = 3411), against vitamin B6 alone or placebo (n = 3426). Median serum folate rose from 3.9 to 27.5 ng/mL on folic acid. Cancer diagnosed in 341 (10.0%) against 288 (8.4%): HR 1.21 (95% CI 1.03 to 1.41); cancer death 136 (4.0%) against 100 (2.9%): HR 1.38 (1.07 to 1.79); death from any cause 548 (16.1%) against 473 (13.8%): HR 1.18 (1.04 to 1.33). Lung cancer 56 against 36 cases (HR 1.59, 99% CI 0.92 to 2.75); colorectal cancer HR 1.00 (0.59 to 1.69). Among MTHFR 677TT carriers (n = 535), HR for cancer death 4.57 (1.55 to 13.5), interaction P = 0.03. The excess in cancer incidence emerged after about 1 year. Classed industry because Alpharma gave a limited grant for the initial phase.
Industry-funded
Lange H, Suryapranata H, De Luca G, Börner C, Dille J, Kallmayer K, et al. Folate therapy and in-stent restenosis after coronary stenting New England Journal of Medicine, 2004.
doi:10.1056/NEJMoa032845 PubMed 15215483
Funding and finding
Participants
636
Follow-up
6 months of treatment and angiography at 6 months; clinical end points to 250 days
Funding
Funded in part by Medice, Iserlohn, Germany. The investigators initiated the study, wrote the article and had full access to the data; data analysis was performed by an independent core laboratory (Diagram).
Finding
Folic acid 1 mg, vitamin B6 5 mg and B12 1 mg intravenously, then daily folic acid 1.2 mg, B6 48 mg and B12 60 µg orally for 6 months (n = 316), against placebo (n = 320). Among 242 and 241 patients with follow-up angiography, restenosis 34.5% against 26.5% (RR 1.30, 95% CI 1.00 to 1.69; P = 0.05); late luminal loss 0.90 against 0.76 mm (P = 0.004). By 250 days, repeat target-vessel revascularisation in 50 (15.8%) against 34 (10.6%) of all patients (P = 0.05). No discontinuation for side effects. The excess was absent in women, patients with diabetes and those with homocysteine of 15 µmol/L or more.
Independent research
Vegrim HM, Dreier JW, Alvestad S, Gilhus NE, Gissler M, Igland J, et al. Cancer risk in children of mothers with epilepsy and high-dose folic acid use during pregnancy JAMA Neurology, 2022.
doi:10.1001/jamaneurol.2022.2977 PubMed 36156660
Funding and finding
Participants
3,379,171
Follow-up
median 7.3 years (IQR 3.5 to 10.9); cancer before age 20 years
Funding
This work was supported by the NordForsk Nordic Program on Health and Welfare (project 83796). The NordForsk Nordic Program on Health and Welfare had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.
Finding
Children of mothers with epilepsy: 5934 (21.4%) exposed to high-dose folic acid (mean 4.3 mg); 18 cancers in exposed and 29 in unexposed children; adjusted HR 2.7 (95% CI 1.2 to 6.3); cumulative risk by age 20 years 1.4% (0.5% to 3.6%) exposed against 0.6% (0.3% to 1.1%) unexposed. Children of mothers without epilepsy: 46,646 (1.4%) exposed (mean 2.9 mg); 69 exposed and 4927 unexposed cases; adjusted HR 1.1 (0.9 to 1.4). Exposure to antiseizure medicines without high-dose folic acid was not associated with cancer (absolute risk 0.6%).
Independent research
Pfeiffer CM, Sternberg MR, Fazili Z, Yetley EA, Lacher DA, Bailey RL, et al. Unmetabolized folic acid is detected in nearly all serum samples from US children, adolescents, and adults Journal of Nutrition, 2015.
doi:10.3945/jn.114.201210 PubMed 25733468
Funding and finding
Participants
2,707
Funding
The authors reported no funding received for this study. All authors were employed by the CDC or the NIH Office of Dietary Supplements.
Finding
Unmetabolised folic acid was detectable (above 0.3 nmol/L) in more than 95% of both users and non-users of folic acid-containing supplements, regardless of demographic group or fasting. Geometric mean UMFA 1.54 nmol/L in users and 0.794 nmol/L in non-users; 5-methyl-THF 48.4 and 30.7 nmol/L. UMFA above 1 nmol/L in 33.2% overall and 21.0% of fasting adults, associated with older age, non-Hispanic black ethnicity, non-fasting, smaller body surface area, higher total folic acid intake and higher red cell folate.
Independent research
Caffrey A, Irwin RE, McNulty H, Strain JJ, Lees-Murdock DJ, McNulty BA, et al. Gene-specific DNA methylation in newborns in response to folic acid supplementation during the second and third trimesters of pregnancy: epigenetic analysis from a randomized controlled trial American Journal of Clinical Nutrition, 2018.
doi:10.1093/ajcn/nqx069 PubMed 29635492
Funding and finding
Registration
ISRCTN19917787
Participants
86
Follow-up
Cord blood at delivery after supplementation from gestational week 14
Funding
Supported by the HSC (Health and Social Care) Research and Development Division of the Public Health Agency, Northern Ireland (Enabling Research Award STL/5043/14) and joint funding (Grant Ref: ES/N000323/1 EpiFASSTT) from the Biology and Biological Sciences Research Council (BBSRC) and the Economic and Social Research Council (ESRC).
Finding
Results read from the published abstract. Continued folic acid, against placebo, gave lower cord blood methylation at LINE-1 (56.3 ± 1.7% against 57.2 ± 2.1%; P = 0.024), IGF2 (48.9 ± 4.4% against 51.2 ± 5.1%; P = 0.021) and BDNF (2.7 ± 0.7% against 3.1 ± 0.8%; P = 0.003); the IGF2 effect was significant only in girls and the BDNF effect only in boys. Individual CpG sites in GRB10 and GRIN3B changed. Nine candidate loci were tested by pyrosequencing; the authors call for a genome-wide approach.
Independent research
Troen AM, Mitchell B, Sorensen B, Wener MH, Johnston A, Wood B, et al. Unmetabolized folic acid in plasma is associated with reduced natural killer cell cytotoxicity among postmenopausal women Journal of Nutrition, 2006.
doi:10.1093/jn/136.1.189 PubMed 16365081
Funding and finding
Participants
105
Funding
Supported by grants from the National Institutes of Health (R01 CA69334; T32 CA80416) and the U.S. Department of Agriculture cooperative research agreement 58-1950-4-401.
Finding
Unmetabolised folic acid was detected in 78% of fasting plasma samples. Natural killer cell cytotoxicity was about 23% lower in women with detectable folic acid (P = 0.04), with a stronger relation at age 60 years or over and with increasing concentrations (P for trend 0.002). Women with a folate-rich diet who also took folic acid supplements above 400 µg/day had lower NK cytotoxicity than those with a low-folate diet and no supplements (P = 0.02), whereas with a low-folate diet (below 233 µg/day) supplement users had greater cytotoxicity (P = 0.01).
Independent research
Paniz C, Bertinato JF, Lucena MR, De Carli E, Amorim PMDS, Gomes GW, et al. A daily dose of 5 mg folic acid for 90 days is associated with increased serum unmetabolized folic acid and reduced natural killer cell cytotoxicity in healthy Brazilian adults Journal of Nutrition, 2017.
doi:10.3945/jn.117.247445 PubMed 28724658
Funding and finding
Registration
RBR-2pr7zp
Participants
30
Follow-up
90 days, samples at baseline, 45 and 90 days
Funding
This study was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP 2012/12912-1) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil; fellowships from CNPq, CAPES and FAPESP.
Finding
Folic acid 5 mg daily raised serum folate about 5-fold; unmetabolised folic acid rose 11.9-fold at 45 days and 5.9-fold at 90 days, exceeding 1.12 nmol/L in 29 of 30 and 26 of 30 participants. The number (P < 0.001) and cytotoxicity (P = 0.003) of natural killer cells fell at 45 and 90 days; mononuclear-cell DHFR mRNA rose at 90 days and IL8 and TNFA mRNA at 45 and 90 days. The authors describe the trial as non-controlled.
Independent research
Ducker GS, Rabinowitz JD. One-Carbon Metabolism in Health and Disease Cell Metabolism, 2017.
doi:10.1016/j.cmet.2016.08.009 PubMed 27641100
Funding and finding
Funding
G.S.D. is supported by a postdoctoral fellowship from the American Cancer Society (PF-15-190-01-TBE). J.D.R. is supported by NIH grants R01CA163591 and P30DK019525 and Stand Up to Cancer.
Finding
Review. Animals require dietary folate. Tetrahydrofolate (THF) binds one-carbon units at N5 and N10 at three oxidation states: 5-methyl-THF, the most reduced, 5,10-methylene-THF and 10-formyl-THF, the most oxidised; folic acid is reduced to dihydrofolate (DHF) and then THF. SHMT1 (cytosol) and SHMT2 (mitochondria) interconvert serine and glycine with THF and 5,10-methylene-THF, reversibly; in rapidly proliferating cultured cells the main direct source of cytosolic one-carbon units is formate released from mitochondria, reassimilated by the trifunctional MTHFD1. TYMS methylates dUMP to dTMP with 5,10-methylene-THF and is the only folate enzyme that alters THF itself, taking two electrons from it and producing DHF; DHFR regenerates THF with NADPH and is the target of trimethoprim and methotrexate, which trap folate as DHF. Purine synthesis incorporates two 10-formyl-THF units; in proliferating cells each new DNA or RNA base needs on average one 10-formyl-THF, and 5,10-methylene-THF is needed for one in four DNA bases. 5-Fluorouracil is a suicide inhibitor of TYMS; methotrexate most potently blocks DHFR, whereas pemetrexed more potently inhibits TYMS and AICAR transformylase; 5-formyl-THF (folinic acid, leucovorin) is given as rescue. 5-Methyl-THF, made by cytosolic NADPH-dependent MTHFR, is consumed only by cobalamin-dependent methionine synthase; BHMT is limited to liver and kidney. SAM inhibits MTHFR and activates CBS; 5-methyl-THF inhibits GNMT. Transsulfuration is the only route that degrades homocysteine. Folate and cobalamin supplementation lower homocysteine without reducing cardiovascular events in the general population. Dietary folate deficiency in adults manifests as macrocytic anaemia.
Independent research
Patanwala I, King MJ, Barrett DA, Rose J, Jackson R, Hudson M, et al. Folic acid handling by the human gut: implications for food fortification and supplementation American Journal of Clinical Nutrition, 2014.
doi:10.3945/ajcn.113.080507 PubMed 24944062
Funding and finding
Registration
NCT02135393
Participants
6
Funding
Supported by the UK Biotechnology and Biological Sciences Research Council (grants BB/F014104/1 and BB/FO14457/1).
Finding
Fifteen minutes after a physiological dose of labelled folic acid, 80 ± 12% of labelled folate in the hepatic portal vein was unmodified folic acid; after labelled 5-formyl-THF, only 4 ± 18% was unmodified and the rest had been converted to 5-methyl-THF. The authors conclude that the human gut converts reduced dietary folates efficiently to 5-methyl-THF but has limited ability to reduce folic acid, so that the liver, with low and variable DHFR activity, carries the reduction and may saturate under chronic exposure.
Independent research
Bailey SW, Ayling JE. The extremely slow and variable activity of dihydrofolate reductase in human liver and its implications for high folic acid intake Proceedings of the National Academy of Sciences of the United States of America, 2009.
doi:10.1073/pnas.0902072106 PubMed 19706381
Funding and finding
Funding
This research was supported by American Heart Association Grant 0150942B and National Institutes of Health Grant HL68165.
Finding
At physiological pH, reduction of folic acid by DHFR per gram of human liver (n = 6) averaged less than 2% of that in rat liver, and DHFR activity varied almost 5-fold among the human samples. In the human liver with the highest activity, folic acid was reduced at 0.02 nmol/min/g, 1300 times more slowly than 7,8-dihydrofolate (26.1 nmol/min/g).
Independent research
Tsang BL, Devine OJ, Cordero AM, Marchetta CM, Mulinare J, Mersereau P, et al. Assessing the association between the methylenetetrahydrofolate reductase (MTHFR) 677C>T polymorphism and blood folate concentrations: a systematic review and meta-analysis of trials and observational studies American Journal of Clinical Nutrition, 2015.
doi:10.3945/ajcn.114.099994 PubMed 25788000
Funding and finding
Funding
Three authors were supported by the CDC and in part by an appointment to the Research Participation Program at CDC administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the US Department of Energy and CDC.
Finding
Results read from the published abstract. In the 6 studies using the microbiological assay, folate followed CC > CT > TT: TT had 13% (95% credible interval 7% to 18%) lower serum or plasma folate and 16% (12% to 20%) lower red cell folate than CC; CT had 7% (1% to 12%) lower serum or plasma folate than CC. The authors suggest that lower blood folate with this variant could have implications for population risk of neural tube defects.
Independent research
Yamada K, Chen Z, Rozen R, Matthews RG. Effects of common polymorphisms on the properties of recombinant human methylenetetrahydrofolate reductase Proceedings of the National Academy of Sciences of the United States of America, 2001.
doi:10.1073/pnas.261469998 PubMed 11742092
Funding and finding
Funding
These studies were supported in part by National Institutes of Health Grant GM24908 (to R.G.M.) and by funds from the Medical Research Council of Canada (to R.R.).
Finding
Human MTHFR is a dimer of about 70 kDa subunits, each with non-covalently bound FAD, that reduces 5,10-methylene-THF to 5-methyl-THF with NADPH; SAM binds the C-terminal regulatory domain as an allosteric inhibitor, and SAH competes with SAM there without inhibiting. Purified recombinant Ala222Val enzyme (677C>T) had the same Km for NADPH and 5,10-methylene-THF, the same Vmax and the same SAM inhibition as wild type, but released FAD about 3-fold faster after dilution to 50 nM subunits at 37 degrees C; 5-methyl-THF and SAM slowed FAD loss. Background cited: cell extracts from TT homozygotes have about 30% of CC activity; about 10-15% of Caucasians of European origin and of Japanese are TT, more in Mediterranean countries and fewer among African Americans; 677C>T is the most frequent genetic cause of mild hyperhomocysteinaemia, seen in TT homozygotes with plasma folate below the median.
Expert consensus
Hickey SE, Curry CJ, Toriello HV. ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing Genetics in Medicine, 2013.
doi:10.1038/gim.2012.165 PubMed 23288205
Funding and finding
Funding
not stated; practice guideline of the American College of Medical Genetics and Genomics.
Finding
More than 25% of Hispanics and 10 to 15% of North American Caucasians are homozygous for the thermolabile c.665C>T (677C>T) variant; c.1286A>C (1298A>C) is milder, and the two are in linkage disequilibrium. Women homozygous for 677C>T should be counselled that they have a modestly increased risk (odds ratio 1.6) of offspring with a neural tube defect, further increased if the fetus is also homozygous. Heterozygosity for 677C>T, homozygosity for 1298A>C and compound heterozygosity appear unlikely to be of clinical significance. There is no evidence that specific treatments reduce risks associated with hyperhomocysteinaemia or MTHFR genotype; an at-risk individual may elect a daily multivitamin or prenatal vitamin. Fortification has raised serum folate and lowered homocysteine and may incidentally reduce risks attributed to the variant. Reaffirmed by the ACMG Board on 27 April 2020 and reclassified as a Clinical Practice Resource because it no longer met the criteria for an evidence-based guideline (Genet Med 2020;22:2125).
Independent research
De-Regil LM, Peña-Rosas JP, Fernández-Gaxiola AC, Rayco-Solon P. Effects and safety of periconceptional oral folate supplementation for preventing birth defects Cochrane Database of Systematic Reviews, 2015.
doi:10.1002/14651858.CD007950.pub3 PubMed 26662928
Funding and finding
Participants
7,391
Follow-up
Supplementation from before conception to up to 12 weeks of pregnancy; outcome at birth or termination
Funding
Internal sources: The University of Liverpool; Evidence and Programme Guidance Unit, Department of Nutrition for Health and Development, World Health Organization; Micronutrient Initiative, Canada. External sources: National Institute for Health Research, UK; The Bill & Melinda Gates Foundation; Centers for Disease Control and Prevention (National Center on Birth Defects and Developmental Disabilities); UNDP/UNFPA/UNICEF/WHO/World Bank Special Programme of Research, Development and Research Training in Human Reproduction. WHO states that donors do not participate in guideline development decisions or in the conduct and interpretation of systematic reviews.
Finding
Neural tube defects, any folate against none (5 trials, 6708 births): RR 0.31 (95% CI 0.17 to 0.58), high-quality evidence; first occurrence (1 trial, 4862 births): RR 0.07 (0.00 to 1.32), no events with folic acid; recurrence (4 trials, 1846 births): RR 0.34 (0.18 to 0.64). The effect did not differ by daily dose (0.36 or 0.4 mg and above) or by use with other micronutrients. Only one trial was judged at low risk of bias. Miscarriage RR 1.10 (0.94 to 1.28; 7391 pregnancies); multiple pregnancy RR 1.38 (0.89 to 2.14; 4 trials, 7280 pregnancies); cleft palate, cleft lip, cardiovascular and other birth defects without clear effect (low-quality evidence). Only one trial reported side effects such as nausea or diarrhoea; no trial assessed neonatal death, maternal blood folate or anaemia at term. The authors state that the best dose, scheme and form, including 5-methyltetrahydrofolate, are not established.
Independent research
MRC Vitamin Study Research Group. Prevention of neural tube defects: results of the Medical Research Council Vitamin Study Lancet, 1991.
doi:10.1016/0140-6736(91)90133-A PubMed 1677062
Funding and finding
Participants
1,817
Follow-up
Each woman until one informative pregnancy or the end of the trial; recruitment from July 1983, stopped in April 1991
Funding
The trial was supported financially by the Medical Research Council.
In-kind support
The capsules were provided free of charge by the Boots Company plc; Hoffmann La Roche checked the potency of the capsule constituents without charge.
Finding
Factorial design: 4 mg folic acid, a mixture of seven other vitamins (A, D, B1, B2, B6, C and nicotinamide), both or neither, one capsule daily from randomisation until 12 weeks of pregnancy. Neural tube defects occurred in 6 of 593 informative pregnancies (1.0%) in the folic acid groups and 21 of 602 (3.5%) in the other groups: relative risk 0.28 (95% CI 0.12 to 0.71), a 72% protective effect; calculated from the counts, an absolute difference of 2.5 percentage points and about 40 women treated per defect prevented. Relative risk for the other vitamins 0.80, not statistically significant. The data monitoring committee recommended stopping the trial on 12 April 1991.
Independent research
Martí-Carvajal AJ, Solà I, Lathyris D, Dayer M. Homocysteine-lowering interventions for preventing cardiovascular events Cochrane Database of Systematic Reviews, 2017.
doi:10.1002/14651858.CD006612.pub5 PubMed 28816346
Funding and finding
Participants
71,422
Follow-up
1 to 7.3 years
Funding
Internal sources: no sources of support provided. External sources: Iberoamerican Cochrane Centre, Spain (academic); Cochrane Heart Group, UK (academic).
Finding
15 RCTs, 71,422 participants, follow-up 1 to 7.3 years. Against placebo, homocysteine-lowering interventions showed no difference in myocardial infarction (7.1% against 6.0%; RR 1.02, 95% CI 0.95-1.10; 12 trials, 46,699 participants), death from any cause (11.7% against 12.3%; RR 1.01, 0.96-1.06; 11 trials, 44,817) or serious adverse events (8.3% against 8.5%; RR 1.07, 1.00-1.14), and were associated with reduced stroke (4.3% against 5.1%; RR 0.90, 0.82-0.99; 10 trials, 44,224). Background: normal total homocysteine is close to 10 umol/L; fasting 12-30 umol/L is termed mild or moderate, 31-100 intermediate and above 100 severe hyperhomocysteinaemia.
Funding unknown
Berry RJ, Li Z, Erickson JD, Li S, Moore CA, Wang H, et al. Prevention of neural-tube defects with folic acid in China New England Journal of Medicine, 1999.
doi:10.1056/NEJM199911113412001 PubMed 10559448
Funding and finding
Participants
247,831
Follow-up
Pregnancies registered from 1 October 1993 to 30 September 1995, with outcome at 20 weeks of gestation or later
Funding
not stated. The project was conducted by the US Centers for Disease Control and Prevention and Beijing Medical University with the Chinese Ministry of Health; women purchased the pills.
Finding
Neural tube defects in 102 fetuses or infants of 130,142 women who took folic acid and 173 of 117,689 who did not. Among women registered before their last menstrual period without folic acid, rates were 4.8 per 1000 pregnancies in the north and 1.0 per 1000 in the south; with periconceptional use, 1.0 and 0.6 per 1000. Reduction in risk with any use 79% in the north and 16% in the south; with periconceptional use 79% and 41% (95% CI 3 to 64); with periconceptional use and over 80% compliance 85% and 40%, rates 0.7 and 0.6 per 1000. Users were about 2 years younger, more often registered before conception and more often primigravid. The authors state that neural tube formation is complete within 28 days after conception.
Industry-funded
Winkels RM, Brouwer IA, Siebelink E, Katan MB, Verhoef P. Bioavailability of food folates is 80% of that of folic acid American Journal of Clinical Nutrition, 2007.
doi:10.1093/ajcn/85.2.465 PubMed 17284745
Funding and finding
Participants
72
Follow-up
4 weeks
Funding
Supported by the Wageningen Centre for Food Sciences [an alliance of major Dutch food industries, the Nederlands Instituut voor Zuivelonderzoek (NIZO), the Nederlandse Organisatie voor toegepast-natuurwetenschappelijk onderzoek (TNO), Maastricht University, Wageningen University, and the Dutch Government].
Finding
Results read from the published abstract. Bioavailability of food folate relative to folic acid was 78% (95% CI 48% to 108%) by the labelled-folate method and 85% (52% to 118%) by change in serum folate. The authors conclude that a diet rich in food folate can improve the folate status of a population more efficiently than generally assumed.
Industry-funded
Brouwer IA, van Dusseldorp M, West CE, Meyboom S, Thomas CM, Duran M, et al. Dietary folate from vegetables and citrus fruit decreases plasma homocysteine concentrations in humans in a dietary controlled trial Journal of Nutrition, 1999.
doi:10.1093/jn/129.6.1135 PubMed 10356077
Funding and finding
Participants
66
Follow-up
4 weeks
Funding
This work was supported by the Zorg Onderzoek Nederland/Dutch Prevention Fund (28-2559), The Hague, and Unilever Research Vlaardingen.
In-kind support
Tablets were supplied by Pharmachemie BV, Haarlem; Abbott Diagnostics provided the IMx diagnostic kits.
Finding
Results read from the published abstract. After 4 weeks folate status improved and homocysteine fell with both the dietary folate and the folic acid diets against placebo. From the additional 350 µg/day food folate and 250 µg/day folic acid, the bioavailability of dietary folate relative to folic acid was calculated as 60% to 98%, depending on the endpoint.
Industry-funded
Ashfield-Watt PA, Pullin CH, Whiting JM, Clark ZE, Moat SJ, Newcombe RG, et al. Methylenetetrahydrofolate reductase 677C-->T genotype modulates homocysteine responses to a folate-rich diet or a low-dose folic acid supplement: a randomized controlled trial American Journal of Clinical Nutrition, 2002.
doi:10.1093/ajcn/76.1.180 PubMed 12081832
Funding and finding
Participants
126
Follow-up
Three 4-month interventions in random order
Funding
Supported by the UK Ministry of Agriculture, Fisheries and Foods (UK Food Standards Agency, project reference N05002). The Kellogg Company of Great Britain provided funding for reimbursement of subjects for the cost of fortified foods.
In-kind support
Folic acid supplements and matched placebos were provided by Peter Black Healthcare Ltd.
Finding
Results read from the published abstract. Exclusion diet with placebo, folate-rich diet of fortified and naturally folate-rich foods providing 400 µg folate/day, and exclusion diet with 400 µg folic acid. Both folate interventions raised plasma folate and lowered homocysteine against the exclusion diet (P ≤ 0.001); plasma folate was higher with the supplement than the diet, without a significant difference in homocysteine (P = 0.72). After the exclusion diet, TT carriers had homocysteine 14.5 against 8.9 µmol/L and plasma folate 14.8 against 19.0 nmol/L in CC; the difference narrowed with increasing folate (P = 0.097 after supplementation).
Funding unknown
Cuskelly GJ, McNulty H, Scott JM. Effect of increasing dietary folate on red-cell folate: implications for prevention of neural tube defects Lancet, 1996.
doi:10.1016/s0140-6736(96)91205-2 PubMed 8596381
Funding and finding
Participants
62
Follow-up
3 months
Funding
not stated in the abstract; full text not accessible
Finding
Results read from the published abstract. Red cell folate rose significantly over 3 months only with the folic acid supplement or fortified foods (P < 0.01 for both). Extra food folate and dietary advice raised food folate intake (P < 0.001 and P < 0.05) without a significant change in red cell folate. The authors judged advice to rely on folate-rich foods to optimise folate status misleading.
Expert consensus
Institute of Medicine, Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Panel on Folate, Other B Vitamins, and Choline, Subcommittee on Upper Reference Levels of Nutrients. Folate. In: Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline National Academies Press, Washington DC, chapter 8, 1998.
doi:10.17226/6015
Funding and finding
Funding
This project was funded by the U.S. Department of Health and Human Services Office of Disease Prevention and Health Promotion; the National Institutes of Health Office of Nutrition Supplements; the Centers for Disease Control and Prevention; Health Canada; the Institute of Medicine; and the Dietary Reference Intakes Corporate Donors' Fund. Contributors to the Fund include Roche Vitamins Inc, Mead Johnson Nutrition Group, Daiichi Fine Chemicals, Inc, Kemin Foods, Inc, M&M Mars, Weider Nutrition Group, and Natural Source Vitamin E Association. The opinions or conclusions expressed herein do not necessarily reflect those of the funders.
Finding
Tolerable Upper Intake Level for adults 1000 µg/day of folate from fortified food or supplements, exclusive of food folate, also applied to pregnancy and lactation. No NOAEL could be set. LOAEL 5 mg/day from case reports of neurological progression in vitamin B12-deficient patients: more than 100 reported cases at 5 mg/day and above, and eight well-documented cases at 0.33 to 2.5 mg/day; in most cases folate maintained haematological remission. Uncertainty factor 5, chosen for the severity of the neurological complications and the use of a LOAEL; not larger because millions of people had taken about 400 µg without reported harm. A UL was set for all adults because the neurological consequences are devastating and irreversible, pernicious anaemia may develop at younger ages in some groups, and B12 deficiency in younger groups is uncertain. Dietary folate equivalents: 1 µg DFE = 1 µg food folate = 0.6 µg folic acid with food = 0.5 µg folic acid on an empty stomach; food folate bioavailability of about 50% rested mainly on Sauberlich et al. (1987), which was not designed as a quantitative bioavailability study, and 85% for folic acid with food on one isotope study, judged probably an underestimate. Under US fortification, it judged regular intake of added folate above 1000 µg unlikely in any group. The periconceptional trials of 0.4 to 5.0 mg were not designed to assess adverse effects, and adverse effects in long-term supplement users or in infants of supplemented mothers had not been investigated systematically. Hypersensitivity reactions are rare but have occurred at 1 mg/day. Folic acid is neurotoxic and epileptogenic in animals, without clear evidence of neurotoxicity in humans. Mental changes, sleep disturbance and gastrointestinal effects at 15 mg/day in one uncontrolled trial were not confirmed by other studies.
Expert consensus
US Preventive Services Task Force, Barry MJ, Nicholson WK, Silverstein M, Chelmow D, Coker TR, et al. Folic acid supplementation to prevent neural tube defects: US Preventive Services Task Force reaffirmation recommendation statement JAMA, 2023.
doi:10.1001/jama.2023.12876 PubMed 37526713
Funding and finding
Funding
The USPSTF is an independent, voluntary body. The US Congress mandates that the Agency for Healthcare Research and Quality (AHRQ) support the operations of the USPSTF.
Finding
Recommends that all persons planning to or who could become pregnant take a daily supplement containing 0.4 to 0.8 mg (400 to 800 µg) of folic acid (A recommendation), starting at least 1 month before conception and continuing through the first 2 to 3 months of pregnancy; high certainty of substantial net benefit. The recommendation does not apply to persons with a previous pregnancy affected by a neural tube defect, at very high risk for other reasons, or taking medicines that block folic acid function, such as methotrexate, carbamazepine and valproic acid. Adequate evidence that folic acid at usual doses is not associated with serious harms. AHRQ staff assisted with the research plan, the commissioning of the evidence review and the writing of the statement, and had no role in its approval or in the decision to submit it.
Independent research
Wald NJ, Morris JK, Blakemore C. Public health failure in the prevention of neural tube defects: time to abandon the tolerable upper intake level of folate Public Health Reviews, 2018.
doi:10.1186/s40985-018-0079-6 PubMed 29450103
Funding and finding
Funding
None
Finding
Argument paper. The IOM derived its LOAEL from 23 studies of patients with pernicious anaemia, mainly from the 1950s, 11 of them single-patient case reports, at a time when folate and B12 deficiency could not be distinguished by assay. The authors attribute the neuropathy to continued B12 deficiency rather than to folic acid; their meta-analysis of the IOM case data by dose showed a non-significant decline, not an increase, in the proportion developing neuropathy with increasing folic acid dose. They conclude that there is no scientific basis for any upper level, that the 1 mg/day limit has obstructed mandatory flour fortification, and that it should be removed. Mandatory fortification was in place in 81 countries in October 2017.
Independent research
McNulty B, McNulty H, Marshall B, Ward M, Molloy AM, Scott JM, et al. Impact of continuing folic acid after the first trimester of pregnancy: findings of a randomized trial of Folic Acid Supplementation in the Second and Third Trimesters American Journal of Clinical Nutrition, 2013.
doi:10.3945/ajcn.112.057489 PubMed 23719554
Funding and finding
Participants
119
Follow-up
Gestational week 14 to delivery
Funding
Supported by the Northern Ireland Department for Employment and Learning (PhD funding to B McNulty).
Finding
Results read from the published abstract. 119 women completed (59 folic acid, 60 placebo). In women who stopped, serum folate fell from 45.7 ± 21.3 to 19.5 ± 16.5 nmol/L between gestational weeks 14 and 36 and homocysteine rose from 6.6 ± 2.3 to 7.6 ± 2.3 µmol/L; continuation prevented these changes and raised red cell folate from 1203 ± 639 to 1746 ± 683 nmol/L. Cord red cell folate 1993 ± 862 against 1418 ± 557 nmol/L (P = 0.001). The authors state that benefits for pregnancy outcomes or early childhood require additional study.

Updated 2026-10-22