Folic acid

Folic Acid (Vitamin B9)

1. Identity: Chemical Names, Natural Sources, and Common Forms

Chemical Identity

Folic acid is an N-acyl-amino acid that is a form of the water-soluble vitamin B9. Folate, formerly known as folacin and sometimes vitamin B9, is the generic term for naturally occurring food folates and folates in dietary supplements and fortified foods, including folic acid. The term "folic" is from the Latin word folium (which means leaf) because it was found in dark-green leafy vegetables.

The crystalline form of folic acid is composed of a pteridine ring, para-aminobenzoic acid, and glutamic acid, and was called pteroylglutamic acid. Folic acid (pteroylmonoglutamic acid) is an orange-yellow crystalline substance that is soluble in water but insoluble in ethanol or less polar organic solvents. The name "folic acid" is reserved for the synthetic form with the fully oxidized pteridine ring and no single-carbon substitution.

Folate (vitamin B9) refers to the many forms of folic acid and its related compounds, including tetrahydrofolic acid (the active form), methyltetrahydrofolate (the primary form found in blood), methenyltetrahydrofolate, folinic acid, folacin, and pteroylglutamic acid. The number of distinct molecular species within the folate family could be as many as 150, due to the combinatorial effect of varying single-carbon and pteridine oxidation states and varying polyglutamate chain lengths.

Distinction Between Folic Acid and Natural Folates

Folic acid is the fully oxidized monoglutamate form of the vitamin that is used in fortified foods and most dietary supplements. More commonly, the primary forms of folate found in natural fresh foods are polyglutamates. Of these polyglutamates, the polyglutamate forms of (6S)-methyltetrahydrofolate and (6S)-5-formyltetrahydrofolate predominate. Whereas the pteridine ring of folic acid is fully oxidized, natural folates in fresh uncooked foods are mostly present as the tetrahydro forms.

Since the primary form of folate which can be absorbed by the human body bears only a single glutamic acid residue, polyglutamates, after ingestion, must be processed enzymatically in the digestive tract prior to absorption. When folic acid is absorbed by the digestive tract, it is eventually reduced to active (6S)-tetrahydrofolic acid by the enzyme dihydrofolate reductase. The first identified folate, folic acid, was likely an artifact due to the lengthy isolation procedures, during which natural folates were largely decomposed by reacting with molecular oxygen, and a minor component among the oxidized products was folic acid.

Physical and Chemical Stability

Folic acid in food is unstable and considerable losses occur during short storage and cooking. However, it is stable to 100°C when protected from light at pH 5.0 to 12.0. The stability of folic acid is greater than naturally occurring folates in most foods. Naturally occurring, reduced folates are chemically labile.

Natural Food Sources

Folate is naturally present in a wide variety of foods, including vegetables (especially dark green leafy vegetables), fruits and fruit juices, nuts, beans, peas, seafood, eggs, dairy products, meat, poultry, and grains. Vegetables (especially asparagus, Brussels sprouts, and dark green leafy vegetables such as spinach and mustard greens), fruits and fruit juices (especially oranges and orange juice), and nuts, beans, and peas (such as peanuts, black-eyed peas, and kidney beans) are particularly rich sources.

Common Supplemental and Pharmaceutical Forms

Folate is available in multivitamins and prenatal vitamins. It is also available in B-complex dietary supplements and supplements containing only folate. Some dietary supplements also contain folate in the monoglutamyl form, 5-MTHF (also known as L-5-MTHF, 5-methyl-folate, L-methylfolate, and methylfolate). The calcium salt of (6S)-methylfolate, also known as L-methylfolate, is commercially available under the trade name Metafolin™ for use as a dietary supplement.

Since 1998, the U.S. Food and Drug Administration (FDA) has required food companies to add folic acid to enriched bread, flour, cornmeal, pasta, rice, and other grain products sold in the United States. Canada has also required the addition of 150 mcg folic acid/100 g to many grains, including enriched pasta, corn meal, and white flour. Many other countries, including Costa Rica, Chile, and South Africa, have also established mandatory folic acid fortification programs.

2. Historical Discovery and Traditional Use

Early Observations and Discovery

Folate was discovered between 1931 and 1943. In the 1930s, Lucy Wills identified a "new hemopoietic factor" in yeast and liver which cured tropical macrocytic anemia in humans and experimental anemia in monkeys. Janet Watson and William B. Castle named the unknown substance, which would ultimately become a form of folate, "Wills' factor." Further studies with this unknown substance showed that it was active against nutritional pancytopenia in monkeys and experimental anemia in chicks, leading to various designations such as vitamin M (monkey) and vitamin Bc (chick).

Other factors with growth-promoting activity for microorganisms such as Lactobacillus casei were given the interim names including folic acid — in recognition of extracts from leafy greens. The saga of folate began when folic acid was isolated from spinach in 1941 and found to function as a growth factor in Streptococcus lactis R.

Isolation, Identification, and Synthesis

Competing pharmaceutical research groups headed by Robert Stokstad at Lederle Laboratories and Joseph John Pfiffner at Parke-Davis Research Laboratory independently isolated factors bearing the biological properties of Wills' factor and other unknown related factors, including folic acid — Lederle Laboratories from a bacterial culture and Parke-Davis Laboratory from yeast and liver as a conjugate of folate. The new vitamin was then crystallized, chemically identified, and synthesized as pteroylglutamic acid and named folic acid between 1943 and 1945. Further studies of the monoglutamic folic acid and the yeast isolate polyglutamyl folate followed through the 1950s and to the present.

Bob Stokstad isolated the pure crystalline form in 1943 and was able to determine its chemical structure while working at Lederle Laboratories. This historical research project, of obtaining folic acid in a pure crystalline form in 1945, was done by the team called the "folic acid boys," under the supervision and guidance of Director of Research Dr. Yellapragada Subbarow, at the Lederle Lab, Pearl River, New York.

Early Pharmacological Applications

This research subsequently led to the synthesis of the antifolate aminopterin, which was used to treat childhood leukemia by Sidney Farber in 1948. In the 1950s and 1960s, scientists began to discover the biochemical mechanisms of action for folate. In 1960, researchers linked folate deficiency to risk of neural tube defects.

Although folic acid is not a botanical herb or traditional medicinal plant with a centuries-long history of folk use in the manner of phytomedicines, the foods richest in folate — particularly dark leafy greens such as spinach, liver, and legumes — have been dietary staples across nearly every human civilization. The recognition of folate's nutritional indispensability emerged from 20th-century science rather than pre-modern herbalism. The discovery of folic acid was first reported in the 1930s, when Wills reported the presence of a substance from yeast called the Wills factor, which could prevent megaloblastic anemia, a potentially fatal condition.

3. Key Constituents, Active Forms, and Mechanisms of Action

Active Folate Metabolites

Almost all of the known physiological functions of folate are performed by tetrahydrofolic acid (THF) or by a one-carbon derivative of it: 5-methyl-THF, 5-formyl-THF, 10-formyl-THF, 5,10-methylene-THF, 5,10-methenyl-THF, and 5-formimino-THF. 5-methylfolate may be the body's preferred form of folate, since it is the predominant form of folate found in humans.

One-Carbon Metabolism

Folate functions as a coenzyme or cosubstrate in single-carbon transfers in the synthesis of nucleic acids (DNA and RNA) and metabolism of amino acids. One-carbon (1C) units can be attached to N5, N10, or both positions of the tetrahydrofolate scaffold. The 1C unit is derived from donors such as serine, exists in three different oxidation states, and is used for various biochemical processes.

10-Formyl groups are passed to intermediates in the de-novo purine synthesis pathway, and 5,10-methylene groups are used to produce thymidylate from deoxyuridine monophosphate (dUMP). Folate polyglutamates are polyglutamated by the addition of up to six to eight glutamate residues. Folate polyglutamates in the form of 10-formyl-THF and 5,10-methylene-THF are the preferred substrates for nucleotide synthesis.

Homocysteine Remethylation

One of the most important folate-dependent reactions is the conversion of homocysteine to methionine. The biologically active forms (tetrahydrofolate and others) are essential for nucleotide biosynthesis and homocysteine remethylation. In this reaction, 5-methyltetrahydrofolate donates its methyl group to homocysteine via the enzyme methionine synthase, requiring vitamin B12 as a cofactor. B-group vitamins (folic acid, pyridoxine, and cobalamin) are involved in homocysteine catabolism; their plasma levels are inversely associated with that of homocysteine.

DNA Synthesis and Cell Division

Folate's biological functions include its roles in one-carbon metabolism, DNA synthesis, red blood cell formation, and cellular replication. Impaired DNA synthesis, which causes precursor cells in the bone marrow to have immature nuclei relative to their cytoplasm, is the hallmark cellular consequence of folate insufficiency. Folic acid is fundamental at the cell level to synthesize DNA, responsible for the transmittal of genetic characters, and also to synthesize RNA, required to form the body's proteins and tissues and other cell processes.

MTHFR Enzyme and Genetic Variation

5-MTHF polyglutamates are formed through the irreversible action of methylenetetrahydrofolate reductase (MTHFR) on 5,10-methylene-THF polyglutamates. Many people with MTHFR mutations are unable to benefit from folic acid in fortified foods or supplements because folic acid is a synthetic form of folate and must be activated in order to be used by the body. Dietary supplements containing methylfolate might be better than folic acid for individuals who have a certain mutation in a gene called MTHFR.

Bioavailability

At least 85% of folic acid is estimated to be bioavailable when taken with food, whereas only about 50% of folate naturally present in food is bioavailable. The FNB (Food and Nutrition Board) developed dietary folate equivalents (DFEs) to reflect the higher bioavailability of folic acid compared with that of food folate. For example, 240 mcg of folic acid and 400 mcg of folate are both equal to 400 mcg DFE.

Folate Receptors and Transport

In specific tissues (e.g., choroid plexus, proximal kidney tubule), a family of receptors named folate receptors (FRs) can transport 5-MTHF or folic acid across cellular barriers by endocytosis, probably linked to the PCFT transporter. Folic acid is mainly absorbed in the jejunum, and the body stores around 5 mg of folate in the liver, which is enough for 3 to 4 months.

4. Scientific Evidence by Area of Use

4.1 Neural Tube Defect Prevention

Evidence strength: Very strong (Grade A recommendation from USPSTF)

Taking folic acid before becoming pregnant and during early pregnancy helps prevent neural tube defects in babies. Neural tube defects are major birth defects in a baby's brain (anencephaly) or spine (spina bifida).

Laurence et al. reported in 1981 the first double-blind randomized controlled trial for the use of folic acid in the periconceptional period, and assessed for the first time that 2 mg/day of folic acid could prevent recurrence of NTDs. However, the methodology used was criticized due to the small number of women included and the a posteriori change of two women with NTD-affected fetuses from the folic acid group to the non-compliers group. In 1991, the Medical Research Council of the United Kingdom published the results from a large multicenter double-blind randomized trial.

The MRC trial was shortly followed by the largest occurrence risk study performed by Czeizel and Dudas, referred to as the "Budapest trial." This randomized, large-scale trial compared NTD prevalence in women receiving 0.8 mg/day of folic acid versus trace element supplements.

A meta-analysis of randomized trials of folic acid for the prevention of recurrent NTDs indicates a 69% reduction in recurrence risk if analyzed on an intention-to-treat basis and an 87% reduction among those women who took supplements prior to the beginning of pregnancy. It is well established that women who have had a pregnancy affected by a neural tube defect have an elevated risk of a subsequent NTD-affected pregnancy and that a high dose (4 mg/day) of folic acid taken around the time of conception prevents most recurrences of NTDs.

The findings of the Hungarian intervention (randomized double-blind and cohort controlled) trials indicated that periconceptional folic acid-containing multivitamin supplementation prevented the major proportion (about 90%) of neural tube defects as well as a certain proportion (about 40%) of congenital heart defects.

In 2017, the US Preventive Services Task Force (USPSTF) concluded that folic acid supplementation in the periconceptional period has substantial benefits in reducing the risk of neural tube defects in the developing fetus and reaffirmed its 2009 recommendation that all persons who are planning or capable of pregnancy take a daily supplement containing 0.4 to 0.8 mg (400–800 μg) of folic acid (an A recommendation). New evidence from observational studies provides continued evidence of benefit of folic acid supplementation for preventing NTDs and no evidence of harms related to multiple gestation, autism, or maternal adverse effects.

The practice of fortifying foods with folate has led to a reduction in the incidence of neural tube defects by as much as 50% in North America.

4.2 Megaloblastic Anemia

Evidence strength: Strong; FDA-approved indication

Folic acid (vitamin B9) is a water-soluble vitamin used to manage and treat megaloblastic anemia. Folic acid has FDA approval for treating megaloblastic and macrocytic anemias due to folic acid deficiency. The classic sign of folic acid deficiency is megaloblastic anemia, characterized by decreased hemoglobin and red cell counts with increased mean corpuscular volume and poikilocytosis.

Low levels of folate lead to macrocytic megaloblastic anemia. A simple blood smear of an individual with a folate deficiency will reveal erythrocyte macrocytosis and hyper-segmented polymorphonuclear cells (PMNs). This abnormal morphology results from impaired DNA synthesis, which causes precursor cells in the bone marrow to have immature nuclei relative to their cytoplasm.

4.3 Cardiovascular Disease and Homocysteine Lowering

Evidence strength: Moderate for homocysteine reduction; inconsistent for clinical cardiovascular endpoints

Since folic acid reduces homocysteine concentrations, to an extent dependent on background folate levels, increasing folic acid consumption might reduce the risk of heart attack and stroke by an amount related to the homocysteine reduction achieved. Meta-analyses of cohort studies show significant positive associations between serum homocysteine concentrations and ischemic heart disease events (fatal and non-fatal myocardial infarction and sudden cardiac death) and stroke.

A 3 μmol/l decrease in serum homocysteine (achievable with 0.8 mg/day folic acid) has been estimated to lower the risk of myocardial infarction by 15% and stroke. However, translating homocysteine lowering into demonstrable clinical event reduction in randomized trials has proven more difficult. Numerous randomized controlled trials have investigated the efficacy of lowering homocysteine with folic acid supplementation for CVD risk, but conflicting results have been reported. Three bibliographic databases were searched from database inception until December 1, 2015. Of the 1,933 references reviewed for eligibility, 30 randomized controlled trials involving 82,334 participants were included in the final analysis.

This meta-analysis indicated a 10% lower risk of stroke and a 4% lower risk of overall CVD with folic acid supplementation. A greater benefit for CVD was observed among participants with lower plasma folate levels and without preexisting CVD. In a separate meta-analysis of 14 RCTs, folic acid supplementation had no effect on overall cardiovascular disease, mortality, and stroke. These divergent meta-analytic findings reflect heterogeneity in patient populations (particularly whether participants were from countries with mandatory fortification), baseline folate status, and concurrent B12 supplementation. Debate remains over whether raised serum homocysteine concentrations cause ischemic heart disease and stroke and whether folic acid, which lowers homocysteine, will help reduce the risk of these disorders.

4.4 Cancer Risk — Colorectal and Other Cancers

Evidence strength: Mixed; observational data suggest protective effects at adequate dietary intake levels; trial data are complex

Observational data suggest that lower folate status is associated with an increased risk of colorectal neoplasia, implying that folate may be useful as a chemopreventive agent. The summary risk estimate for case-control studies comparing high versus low total folate intake was 0.85 (95% CI 0.74–0.99), with no significant heterogeneity among studies, suggesting a modest protective association.

However, findings from randomized supplementation trials are less consistent. One meta-analysis suggested that folic acid treatment was not associated with colorectal cancer risk in the total population (RR = 1.00, 95% CI = 0.82–1.22). Whether or not possible deleterious effects of folic acid supplementation (for example, cancer-promoting effect on established preneoplastic and neoplastic lesions) outweigh the known and potential health benefits is largely unknown at present.

Adverse effects of folate were observed for prostate cancer in some analyses. Current evidence allows for the conclusion that folate is associated with decreased risk of all-cause mortality and a wide range of chronic diseases; however, folate may be associated with an increased risk of prostate cancer. The overall picture is one of a "dual role" for folate in cancer biology: adequate dietary folate intake in normal tissue may suppress early carcinogenesis, whereas pharmacological supplementation in people with established (pre)neoplastic lesions may conceivably promote tumor progression. Both the overall evidence base and the mechanistic basis for this remain under active investigation.

4.5 Depression and Neuropsychiatric Conditions

Evidence strength: Preliminary; supportive associations, limited by small or methodologically heterogeneous trial designs

It is becoming clear that folic acid affects mood and cognitive function, especially in older people. Neuropsychiatric manifestations such as depression, irritability, or cognitive changes may occur in folate deficiency. Neuropsychiatric symptoms may include depression, irritability, cognitive decline, insomnia, fatigue, and psychosis.

Some evidence suggests that folic acid might be a helpful supplement in treating depression, though this evidence is not sufficiently robust for a definitive clinical recommendation independent of folate-deficiency states. Research into L-methylfolate (5-MTHF) as an adjunct to antidepressant therapy has proceeded separately and is outside the scope of folic acid per se.

4.6 Cognitive Function and Dementia

Evidence strength: Inconsistent; some positive signals in specific populations, but no consistent benefit in general dementia prevention

Clinical trials conducted on older adults indicated that oral supplementation with folic acid improved cognitive function in relation to daily-life activities, mental condition, and inflammatory markers, such as interleukin-6 (IL-6) and C-reactive protein (CRP). However, larger trials have produced null results. In one placebo-controlled trial (n = 279) in older adults with mild cognitive impairment and elevated homocysteine, the supplement effectively lowered serum homocysteine (mean 13.9 ± 3.5 μmol/L at baseline to 9.3 ± 2.4 μmol/L at month 24), but at month 24, there was no significant group difference in the clinical dementia rating scale sum of boxes or any secondary outcomes. Vitamin B12 and folic acid supplementation did not reduce cognitive decline in older people with mild cognitive impairment and elevated serum homocysteine. There is not enough evidence to support taking folic acid to prevent dementia.

4.7 Autism Spectrum Disorder (ASD) — Prenatal Exposure

Evidence strength: Preliminary and inconclusive; cannot be considered established

Some studies have shown that taking recommended amounts of folic acid before and during early pregnancy may help reduce the risk of ASD in the child. However, because the study results are inconclusive, more research is needed to understand the potential role of folic acid in lowering the risk of ASD. Inadequate folate during pregnancy has also been associated with an increased risk of autism spectrum disorders, though the direction of this association in the context of supplementation (whether it prevents or is unrelated to ASD risk) remains debated in the literature.

4.8 Folic Acid and Methotrexate Toxicity Mitigation

Evidence strength: Moderate; clinically applied in rheumatological practice

Toxicity caused by the drug methotrexate is a well-documented concern; taking folic acid by mouth seems to reduce nausea and vomiting from methotrexate treatment. This is a recognized clinical application in patients receiving low-dose methotrexate for rheumatoid arthritis and related inflammatory conditions, where folic acid supplementation helps maintain blood folate levels without fully negating methotrexate's anti-inflammatory action.

4.9 Male Reproductive Health

Evidence strength: Preliminary; limited by small trial sizes

Folic acid plays an essential role in the treatment of decreased reproductive function and in the improvement of spermatic quality. In a placebo-controlled study, folic acid and zinc supplementation caused a significant increase in the concentration of spermatozoa while improving the morphology of the same.

5. Body Systems and Health Areas

• Hematopoietic system: Deficiencies of vitamin B12 and folic acid are the leading causes of megaloblastic anemia. Hematological effects of deficiency include megaloblastic anemia, pancytopenia, leukopenia, and thrombocytopenia.
• Reproductive and fetal development: Folate deficiency is a leading cause of megaloblastic anemia during pregnancy in developing countries. Folate requirements may increase by 5- to 10-fold during the perinatal period. Folate deficiency can cause congenital malformations, most notably neural tube defects.
• Cardiovascular system: Folic acid works with vitamins B-6 and B-12 to control high levels of homocysteine in the blood. Too-high homocysteine levels might raise your risk of heart and blood vessel conditions, also called cardiovascular disease.
• Neurological and psychiatric: Folic acid is important for functioning of the nervous system at all ages. Folate deficiency leads to the incorporation of nitrogenous bases in DNA, impairing repair mechanisms in neuronal cells and sensitizing them to oxidative damage and beta-amyloid toxicity. These inflammatory processes contribute to the development of neurodegenerative diseases.
• Gastrointestinal system: Mucocutaneous and gastrointestinal manifestations may present as glossitis, angular stomatitis, and oral ulcers. More generalized signs of deficiency include stomatitis, gingivitis, and diarrhea.
• Oncological context: Folate participates in thymidylate and purine synthesis; both inadequacy and excess have been investigated in the context of cancer biology, particularly colorectal and prostate cancer, with complex and context-dependent results as described above.
• Male reproductive system: In a murine model, paternal insufficient folate intake alters the sperm epigenome, negatively impacting pregnancy outcomes. In oligospermia and asthenospermia, a pathologic increase in homocysteine has been observed.

6. Deficiency: Causes, Populations at Risk, and Clinical Manifestations

Causes of Deficiency

Folic acid deficiency results from inadequate intake, malabsorption, increased physiological demand, or drug interference, and is defined by low folate levels in serum, plasma, or red blood cells. Common causes include poor dietary intake, chronic alcohol use, malabsorption syndromes (e.g., celiac disease), certain medications (e.g., methotrexate and phenytoin), and increased physiological demands (e.g., pregnancy and hemolysis).

Folic acid deficiency may be related to decreased intake in the case of alcohol use disorder or malnutrition (elderly patients, institutionalized patients, poverty, special diets, etc.), increased demand particularly in cases of pregnancy, hemolysis, hemodialysis, and malabsorption (tropical sprue, celiac disease, jejunal resection, Crohn disease, etc.). In some cases, medications such as anticonvulsants and anticancer agents can cause megaloblastic anemia due to folate deficiency by affecting folate metabolism.

Clinical Signs and Symptoms

Deficiency leads to megaloblastic anemia, neuropsychiatric symptoms, and pregnancy complications, including neural tube defects. Symptoms may include fatigue, pallor, weakness, glossitis, and oral ulcers. Neurological symptoms are typically absent in folate deficiency (unlike vitamin B12 deficiency). Anemia without neurological symptoms suggests folate deficiency. Anemia with neurological symptoms suggests a vitamin B12 deficiency.

Other symptoms of vitamin B12 or folate deficiency may include fatigue, weakness, glossitis, gastrointestinal problems (e.g., diarrhea), decreased appetite, changes in taste and smell, and weight loss. These symptoms sometimes precede anemia, in some cases by many years. Megaloblastic anemia, however, is often asymptomatic until the condition is quite severe.

Populations at Elevated Risk

Although most people consume adequate amounts of folate, certain groups, including women of childbearing age and non-Hispanic Black women, are at risk of insufficient folate intakes. Even when intakes of folic acid from dietary supplements are included, 19% of female adolescents age 14–18 years and 17% of women age 19–30 years do not meet the estimated average requirement (EAR). Similarly, 23% of non-Hispanic Black women have inadequate total intakes, compared with 13% of non-Hispanic White women.

7. Dosage: Recommended Intakes and Doses Reported in Studies

Recommended Dietary Allowances (RDA)

The Daily Value (DV) for folate is 400 mcg DFE for adults and children age 4 years and older, where mcg DFE = mcg naturally occurring folate + (1.7 × mcg folic acid). The recommended dietary allowance is 400 mcg in adults and 600 mcg in pregnant women.

Dosages in Neural Tube Defect Prevention Studies

• All women and teen girls who could become pregnant should consume 400 mcg of folic acid daily from supplements, fortified foods, or both, in addition to the folate they get from following a healthy eating pattern.
• The largest occurrence-risk randomized trial used 0.8 mg/day of folic acid compared to trace element supplements.
• A high dose of 4 mg/day of folic acid taken around the time of conception prevents most recurrences of NTDs in women with a prior affected pregnancy.
• Consuming folic acid 600–800 mcg by mouth daily during pregnancy reduces the risk of neural tube defects. Some people who are at high risk should get 4,000–5,000 mcg daily.

Dosages in Cardiovascular Studies

Folic acid doses in cardiovascular randomized controlled trials ranged from 0.5 mg to 5 mg per day. A 3 μmol/l decrease in serum homocysteine — achievable with 0.8 mg/day folic acid — was associated with estimated reductions in myocardial infarction and stroke risk in meta-analytic models.

Dosages in Deficiency Treatment

Treatment of folic acid deficiency involves oral folic acid supplementation (1–5 mg/day), with higher doses recommended during pregnancy or in high-risk groups. Pregnant women should take at least 0.4 mg of folic acid daily, with an increased dosage of 4 to 5 mg/day recommended for those at high risk.

Cognitive Function Studies

In one placebo-controlled RCT in older adults with mild cognitive impairment and elevated homocysteine, 279 outpatients aged ≥65 years with serum homocysteine ≥10.0 μmol/L were randomly assigned to take either methylcobalamin 500 μg and folic acid 400 μg once daily, or two placebo tablets for 24 months.

8. Safety, Tolerable Upper Intake Levels, and Drug Interactions

Tolerable Upper Intake Level

Folic acid intake below the established tolerable upper intake level (UL) of 1,000 μg/day for the general population is not associated with any adverse health outcomes. There is no UL for natural reduced folates found in foods. The UL for the provitamin folic acid was established to avoid a delayed diagnosis of vitamin B12 deficiency, as assessed by hematological indices, and thereby minimize the risk of neurological complications in vitamin B12-deficient individuals.

The Institute of Medicine (IOM) conducted a comprehensive review of folic acid safety and concluded that progression of neuropathy among vitamin B12-deficient individuals receiving folic acid doses ≥5,000 μg/day was a potential adverse effect. The IOM identified 5,000 μg/day as the lowest observed adverse effect level (LOAEL) for folic acid and then established a tolerable upper intake level (UL) of 1,000 μg/day for adults by dividing the LOAEL by an uncertainty factor of five.

Masking of Vitamin B12 Deficiency

Taking large amounts of folate supplements might hide a vitamin B12 deficiency because these supplements can correct the anemia that the vitamin B12 deficiency causes but not the nerve damage that the vitamin B12 deficiency also causes. The vitamin B12 deficiency can lead to permanent damage of the brain, spinal cord, and nerves. Vitamin B12 levels should always be checked before initiating folic acid treatment.

Unmetabolized Folic Acid

The upper tolerable limit of folic acid is 1,000 mcg per day; however, this level was determined to avoid masking a vitamin B12 deficiency and not based on folic acid bioavailability and metabolism. There are two main ways in which a buildup of folic acid in the blood can occur: the first is through excess intake beyond the body's needs, and the second is the inability to fully metabolize folic acid due to genetic mutations, such as those affecting the MTHFR gene. Women of childbearing age do not receive sufficient folate intake from food sources alone even when consuming fortified food products; however, almost all women taking a folic acid-based supplement exceed the upper tolerable limit of folic acid intake.

Fortification Safety

The totality of the evidence examined by expert panels has not established risks for adverse consequences resulting from existing mandatory folic acid fortification programs that have been implemented in many countries. Current folic acid fortification programs have been shown to support public health in populations, and the exposure levels are informed by and adherent to the precautionary principle.

Drug Interactions

• Anticonvulsants (phenytoin, carbamazepine, phenobarbital, valproate): Taking folic acid with carbamazepine, phenytoin, or valproate might lower the amount of the medicine in the blood. Conversely, these same anticonvulsants can reduce folate absorption and increase folate requirements. Anticonvulsants can cause megaloblastic anemia due to folate deficiency by affecting folate metabolism.
• Methotrexate: Taking folic acid with methotrexate when used to treat cancer could affect how well it works. The interaction is context-dependent: folic acid supplementation helps reduce methotrexate toxicity in inflammatory conditions but may compromise its efficacy in oncological use.
• Pyrimethamine: Taking folic acid with pyrimethamine, used to prevent malaria, might affect how well it works.
• Barbiturates: A barbiturate is a medicine that acts as a central nervous system depressant. If you take folic acid with a barbiturate, the medicine might not work as well.
• Methotrexate and other folate antagonists (sulfasalazine, trimethoprim): Certain drugs, such as methotrexate, phenytoin, sulfasalazine, and trimethoprim, interfere with folate utilization or increase breakdown.

MTHFR Variant Considerations

Folic acid supplements can result in intakes above the tolerable upper intake level due to poor metabolism in the body in individuals with MTHFR mutations. All women and teen girls who could become pregnant should consume 400 mcg of folic acid daily from supplements, fortified foods, or both — even if they have an MTHFR C677T gene variant — in addition to the folate they get from following a healthy eating pattern.

Cancer Caution

The EFSA Panel on Nutrition, Novel Foods and Food Allergens conducted systematic reviews to assess evidence on priority adverse health effects of excess intake of folate (including folic acid and other authorized forms), namely risk of cobalamin-dependent neuropathy, cognitive decline among people with low cobalamin status, and colorectal cancer and prostate cancer. Additional research is needed to assess the health effects of folic acid supplement use when the current upper limit for folic acid is exceeded.

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