Vitamin B9 (folate)

Vitamin B9 (Folate): A Comprehensive Reference

1. Identity, Chemical Nomenclature, and Natural Sources

Names and Terminology

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. 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. Historic names included L. casei factor, vitamin Bc, and vitamin M.

The crystalline form of folic acid was found to be composed of a pteridine ring, para-aminobenzoic acid, and glutamic acid, and was called pteroylglutamic acid. Folate is a water-soluble B vitamin that is naturally present in some foods, added to others, and available as a dietary supplement. As the human body cannot make folate, it is required in the diet, making it an essential nutrient.

Chemical Forms

Food folates are in the tetrahydrofolate (THF) form and usually have additional glutamate residues, making them polyglutamates. Folic acid is the fully oxidized monoglutamate form of the vitamin that is used in fortified foods and most dietary supplements. 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).

Dietary folates exist predominantly in the polyglutamyl form (containing several glutamate residues), whereas folic acid — the synthetic vitamin form — is a monoglutamate, containing just one glutamate moiety. In addition, natural folates are reduced molecules, whereas folic acid is fully oxidized. These chemical differences have major implications for the bioavailability of the vitamin such that folic acid is considerably more bioavailable than naturally occurring food folates at equivalent intake levels.

Other synthetic forms include folinic acid and levomefolic acid. Folic acid has no biological activity unless converted into folates. The intestinal absorption of dietary folates is a two-step process that involves the hydrolysis of folate polyglutamates to the corresponding monoglutamyl derivatives, followed by their transport into intestinal cells. There, folic acid is converted into a naturally occurring folate, namely 5-methyltetrahydrofolate, which is the major circulating form of folate in the human body.

Bioavailability and the Dietary Folate Equivalent (DFE)

Because of the difference in bioavailability between supplemented folic acid and the different forms of folate found in food, the dietary folate equivalent (DFE) system was established. One DFE is defined as 1 μg of dietary folate. 1 μg of folic acid supplement counts as 1.7 μg DFE. The reason for the difference is that when folic acid is added to food or taken as a dietary supplement with food it is at least 85% absorbed, whereas only about 50% of folate naturally present in food is absorbed.

DFEs adjust for the nearly 50% lower bioavailability of food folate compared with that of folic acid: 1 μg of dietary folate equivalent = 0.6 μg of folic acid from fortified food or as a supplement taken with meals = 1 μg of food folate = 0.5 μg of a supplement taken on an empty stomach.

Natural Food Sources

The richest natural food sources of folate include yeast, organ meats (e.g., liver, kidney, tongue), green leafy vegetables (e.g., spinach, collard greens), legumes, beans, and some fruits. Folate is found mainly in dark green leafy vegetables, beans, peas, and nuts. Fruits rich in folate include oranges, bananas, melons, and papayas. In adults, normal total body folate is between 10 and 30 mg with about half of this amount stored in the liver and the remainder in blood and body tissues.

The highest reported intakes of folate occur in populations with the highest consumption of vegetables, such as in countries in which the diet is similar to the Mediterranean diet. In January 1998, the U.S. Food and Drug Administration required food manufacturers to add folic acid to foods commonly eaten, including breads, cereals, pasta, rice, and other grain products, to reduce the risk of neural tube defects. This program has helped to increase the average folic acid intake by about 100 mcg/day.

Approved Supplement Forms

Several forms of synthetic folates are used in supplements, many of which are approved by the EFSA for use as a food additive or food supplement, including folic acid, calcium l-methylfolate, and (6S)-5-methyltetrahydrofolic acid, glucosamine salt. Folate is available in multivitamins and prenatal vitamins. It is also available in B-complex dietary supplements and supplements containing only folate. In dietary supplements, folate is usually in the form of folic acid, but methylfolate (5-MTHF) is also used.

2. Historical Discovery and Traditional Use

Scientific Discovery (1931–1945)

Folate was discovered between 1931 and 1943. In 1931, researcher Lucy Wills made a key observation that led to the identification of folate as the nutrient required to prevent anemia during pregnancy. Wills demonstrated that anemia could be reversed with brewer's yeast. 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 B(c) (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. Bob Stokstad isolated the pure crystalline form in 1943 and was able to determine its chemical structure while working at the Lederle Laboratories of the American Cyanamid Company. 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.

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.

Early Medical Use and Public Health Application

Folic acid has a long history of use, in conjunction with vitamin B12, for treatment of macrocytic (or megaloblastic) anemia. In 1960, researchers linked folate deficiency to risk of neural tube defects, an observation that eventually drove landmark clinical trials and public health policy. Folate is on the World Health Organization's List of Essential Medicines. In 2023, it was the 94th most commonly prescribed medication in the United States, with more than 7 million prescriptions.

3. Key Constituents and Mechanisms of Action

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. The only function of folate coenzymes in the body appears to be in mediating the transfer of one-carbon units. Folate coenzymes act as acceptors and donors of one-carbon units in a variety of reactions critical to the metabolism of nucleic acids and amino acids.

Tetrahydrofolate's main function in metabolism is transporting single-carbon groups (i.e., a methyl group, methylene group, or formyl group). These carbon groups can be transferred to other molecules as part of the modification or biosynthesis of a variety of biological molecules. Folates are essential for the synthesis of DNA, the modification of DNA and RNA, the synthesis of methionine from homocysteine, and various other chemical reactions involved in cellular metabolism. These reactions are collectively known as folate-mediated one-carbon metabolism.

Homocysteine Conversion and the Methyl Trap

Methyl-THF converts vitamin B12 to methyl-B12 (methylcobalamin). Methyl-B12 converts homocysteine, in a reaction catalyzed by homocysteine methyltransferase, to methionine. A defect in homocysteine methyltransferase or a deficiency of B12 may lead to a so-called "methyl-trap" of THF, in which THF converts to methyl-THF, causing a deficiency in folate. Thus, a deficiency in B12 can cause accumulation of methyl-THF, mimicking folate deficiency.

MTHFR Enzyme and Genetic Variation

Integral to methionine production is the enzyme 5,10-methylenetetrahydrofolate reductase (MTHFR). MTHFR generates the folate-derived methyl-group for the creation of methionine by catalyzing the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate. This reaction is part of the folate cycle, which additionally includes the enzyme methylenetetrahydrofolate dehydrogenase (MTHFD1). Moderately raised homocysteine concentrations occur as a result of a mutation in the methylenetetrahydrofolate reductase gene (MTHFR). A base pair substitution of cytosine for thymidine reduces the activity of the enzyme. People who are homozygous for the mutant gene (TT) have homocysteine concentrations about 25% higher than people who are homozygous for the normal gene (CC), although the effect varies between populations because it is dependent on environmental factors such as dietary folate.

Cell Division and DNA Synthesis

Folate is a required coenzyme for the synthesis of the amino acid methionine, and for making RNA and DNA. Therefore, rapidly dividing cells are most affected by folate deficiency. Red blood cells, white blood cells, and platelets are continuously being synthesized in the bone marrow from dividing stem cells. When folate is deficient, cells cannot divide normally. A consequence of folate deficiency is macrocytic or megaloblastic anemia. Macrocytic and megaloblastic mean "big cell," and anemia refers to fewer red blood cells or red blood cells containing less hemoglobin. Macrocytic anemia is characterized by larger and fewer red blood cells. It is caused by red blood cells being unable to produce DNA and RNA fast enough — cells grow but do not divide, making them large in size.

4. Scientific Evidence by Area of Use

4.1 Neural Tube Defect Prevention

Evidence Strength: Strong — supported by landmark RCTs and endorsed by major health authorities.

Neural tube defects (NTDs) are common complex congenital malformations resulting from failure of the neural tube closure during embryogenesis. It is established that folic acid supplementation decreases the prevalence of NTDs, which has led to national public health policies regarding folic acid.

In 1991, the Medical Research Council (MRC) Vitamin Study Group reported the results of a well-designed, prospective, randomized trial of folic acid supplementation for the prevention of NTDs in pregnancies of women who had a previous child with an NTD; the results of the MRC study conclusively demonstrated that a daily dosage of 4000 μg (4 mg) of folic acid, in addition to folate in the diet, before and during early pregnancy resulted in a 71% reduction of recurrence of NTDs.

The results of 2 randomized controlled trials and several observational studies showed that 50% or more of NTDs can be prevented if women consume a folic acid–containing supplement before and during the early weeks of pregnancy in addition to the folate in their diet. The evidence on anaemia consisted of four intervention trials demonstrating effectiveness of folic acid supplementation during pregnancy in reducing the risk of megaloblastic anaemia (relative risk (RR) = 0.21; 95% CI = 0.11, 0.38). Maternal folic acid use was also significantly inversely related to the prevention of NTD at birth (RR = 0.31; 95% CI = 0.16, 0.60) and NTD recurrence (RR = 0.30; 95% CI = 0.14, 0.65).

A cohort study evaluated the pregnancy outcomes of 130,142 women in 3 provinces in China who were asked during their premarital medical examination to take a 0.4-mg daily folic acid supplement. Periconceptional use of a folic acid supplement was associated with an approximately 40% to 80% reduction in risk for NTD-affected pregnancies.

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 cancer, and is consistent with the previously reviewed evidence on this topic. Ethical and logistical issues constrain the conduct of new randomized, controlled trials of folate supplementation versus placebo. All newly available evidence is observational and offers limited ability to control for confounding (including from mandatory food fortification), selection bias, recall bias, and attrition.

Dietary supplements containing 5-MTHF might be better than folic acid for some individuals who have a gene variant called MTHFR C677T because their bodies can use this form more easily. However, all women and teen girls who could become pregnant should get 400 mcg a day of folic acid, not 5-MTHF, even if they have an MTHFR C677T gene variant. No clinical trials show that other types of folate supplements, like MTHF, can prevent neural tube defects.

4.2 Megaloblastic (Macrocytic) Anemia

Evidence Strength: Well established; direct clinical evidence supports treatment of folate-deficiency anemia.

Folate in the form of folic acid is used to treat anemia caused by folate deficiency. Folate is needed to produce healthy red blood cells and is critical during periods of rapid growth, such as during pregnancy and fetal development. An important clinical caveat is that taking folic acid supplements can help correct low blood counts, but it won't repair nerve damage from not having enough vitamin B-12. The interaction between folate and B12 status is therefore clinically important: a complex interaction occurs between folic acid, vitamin B12, and iron. A deficiency of folic acid or vitamin B12 may mask the deficiency of iron; so when taken as dietary supplements, the three need to be in balance.

4.3 Cardiovascular Disease and Stroke

Evidence Strength: Mixed. Folate reliably lowers homocysteine, but this does not consistently translate into reduced cardiovascular events. A possible modest benefit for stroke prevention has emerged from meta-analyses.

Folate-rich diets have been associated with decreased risk of CVD, including coronary artery disease, myocardial infarction (heart attack), and stroke. 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.

Three bibliographic databases were searched; 30 randomized controlled trials involving 82,334 participants were included in the final analysis. The pooled relative risks of folic acid supplementation compared with controls were 0.90 (95% CI 0.84–0.96; P=0.002) for stroke, 1.04 (95% CI 0.99–1.09; P=0.16) for coronary heart disease, and 0.96 (95% CI 0.92–0.99; P=0.02) for overall CVD. 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.

Compared to control, folic acid decreased homocysteine levels in all trials. The pooled net reduction was 2.9 μmol/L (95% CI 2.4 to 3.4). Compared to control, folic acid had no effect on the primary clinical outcomes, risk of cardiovascular disease, coronary heart disease, stroke, or all-cause mortality.

In a Cochrane review update, there were no differences in effects of homocysteine-lowering interventions in the form of supplements of vitamins B6, B9, or B12 given alone or in combination compared with placebo on myocardial infarction or death from any cause. In terms of stroke, this review found a small difference in effect favouring homocysteine-lowering interventions. The review showed that supplementary vitamin B6, B12, and folic acid administration did not prevent cardiovascular events in participants with or without pre-existing cardiovascular disease.

The analysis of clinical trials of B-vitamin supplementation has shown that lowering homocysteine concentrations did not prevent the occurrence of a second cardiovascular event in patients with existing CVD. Consequently, the American Heart Association recommends screening for elevated total homocysteine concentrations only in "high risk" individuals.

4.4 Cognitive Function and Neurological Health

Evidence Strength: Preliminary to moderate. Associations between low folate and cognitive decline are consistent in observational studies; intervention evidence is limited and context-dependent.

Folates are important in the nervous system at all ages and there is growing evidence of their involvement in the ageing brain, especially in mood and cognitive function. Older individuals with low folate status are at higher risk of cognitive impairment, dementia, and/or Alzheimer's disease, and it has been postulated that the effect of folate deficiency on brain function is mediated by homocysteine.

In a trial reported in The Lancet, Jane Durga and colleagues found a favourable effect of folic acid supplementation on cognitive decline in adults aged 50–70 years. By design, the trial was focused on people whose folate status was inadequate, as shown by a raised homocysteine concentration in the absence of other disorders or diseases. The trial was well designed and unique in its approach of targeting individuals who might benefit from folate supplementation.

Evidence for the efficacy of folate in improving cognitive symptoms is equivocal, but most studies used folic acid. Low concentrations of folate in serum, red cells, and cerebrospinal fluid are associated with depression and dementia in a wide range of clinical neuropsychiatric settings, as is raised plasma homocysteine.

4.5 Depression and Psychiatric Disorders

Evidence Strength: Preliminary to moderate. Observational data are consistent; trials exploring folate as adjunct therapy are promising but limited.

Depressive symptoms are the most common neuropsychiatric manifestation of folate deficiency. Conversely, borderline low or deficient serum or red blood cell folate levels have been detected in 15–38% of adults diagnosed with depressive disorders. Recently, low folate levels have been linked to poorer antidepressant response to selective serotonin reuptake inhibitors.

Individuals with depression have lower serum levels of folate and dietary folate intake than individuals without depression. Given that previous literature suggested folate supplementation improved the efficacy of traditional antidepressant medications, future research on folate supplementation in depression is warranted.

Multiple studies and systematic reviews have investigated the efficacy of folic acid and its derivative, L-methylfolate, which can cross the blood–brain barrier, as stand-alone or adjunct therapies for depression. Although findings have been mixed, the available evidence generally supports the use of these compounds in depressed individuals. Recent studies have established links between the one-carbon cycle, folate–homocysteine balance, immune system function, glutamate excitation via NMDA (N-methyl-D-aspartate) receptors, and gut microbiome eubiosis in mood regulation.

The limited available evidence suggests folate may have a potential role as a supplement to other treatment for depression. It is currently unclear if this is the case both for people with normal folate levels, and for those with folate deficiency.

Evidence of a possible role of impaired folate metabolism in depression is suggested by a finding that patients homozygous for an abnormal variant of the methylenetetrahydrofolate reductase gene experience more severe depression (odds ratio = 1.69; 95% CI = 1.09–2.62). This study has not been replicated, however, and was associated with a relatively modest odds ratio. The use of genotyping to predict the effectiveness of folate supplementation of antidepressants thus needs further critical examination in appropriately powered studies.

4.6 Cancer — Colorectal and Other Cancers

Evidence Strength: Mixed and context-dependent. Dietary folate is associated with lower colorectal cancer risk in observational data, but supplemental folic acid — especially at high doses after preneoplastic lesions are established — may promote cancer progression. The overall picture is one of a "dual-modulator" effect.

Several epidemiological studies have suggested an inverse association between folate intakes and status and the risk of colorectal, lung, pancreatic, esophageal, stomach, cervical, ovarian, breast, bladder, and other cancers. Research has not established the precise nature of folate's effect on carcinogenesis, but scientists hypothesize that folate might influence cancer development through its role in one-carbon metabolism and subsequent effects on DNA replication and cell division.

Evidence also indicates that folate might play a dual role in cancer initiation and progression. That is, folate might suppress some types of cancer during the early stages of development, whereas high doses of folic acid taken after preneoplastic lesions have been established might promote cancer development and progression. Results from clinical trials involving folic acid supplementation have been mixed.

Several epidemiological studies have found inverse associations between high dietary folate intakes and the risk of colorectal adenoma and cancer. For example, in the NIH-AARP Diet and Health Study, a cohort study of more than 525,000 people age 50 to 71 years in the United States, individuals with total folate intakes of 900 mcg/day or higher had a 30% lower risk of colorectal cancer compared with those with lower intakes.

In a combined analysis of two trials in Norway (where foods are not fortified with folic acid), supplementation with 800 mcg/day folic acid plus 400 mcg/day vitamin B12 for a median of 39 months in 3,411 people with ischemic heart disease increased cancer incidence rates by 21% and cancer mortality rates by 38% compared with no supplementation. Findings from these Norwegian trials have raised concerns about folic acid supplementation's potential to raise cancer risk.

A "dual-modulator" role for folate in colorectal carcinogenesis has been proposed in which moderate dietary increases initiated before the establishment of neoplastic foci have a protective influence, whereas excessive intake or increased intake once early lesions are established increases tumorigenesis.

A higher folate intake is associated with a decreased colorectal cancer risk in observational studies, but recent evidence suggests that excessive folate supplementation may increase colorectal cancer risk in some individuals. In the large NIH-AARP Diet and Health Study that included 8.5 years of postfortification follow-up, folate intake was associated with a decreased colorectal cancer risk. Given that the adenoma-carcinoma sequence may take ≥10 years, additional follow-up time is needed to fully examine the effect of folic acid fortification.

5. Body Systems and Health Areas Associated with Folate

• Hematological system: Folate is required for the body to make DNA and RNA and metabolise amino acids necessary for cell division and maturation of blood cells. Deficiency causes megaloblastic anemia.
• Reproductive and developmental: Sufficient intakes of folate during pregnancy are recommended to reduce the risk of neural tube defects (NTDs) and other birth defects (e.g., congenital heart defects and cleft lip palate).
• Cardiovascular system: Elevated plasma homocysteine is one of the primary consequences of folate deficiency, which is a recognized risk factor for cardiovascular disease.
• Nervous system: In the fetus, the relation between maternal folate status and the risk of neural tube defects is well established. In neonates, infants, children, and adolescents, inborn errors of folate transport and metabolism are associated with a variety of overlapping syndromes which are influenced by age of clinical presentation.
• Mental health: Patients with depression often have a functional folate deficiency; the severity of such deficiency, indicated by elevated homocysteine, correlates with depression severity; low folate is associated with poor antidepressant response, and folate is required for the synthesis of neurotransmitters implicated in the pathogenesis and treatment of depression.
• Oncological: Folate's role in DNA synthesis and methylation intersects with cancer biology in a complex, dose- and timing-dependent manner, as described above.
• Gastrointestinal: People who have conditions that prevent the small intestine from taking in nutrients from foods may have low folate levels. Examples include celiac disease and inflammatory bowel disease.

6. Dosage Forms and Doses Reported in Studies

The RDA for both men and women is 400 μg/day of dietary folate equivalents (DFEs). Pregnant and lactating women require 600 mcg DFE and 500 mcg DFE, respectively. To reduce the risk of neural tube defects for women capable of becoming pregnant, the recommendation is to take 400 μg of folic acid daily from fortified foods, supplements, or both in addition to consuming food folate from a varied diet.

Doses studied in major clinical trials include:

• A daily dosage of 4000 μg (4 mg) of folic acid in the 1991 MRC trial for recurrence prevention of NTDs resulted in a 71% reduction of recurrence.
• 800 mcg/day folic acid plus 400 mcg/day vitamin B12 for a median of 39 months in 3,411 people with ischemic heart disease (the Norwegian trials).
• Marketed dietary supplements contain folic acid and 5-methyltetrahydrofolic acid (5-MTHF) with recommended dose levels ranging from 0.4 to 0.8 mg per day.
• Women with obesity are recommended a higher dose of 5 mg folic acid to mitigate the risk of NTDs.
• In the rheumatoid arthritis/methotrexate context, patients were started on oral methotrexate at a dose of 10 mg per week and randomized to receiving folic acid at a dose of 10 mg/week or 30 mg/week for 24 weeks.

Even a small dose of folic acid, such as 200 to 400 mcg per day, may not be completely metabolized until the next dose is taken, raising issues of unmetabolized folic acid.

7. Safety Considerations and Drug Interactions

Tolerable Upper Intake Level and Cancer Concern

Systematic reviews assessed evidence on priority adverse health effects of excess intake of folate, including risk of cobalamin-dependent neuropathy, cognitive decline among people with low cobalamin status, and colorectal cancer and prostate cancer. The evidence is insufficient to conclude on a positive and causal relationship between the dietary intake of folate and impaired cognitive function, risk of colorectal and prostate cancer. The risk of progression of neurological symptoms in cobalamin-deficient patients is considered as the critical effect to establish an upper limit (UL) for folic acid.

High doses of folic acid might increase the risk of colorectal cancer and possibly other cancers in some people. High doses can also lead to more folic acid in the body than it can use, but whether these increased folic acid levels are harmful is not completely clear.

Masking Vitamin B12 Deficiency

A defect in homocysteine methyltransferase or a deficiency of B12 may lead to a so-called "methyl-trap" of THF, in which THF converts to methyl-THF, causing a deficiency in folate. Thus, a deficiency in B12 can cause accumulation of methyl-THF, mimicking folate deficiency. A clinically important consequence is that taking folic acid supplements can help correct low blood counts, but it won't repair nerve damage from not having enough vitamin B-12. This means that high-dose folic acid supplementation can correct the hematological presentation of B12 deficiency while masking the progressive neurological damage.

Unmetabolized Folic Acid (UMFA)

Unlike folate, not all of the folic acid consumed is converted into the active form of vitamin B9 (5-MTHF) in the digestive system. Instead, some folic acid is converted to 5-MTHF in the liver. Yet, this process is slow and inefficient in some people. After taking a folic acid supplement, it takes time for the body to convert all of it to 5-MTHF.

Alcohol Interactions

Folate deficiency is common in alcoholics, attributed to both inadequate diet and an inhibition in intestinal processing of the vitamin. Chronic alcohol use inhibits both the digestion process of dietary folate polyglutamates and the uptake phase of liberated folate monoglutamates.

Drug Interactions

Folate supplements could interfere with methotrexate (Rheumatrex, Trexall) when taken to treat cancer. Taking antiepileptic or antiseizure medications, such as phenytoin (Dilantin), carbamazepine (Carbatrol, Tegretol, Equetro, Epitol), and valproate (Depacon), could reduce blood levels of folate.

The coadministration of folic acid or its derivatives may reduce methotrexate's effectiveness in treating neoplastic diseases by competing for active transport across cell membranes. By contrast, these side effects are similar to folate deficiency and can be prevented by supplementing methotrexate with folic acid when methotrexate is used at low doses for autoimmune diseases such as rheumatoid arthritis. The important clinical distinction is the dose and indication: large doses of methotrexate are used to treat some cancers, and the drug's anti-cancer activity results from its interference with folate, so cancer patients taking methotrexate should not take supplemental folic acid. However, the actions of MTX on folic acid are not related to its ability to reduce inflammation and joint damage at the low doses used in treating rheumatoid arthritis.

Pyrimethamine (Daraprim), a medicine used to prevent malaria, may be affected by taking folic acid with this medicine.

Populations at Particular Risk of Deficiency

Individuals in the early stages of folate deficiency may not show obvious symptoms, but blood concentrations of homocysteine may increase. Yet, the concentration of circulating homocysteine is not a specific indicator of folate status, as elevated homocysteine can be the result of vitamin B12 and other B-vitamin deficiencies, lifestyle factors, and renal insufficiency. Populations with documented higher risk include pregnant women, individuals with malabsorption syndromes, heavy alcohol users, and those with MTHFR polymorphisms. Some studies show iron–folic acid supplementation in children under five may result in increased mortality due to malaria; this has prompted the World Health Organization to alter their iron–folic acid supplementation policies for children in malaria-prone areas, such as India.

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