Folate is a generic term referring to both natural folates in food and folic acid, the synthetic form used in supplements and fortified food. Folate is critical in the metabolism of nucleic acid precursors and several amino acids, as well as in methylation reactions.
Function and One-Carbon Metabolism
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.
Nucleic Acid Metabolism
Folate coenzymes play a vital role in DNA metabolism through two different pathways: (1) The synthesis of DNA from its precursors (thymidine and purines) is dependent on folate coenzymes. (2) A folate coenzyme is required for the synthesis of methionine from homocysteine, and methionine is required for the synthesis of S-adenosylmethionine (SAM).
Amino Acid Metabolism
Folate coenzymes are required for the metabolism of several important amino acids, namely methionine, cysteine, serine, glycine, and histidine. The synthesis of methionine from homocysteine is catalyzed by methionine synthase, an enzyme that requires not only folate (as 5-methyltetrahydrofolate) but also vitamin B12.
SAM is a methyl group (one-carbon unit) donor used in most biological methylation reactions, including the methylation of a number of sites within DNA, RNA, proteins, and phospholipids. The methylation of DNA plays a role in controlling gene expression and is critical during cell differentiation. Aberrations in DNA methylation have been linked to the development of cancer.
Nutrient Interactions
The metabolism of homocysteine, an intermediate in the metabolism of sulfur-containing amino acids, provides an example of the interrelationships among nutrients necessary for optimal physiological function and health. Healthy individuals utilize two different pathways to metabolize homocysteine.
Vitamin B12 and B6
One pathway (methionine synthase) synthesizes methionine from homocysteine and is dependent on both folate and vitamin B12 as cofactors. The other pathway converts homocysteine to another amino acid, cysteine, and requires two vitamin B6-dependent enzymes.
Riboflavin
Although less well recognized, folate has an important metabolic interaction with riboflavin. Riboflavin is a precursor of flavin adenine dinucleotide (FAD), a coenzyme required for the activity of the folate-metabolizing enzyme, 5,10-methylenetetrahydrofolate reductase (MTHFR).
Thus, the concentration of homocysteine in the blood is regulated by three B-vitamins: folate, vitamin B12, and vitamin B6. In some individuals, riboflavin (vitamin B2) is also involved in the regulation of homocysteine concentrations.
Bioavailability and Transport
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.
Bioavailability
The bioavailability of naturally occurring folates is inherently limited and variable. There is much variability in the ease with which folates are released from different food matrices, and the polyglutamyl “tail” is removed (de-conjugation) before uptake by intestinal cells. The bioavailability of folic acid, in contrast, is assumed to be 100% when ingested as a supplement, while folic acid in fortified food is estimated to have about 85% the bioavailability of supplemental folic acid.
Transport
Folate and its coenzymes require transporters to cross cell membranes. Folate transporters include the reduced folate carrier (RFC), the proton-coupled folate transporter (PCFT), and the folate receptor proteins, FRα and FRβ. Folate homeostasis is supported by the ubiquitous distribution of folate transporters, although abundance and importance vary among tissues.
Of note, folate recommendations in the US and certain other countries are now expressed as Dietary Folate Equivalents (DFEs), a calculation that was devised to take into account the greater bioavailability of folic acid compared to naturally occurring dietary folates.
Deficiency
Although folate deficiency is uncommon in the United States, it is estimated to be more prevalent worldwide. Most often caused by a dietary insufficiency, folate deficiency can also occur in a number of other situations.
Causes
- Chronic, heavy alcohol consumption is associated with diminished absorption of folate
- Smoking is also associated with low folate status
- Pregnancy is a time when the folate requirement is greatly increased
- Conditions such as cancer or inflammation can result in increased rates of cell division and metabolism
- Folate deficiency can result from some malabsorptive conditions, including inflammatory bowel diseases and celiac disease
- Several medications can contribute to folate deficiency
Symptoms
Clinical folate deficiency leads to megaloblastic anemia, which is reversible with folic acid treatment. Rapidly dividing cells like those derived from bone marrow are most vulnerable to the effects of folate deficiency since DNA synthesis and cell division are dependent on folate coenzymes.
When folate supply to the rapidly dividing cells of the bone marrow is inadequate, blood cell division is reduced, resulting in fewer but larger red blood cells. This type of anemia is called megaloblastic or macrocytic anemia, referring to the enlarged, immature red blood cells.
Progression of such an anemia leads to a decreased oxygen carrying capacity of the blood and may ultimately result in symptoms of fatigue, weakness, and shortness of breath. It is important to point out that megaloblastic anemia resulting from folate deficiency is identical to the megaloblastic anemia resulting from vitamin B12 deficiency, and further clinical testing is required to diagnose the true cause of megaloblastic anemia.
Recommended Dietary Allowance and Genetic Variation
The most recent recommended dietary allowance (RDA) (1998) was based primarily on the adequacy of red blood cell folate concentrations at different levels of folate intake, as judged by the absence of abnormal hematological indicators. Red cell folate has been shown to correlate with liver folate stores and is used as an indicator of long-term folate status.
| Life Stage | Age | Males (μg/day) | Females (μg/day) |
| Infants | 0-6 months | 65 (AI) | 65 (AI) |
| Infants | 7-12 months | 80 (AI) | 80 (AI) |
| Children | 1-3 years | 150 | 150 |
| Children | 4-8 years | 200 | 200 |
| Children | 9-13 years | 300 | 300 |
| Adolescents | 14-18 years | 400 | 400 |
| Adults | 19 years and older | 400 | 400 |
| Pregnancy | all ages | – | 600 |
| Breastfeeding | all ages | – | 500 |
Genetic Variation in Folate Requirements
A common polymorphism or variation in the sequence of the gene for the enzyme, 5, 10-methylenetetrahydrofolate reductase (MTHFR), known as the MTHFR c.677C>T polymorphism, results in a thermolabile enzyme. Depending on the population, 20% to 53% of individuals may have inherited one T copy (677C/T genotype), and 3% to 32% of individuals may have inherited two T copies (677T/T genotype) for the MTHFR gene.
MTHFR activity is greatly diminished in heterozygous 677C/T (-30%) and homozygous 677T/T (-65%) individuals compared to those with the 677C/C genotype. Homozygosity for the mutation (677T/T) is linked to lower concentrations of folate in red blood cells and higher blood concentrations of homocysteine.
Disease Prevention: Adverse Pregnancy Outcomes
Fetal growth and development are characterized by widespread cell division. Adequate folate is critical for DNA and RNA synthesis. Neural tube defects (NTDs) arise from failure of embryonic neural tube closure between the 21st and 28th days after conception, a time when many women may not even realize they are pregnant.
Neural Tube Defects
NTDs include various malformations, such as lesions of the brain (e.g., anencephaly, encephalocele) or lesions of the spine (spina bifida), which are devastating and life-threatening. The worldwide prevalence of NTDs is estimated to be 2 per 1,000 births, which translates to 214,000-322,000 cases annually.
Prevention Through Supplementation
Results of randomized trials have demonstrated 60% to 100% reductions in NTD cases when women consumed folic acid supplements in addition to a varied diet during the periconceptional period (about one month before and at least one month after conception).
Fortification Programs
To decrease the incidence of NTDs in the US, the Food and Drug Administration implemented legislation in 1998 requiring the fortification of all enriched grain products with 1.4 mg of folic acid per kg of grain. As a result of this fortification mandate, a 30% decrease in the prevalence of NTDs was noted compared to the pre-fortification period.
The US Public Health Service recommends that all persons capable of becoming pregnant consume 400 μg of folic acid daily to prevent NTDs. Those with a previously affected pregnancy were also advised to receive 4,000 μg (4 mg) of folic acid daily in order to reduce NTD recurrence. To date, 58 nations worldwide have established mandatory programs of folic acid fortification of staple grains, and an estimated 22% of NTDs have been prevented through such programs.
Other Pregnancy Outcomes
Several studies have investigated the role of folic acid supplementation in the prevention of congenital anomalies other than NTDs. Using data from the European Registration of Congenital Anomalies and Twins (EUROCAT) database, a case-control study found that consumption of at least 400 μg/day of folic acid during the periconceptual period was associated with an 18% reduced risk of congenital heart defects. In a recent systematic review and meta-analysis of 39 observational studies, use of folic acid-containing supplements preconceptionally or during pregnancy was associated with a 42% lower risk of cleft lip with or without cleft palate.
Disease Prevention: Cardiovascular Disease and Cancer
The results of more than 80 observational studies indicate that even moderately elevated concentrations of homocysteine in the blood increase the risk of cardiovascular disease (CVD). Possible predispositions to vascular accidents have also been linked to genetic deficiencies in homocysteine metabolism in certain populations.
Cardiovascular Disease
Folate-rich diets have been associated with decreased risk of CVD, including coronary artery disease, myocardial infarction (heart attack), and stroke. Of the three B vitamins that regulate homocysteine concentrations, folic acid has been shown to have the greatest effect in lowering basal concentrations of homocysteine in the blood when there is no coexisting deficiency of vitamin B12 or vitamin B6.
A meta-analysis of 25 randomized controlled trials, including almost 3,000 subjects, found that folic acid supplementation with 800 μg/day or more could achieve a maximal 25% reduction in plasma homocysteine concentrations. However, more recent evidence suggests that homocysteine-lowering therapy with B vitamins may help prevent stroke in particular.
Cancer Prevention
Cancer is thought to arise from DNA damage in excess of ongoing DNA repair and/or the inappropriate expression of critical genes. Because of the important roles played by folate in DNA and RNA synthesis and methylation, it is possible that inadequate folate intake contributes to genome instability and chromosome breakage that often characterize cancer development.
The consumption of at least five servings of fruit and vegetables daily has been consistently associated with a decreased incidence of cancer. Fruit and vegetables are excellent sources of folate, which may play a role in their anti-carcinogenic effect. Observational studies have found diminished folate status to be associated with site-specific cancers.
However, meta-analyses of folic acid intervention trials (supplemental doses ranging from 500 to 5,000 μg/day for at least one year) did not show any specific benefit or harm regarding total and site-specific cancer incidence. In 2015, an expert panel convened by the US National Toxicology Program and the US Office of Dietary Supplements concluded that folic acid supplementation does not reduce cancer incidence in folate replete individuals and that further research is needed to determine whether supplemental folic acid might promote cancer growth.
Disease Prevention: Alzheimer’s Disease and Disease Treatment
Alzheimer’s disease (AD) is the most common form of dementia, affecting more than 5 million people 65 years or older in the US. β-amyloid plaque deposition, Tau protein-forming tangles, and increased cell death in the brain of AD patients have been associated with cognitive decline and memory loss.
Cognitive Function
One study associated increased consumption of fruit and vegetables, which are abundant sources of folate, with a reduced risk of developing dementia and AD in women. In the Baltimore Longitudinal Study of Aging that followed 579 nondemented older adults for an average of 9.3 years, daily folate intakes of at least 400 μg at baseline were associated with a 55% lower risk of developing Alzheimer’s disease compared to lower intakes.
Metabolic Diseases Treatment
Folinic acid (also known as leucovorin), a tetrahydrofolic acid derivative, is used in the clinical management of rare inborn errors that affect folate transport or metabolism. Such conditions are of autosomal recessive inheritance, meaning only individuals receiving two copies of the mutated gene (one from each parent) develop the disease.
Several investigators have described associations between increased homocysteine concentrations and cognitive impairment in the elderly, but prospective cohort studies examining dietary or total daily folate intake and cognition have reported mixed results. Over the past few decades, hyperhomocysteinemia in older adults has become recognized as a modifiable risk factor for cognitive decline, dementia, and Alzheimer’s disease.
Specific Metabolic Conditions
- Hereditary folate malabsorption is caused by mutations in the SLC46A1 gene coding for the folate transporter PCFT and typically affects gastrointestinal folate absorption and folate transport into the brain
- Cerebral Folate Deficiency (CFD) syndrome is characterized by low levels of folate coenzymes in cerebrospinal fluid despite often normal concentrations of folate in blood
- Dihydrofolate reductase (DHFR) deficiency is characterized by megaloblastic anemia and cerebral folate deficiency causing intractable seizures and mental deficits
Sources, Safety, and Recommendations
Green leafy vegetables (foliage) are rich sources of folate and provide the basis for its name. Citrus fruit juices, legumes, and fortified foods are also excellent sources of folate; the folate content of fortified cereal varies greatly.
| Food | Serving | Folate (μg DFEs) |
| Lentils (cooked, boiled) | ½ cup | 179 |
| Garbanzo beans (chickpeas, boiled) | ½ cup | 141 |
| Asparagus (boiled) | ½ cup (~6 spears) | 134 |
| Spinach (boiled) | ½ cup | 132 |
| Lima beans (boiled) | ½ cup | 78 |
| Orange juice | 6 fl. oz. | 56 |
| Bread (enriched) | 1 slice | 50* |
| Spaghetti (enriched, cooked) | 1 cup | 180* |
| White rice (enriched, cooked) | 1 cup | 153-180* |
*To help prevent neural tube defects, the US FDA required the addition of 1.4 milligrams (mg) of folic acid per kilogram (kg) of grain to be added to refined grain products, which were already enriched with niacin, thiamin, riboflavin, and iron, as of January 1, 1998.
Safety and Drug Interactions
No adverse effects have been associated with the consumption of excess folate from food. Concerns regarding safety are limited to synthetic folic acid intake. There is concern that large doses of folic acid given to an individual with an undiagnosed vitamin B12 deficiency could correct megaloblastic anemia without correcting the underlying vitamin B12 deficiency, leaving the individual at risk of developing irreversible neurologic damage.
In order to be very sure of preventing irreversible neurologic damage in vitamin B12-deficient individuals, the Food and Nutrition Board of the US National Academy of Medicine advises that all adults limit their intake of folic acid (supplements and fortification) to 1,000 μg (1 mg) daily.
LPI Recommendation
The Linus Pauling Institute recommends that adults take a daily multivitamin/mineral supplement, which typically contains 400 μg of folic acid, the Daily Value (DV). Even with a larger than average intake of folic acid from fortified food, it is unlikely that an individual’s daily folic acid intake would regularly exceed the tolerable upper intake level of 1,000 μg/day established by the Institute of Medicine.
The recommendation for 400 μg/day of supplemental folic acid as part of a daily multivitamin/mineral supplement, in addition to a folate-rich diet, is especially important for older adults because blood homocysteine concentrations tend to increase with age.
