Biological roles

DNA and cell division
Folate is necessary for the production and maintenance of new cells, for DNA synthesis and RNA synthesis, and for preventing changes to DNA, and, thus, for preventing cancer. It is especially important during periods of rapid cell division and growth, such as infancy and pregnancy. Folate is needed to carry one-carbon groups for methylation reactions and nucleic acid synthesis (the most notable one being thymine, but also purine bases). Thus, folate deficiency hinders DNA synthesis and cell division, affecting hematopoietic cells and neoplasms the most because of rapid cell division. RNA transcription, and subsequent protein synthesis, are less affected by folate deficiency, as the mRNA can be recycled and used again (as opposed to DNA synthesis, where a new genomic copy must be created). Since folate deficiency limits cell division, erythropoiesis, production of red blood cells, is hindered and leads to megaloblastic anemia, which is characterized by large immature red blood cells. This pathology results from persistently thwarted attempts at normal DNA replication, DNA repair, and cell division, and produces abnormally large red cells called megaloblasts (and hypersegmented neutrophils) with abundant cytoplasm capable of RNA and protein synthesis, but with clumping and fragmentation of nuclear chromatin. Some of these large cells, although immature (reticulocytes), are released early from the marrow in an attempt to compensate for the anemia. Both adults and children need folate to make normal red and white blood cells and prevent anemia.Deficiency of folate in pregnant women has been implicated in neural tube defects (NTD); therefore, many developed countries have implemented mandatory folic acid fortification in cereals, etc. It must be noted that NTDs occur early in pregnancy (first month), therefore women must have abundant folate upon conception. Folate is required to make red blood cells and white blood cells and folate deficiency may lead to anemia, which further leads to fatigue and weakness and inability to concentrate.

Biochemistry of DNA base and amino acid production
In the form of a series of tetrahydrofolate (THF) compounds, folate derivatives are substrates in a number of single-carbon-transfer reactions, and also are involved in the synthesis of dTMP (2'-deoxythymidine-5'-phosphate) from dUMP (2'-deoxyuridine-5'-phosphate). It is a substrate for an important reaction that involves vitamin B12 and it is necessary for the synthesis of DNA, and so required for all dividing cells.
The pathway leading to the formation of tetrahydrofolate (FH4) begins when folate (F) is reduced to dihydrofolate (DHF) (FH2), which is then reduced to THF. Dihydrofolate reductase catalyses the last step. Vitamin B3 in the form of NADPH is a necessary cofactor for both steps of the synthesis.
Methylene-THF (CH2FH4) is formed from THF by the addition of methylene groups from one of three carbon donors: formaldehyde, serine, or glycine. Methyl tetrahydrofolate (CH3-THF) can be made from methylene-THF by reduction of the methylene group with NADPH. It is important to note that Vitamin B12 is the only acceptor of methyl-THF. There is also only one acceptor for methyl-B12, which is homocysteine in a reaction catalyzed by homocysteine methyltransferase. This is important because a defect in homocysteine methyltransferase or a deficiency of B12 can lead to a methyl-trap of THF and a subsequent deficiency. Thus, a deficiency in B12 can generate a large pool of methyl-THF that is unable to undergo reactions and will mimic folate deficiency. Another form of THF, formyl-THF or folinic acid results from oxidation of methylene-THF or is formed from formate donating formyl group to THF. Finally, histidine can donate a single carbon to THF to form methenyl-THF.
In other words:
folate ? dihydrofolate ? tetrahydrofolate ? methylene-THF ? methyl-THF

Overview of drugs that interfere with folate reactions
A number of drugs interfere with the biosynthesis of folic acid and THF. Among them are the dihydrofolate reductase inhibitors such as trimethoprim, pyrimethamine, and methotrexate; the sulfonamides (competitive inhibitors of 4-aminobenzoic acid in the reactions of dihydropteroate synthetase).
Valproic acid, one of the most commonly prescribed anticonvulsants that is also used to treat certain psychological conditions, is a known inhibitor of folic acid, and as such, has been shown to cause neural tube defects and cases of spina bifida and cognitive impairment in the newborn. Because of this considerable risk, those mothers who must continue to use valproic acid or its derivatives during pregnancy to control their condition (as opposed to stopping the drug or switching to another drug or to a lesser dose) should take folic acid supplements under the direction and guidance of their health care providers.
The National Health and Nutrition Examination Survey (NHANES III 1988–91) and the Continuing Survey of Food Intakes by Individuals (1994–96 CSFII) indicated most adults did not consume adequate folate. However, the folic acid fortification program in the United States has increased folic acid content of commonly eaten foods such as cereals and grains, and as a result, diets of most adults now provide recommended amounts of folate equivalents.

Health issues

Human reproduction
Adequate folate intake during the periconception period, the time right before and just after a woman becomes pregnant, helps protect against a number of congenital malformations, including neural tube defects (which are the most notable birth defects that occur from folate deficiency). Neural tube defects produce malformations of the spine, skull, and brain including spina bifida and anencephaly. The risk of neural tube defects is significantly reduced when supplemental folic acid is consumed in addition to a healthy diet prior to and during the first month following conception.Supplementation with folic acid has also been shown to reduce the risk of congenital heart defects, cleft lips, limb defects, and urinary tract anomalies.Folate deficiency during pregnancy may also increase the risk of preterm delivery, infant low birth weight and fetal growth retardation, as well as increasing homocysteine level in the blood, which may lead to spontaneous abortion and pregnancy complications, such as placental abruption and pre-eclampsia. Women who could become pregnant are advised to eat foods fortified with folic acid or take supplements in addition to eating folate-rich foods to reduce the risk of serious birth defects. Taking 400 micrograms of synthetic folic acid daily from fortified foods and/or supplements has been suggested. The RDA for folate equivalents for pregnant women is 600–800 micrograms, twice the normal RDA of 400 micrograms for women who are not pregnant. The mechanisms and reasons why folic acid prevents birth defects is unknown It is hypothesized that the insulin-like growth factor 2 gene is differentially methylated and these changes in IGF2 result in improved intrauterine growth and development. Approximately 85% of women in an urban Irish study reported using folic acid supplements before they become pregnant, but only 18% used enough folic acid supplements to meet the current folic acid requirements due, reportedly, to socio-economic challenges. Folic acid supplements may also protect the fetus against disease when the mother is battling a disease or taking medications or smoking during pregnancy.
Folic acid may also reduce chromosomal defects in sperm. A benefit is indicated even for more than 700 mcg folate per day, which, though below the tolerable upper intake levels of 1,000 µg/day, was 1.8 times the recommended dietary allowance.

Heart disease
An estimated 13,500 deaths occur annually due to folate deficiency's effect on coronary artery disease and the risk of ischemic heart disease, and stroke has been reduced by 15% since folate fortification regulations were enforced. Adequate concentrations of folate, vitamin B12, or vitamin B6 may decrease the circulating level of homocysteine, an amino acid normally found in blood. There is evidence an elevated homocysteine level is an independent risk factor for heart disease and stroke. The evidence suggests high levels of homocysteine may damage coronary arteries or make it easier for blood platelets to clump together and form a clot. However, there is currently no evidence available to suggest lowering homocysteine with vitamins will reduce risk of heart disease. The NORVIT trial suggests folic acid supplementation may do more harm than good.As of 2006, studies have shown giving folic acid to reduce levels of homocysteine does not result in clinical benefit. One of these studies suggests folic acid in combination with B12 may even increase some cardiovascular risks.
However, a 2005 study found 5 mg of folate daily over a three-week period reduced pulse pressure by 4.7 mmHg compared with a placebo, and concludedfolic acid is a safe and effective supplement that targets large artery stiffness and may prevent isolated systolic hypertension.
Also, as a result of new research, "heart experts" at Johns Hopkins Medical Center reported in March 2008 in favour of therapeutic folate, although they cautioned that it is premature for people to begin to self-medicate by taking high doses of folic acid."
Hyperhomocysteinemia is a predictor of cardiovascular disease and hypertension among children and folic acid is a safe and effective supplement because it reduces serum homocysteine levels as well as systolic and diastolic blood pressure, thus preventing cardiovascular disease in children.
Folic acid supplements may improve the integrity of the vascular endothelium. Folic acid supplements consumed before and during pregnancy may reduce the risk of heart defects in infants, and may reduce the risk for children to develop metabolic syndrome. Thet may, however, worsen the outcomes in patients with cardiovascular disease such as angina and myocardial infarction.

Stroke
Folic acid appears to reduce the risk of stroke. The reviews indicate the risk of stroke appears to be reduced only in some individuals, but a definite recommendation regarding supplementation beyond the current RDA has not been established for stroke prevention. Observed stroke reduction is consistent with the reduction in pulse pressure produced by folate supplementation of 5 mg per day, since hypertension is a key risk factor for stroke. Folic supplements are inexpensive and relatively safe to use, which is why stroke or hyperhomocysteinemia patients are encouraged to consume daily B vitamins including folic acid.
Cancer
Folate deficiency decreases intracellular S-adenosylmethionine (SAM), which inhibits cytosine methylation in DNA, activates proto-oncogenes, induces malignant transformations, causes DNA precursor imbalances, misincorporates uracil into DNA, and promotes chromosome breakage; all of these mechanisms increase the risk of prostate cancer development.
The association between folate and cancer appears to be complex and mixed. There are theoretical reasons that folate may help prevent cancer, and a meta-analysis published in 2010 failed to find a statistically significant cancer risk due to folic acid treatments, but a 1995 study found supplementation increases rates of cancer.
Some investigations have proposed good levels of folic acid may be related to lower risk of esophageal, stomach, and ovarian cancers, but the benefits of folic acid against cancer may depend on when it is taken and on individual conditions. In addition, folic acid may not be helpful, and could even be damaging, in people already suffering from cancer or from a precancerous condition. Likewise, it has been suggested excess folate may promote tumor initiation. Folate has shown to play a dual role in cancer development; low folate intake protects against early carcinogenesis, and high folate intake promotes advanced carcinogenesis. Therefore, public health recommendations should be careful not to encourage too much folate intake.
Diets high in folate are associated with decreased risk of colorectal cancer; some studies show the association is stronger for folate from foods alone than for folate from foods and supplements, while other studies find that folate from supplements is more effective due to greater bioavailability. A 2007 randomized clinical trial found folate supplements did not reduce the risk of colorectal adenomas, and in fact increase the presence of advanced lesions and adenoma multiplicity. Colorectal cancer is the most studied type of cancer in relation to folate and one carbon metabolism. For example, folic acid supplement intake increased advanced colorectal cancer development by 67% in a 14-year European research study involving 520,000 men.
A 2006 prospective study of 81,922 Swedish adults found diets great in folate from foods, but not from supplements, were associated with a reduced risk of pancreatic cancer.
Most epidemiologic studies suggest diets high in folate are associated with decreased risk of breast cancer, but results are not uniformly consistent. One broad cancer screening trial reported a potential harmful effect of much folate intake on breast cancer risk, suggesting routine folate supplementation should not be recommended as a breast cancer preventive, but a 2007 Swedish prospective study found much folate intake was associated with a lower incidence of postmenopausal breast cancer. A 2008 study has shown no significant effect of folic acid on overall risk of total invasive cancer or breast cancer among women.Folate intake may not have any effect on the risk of breast cancer but may have an effect for women who consume at least 15 g/d of alcohol. Folate intake of more than 300 µg/d may reduce the risk of breast cancer in women who consume alcohol.
Most research studies associate high dietary folate intake with a reduced risk of prostate cancer; however, in men, folic acid supplementation appears to double the risk of prostate cancer. Recently, a clinical trial showed daily supplementation of 1 mg of folic acid increased the risk of prostate cancer, while dietary and plasma folate levels among vitamin nonusers actually decreased the risk of prostate cancer.A Finnish study consisting of 29,133 older male smokers observed prostate cancer risk had no relationship with serum folate levels.
The reason high levels of folic acid may increase cancer is because of its role in nucleotide synthesis (proliferating neoplastic cells need this and folate receptors are increased in cancers)Folate's role in DNA methylation is important in prostate cancer. Unmetabolized folic acid is associated with a reduction in natural killer cell cytotoxicity, which reduces the immune system's ability to defend against malignant cells. However, the study also showed dietary baseline intake of folate may have an inverse relationship to prostate cancer occurrence.
Although the relationship between folate and prostate cancer is not yet clear, suicide gene studies show a target vector for folate to prostate and nasopharyngeal cancer cells.Growth of tumor cells is significantly inhibited when a folate-linked nanoparticle is injected intratumorally. The mechanism might be the interference of transfection and communication failures of intracellular gap junctions.
The cancer drug methotrexate is designed to inhibit the metabolism of folic acid. Folic acid may interact unexpectedly with the cancer drug fluorouracil. The exact mechanism of interaction is unknown.
The low dihydrofolate reductase activity in the liver of humans compared to other animals and so the low conversion of folic acid into its active derivatives might be due to the control of this enzyme by transcription factors, such as E2F-1 involved in cell proliferation. It has been suggested "the low level of DHFR, and the other proteins under the control of E2F-1, in humans may have evolved to hinder the development of cancer. If this is the case, other animals with slow tissue turnover rates, possibly related to long life span, might also have low DHFR activity."
Folic acid supplements prevent mistakes (inserting uracils into the DNA, for example) from occurring during DNA replication and repair.This is a proposed mechanism for folic acid's protection against colorectal cancer.
Folic acid supplements stimulate the PI3k/Akt signaling cascade, which leads to improved cell survival, but this could be beneficial or harmful for the body because cancer cells may use this pathway to survive. Folic acid may also reduce the levels of PTEN (a tumor suppressor gene), making this relationship even more controversial.

Antifolates
Main article: Antifolate
Folate is important for cells and tissues that rapidly divide. Cancer cells divide rapidly, and drugs that interfere with folate metabolism are used to treat cancer. The antifolate methotrexate is a drug often used to treat cancer because it inhibits the production of the active form of THF from the inactive dihydrofolate (DHF). However, methotrexate can be toxic, producing side effects, such as inflammation in the digestive tract that make it difficult to eat normally. Also, bone marrow depression (inducing leukopenia and thrombocytopenia), and acute renal and hepatic failure have been reported.
Folinic acid, under the drug name leucovorin,a form of folate (formyl-THF), can help "rescue" or reverse the toxic effects of methotrexate. Folinic acid is not the same as folic acid. Folic acid supplements have little established role in cancer chemotherapy. There have been cases of severe adverse effects of accidental substitution of folic acid for folinic acid in patients receiving methotrexate cancer chemotherapy. It is important for anyone receiving methotrexate to follow medical advice on the use of folic or folinic acid supplements. The supplement of folinic acid in patients undergoing methotrexate treatment is to give cells dividing less rapidly enough folate to maintain normal cell functions. The amount of folate given will be depleted by rapidly dividing cells (cancer) very fast and so will not negate the effects of methotrexate.

Obesity
Folic acid increases lipolysis in adipocytes, and may have a role in the prevention of obesity and type 2 diabetes. This mechanism involves the beta adrenoceptors in the adbdominal adipocytes. Folic acid supplements may reduce the accumulation of cholesterol in the liver and in the blood; this may be due to folic acid's role in incorporating cholesterol into bile acid. Folic acid supplements have been shown to increase bile acid production and flow.

Depression
Some evidence links a shortage of folate with depression. Limited evidence from randomised controlled trials showed using folic acid in addition to antidepressants, specifically SSRIs, may have benefits. Research at the University of York and Hull York Medical School has found a link between depression and low levels of folate.One study by the same team involved 15,315 subjects. However, the evidence is probably too limited at present for this to be a routine treatment recommendation. Folic acid supplementation affects noradrenaline and serotonin receptors within the brain which could be the cause of folic acid's possible ability to act as an antidepressant.

Memory and mental agility
In a three-year trial on 818 people over the age of 50, short-term memory, mental agility, and verbal fluency were all found to be better among people who took 800 micrograms of folic acid daily, twice the current RDA, than those who took placebo. The study was reported in The Lancet on 20 January 2007.

Schizophrenia
Folate deficiency may increase the risk of schizophrenia because, by increasing homocysteine levels, folate also increases interleukin 6 and tumor necrosis factor alpha levels, and these two cytokines are involved in the development of schizophrenia. The exact mechanisms involved in the development of schizophrenia are not entirely clear, but may have something to do with DNA methylation and one carbon metabolism, and these are the precise roles of folate in the body.

Allergic diseases
There is a relationship between folic acid and allergic diseases. In one study that examined the relationship between serum folate levels and markers of atopy, wheeze, and asthma in 8,083 subjects, serum folate levels were found to be inversely related to IgE level, atopy, and wheeze in a dose-response relationship. Increased folate levels were also associated with decreased risk of doctor-diagnosed asthma. Folic acid supplementation during late pregnancy is associated with an increased risk of childhood asthma, increased risk of persistent asthma, and poorer respiratory function in young children.

Rheumatoid arthritis
Folic acid in a dose of 5–27 mg per week is used to protect patients with rheumatoid arthritis who are taking methotrexate from the toxic effects of this drug.

Fertility
Folate is necessary for fertility in both men and women. In men, it contributes to spermatogenesis. In women, on the other hand, it contributes to oocyte maturation, implantation, placentation, in addition to the general effects of folic acid and pregnancy. Therefore, it is necessary to receive sufficient amounts through the diet to avoid subfertility. Also, polymorphisms in genes of enzymes involved in folate metabolism could be one reason for fertility complications in some women with unexplained infertility.

Renal disease
Folic acid supplements may reduce the risk of children developing renal diseases or injuries, such as microalbuminuria.
Type 1 diabetes mellitus
Type 1 diabetes mellitus patients have lower plasma levels of folic acid, and may benefit from folic acid supplements or folic acid-fortified food products.

Macular degeneration
A substudy of the Women's Antioxidant and Folic Acid Cardiovascular Study published in 2009 reported use of a nutritional supplement containing folic acid, pyridoxine, and cyanocobalamin decreased the risk of developing age-related macular degeneration by 34.7%.

Bone health
Folate deficiency has been hypothesized to lead to elevated homocysteine levels, which, in turn, leads to an increased risk of bone fractures, osteoporosis, and reduction in bone mineral density, but research studies so far show controversial results. It aids in bone health by promoting the regeneration of bone marrow.

Menopause
Folic acid supplements help relieve hot flashes in postmenopausal women. Just as in estrogen hormone replacement therapy, folic acid interacts with neurotransmitters (norepinephrine, serotonin) in the brain to reduce hot flashes.

Infectious disease
Folate deficiency is linked to anemia-causing Plasmodium falciparum malaria in areas such as Colombia, where malaria has reached endemic proportions.

Bone loss in Parkinson's disease (PD)
Folate lowers homocysteine levels, which, in turn, prevents bone loss in Parkinson's disease (PD) patients taking levodopa (a psychoactive drug taken to treat Parkinson's disease). Improvements in bone health include increased BMD at the lumbar spine, total femur, and femur shaft.

Skin Health
Folate is destroyed by exposure to ultraviolet radiation (UVR), which puts the skin under increased demands for optimal folate intake. A 2010 cell culture study showed that folic acid supplementation may positively influence skin health.

Folic acid supplements and masking of B12 deficiency
There has been concern about the interaction between vitamin B12 and folic acid. The National Institutes of Health has found that "Large amounts of folic acid can mask the damaging effects of vitamin B12 deficiency by correcting the megaloblastic anemia caused by vitamin B12 deficiency without correcting the neurological damage that also occurs", there are also indications that "high serum folate levels might not only mask vitamin B12 deficiency, but could also exacerbate the anemia and worsen the cognitive symptoms associated with vitamin B12 deficiency". Due to the fact that in the United States legislation has required enriched flour to contain folic acid to reduce cases of fetal neural-tube defects consumers may be ingesting more than they realize. To counter the masking effect of B12 deficency the NIH recommends "folic acid intake from fortified food and supplements should not exceed 1,000 micrograms (1000 µg = 1 mg) daily in healthy adults."
In fact, to date the evidence such masking actually occurs is scarce, and there is no evidence folic acid fortification in Canada or the U.S. has increased the prevalence of vitamin B12 deficiency or its consequences. However, one recent study has demonstrated high folic or folate levels, when combined with low B12 levels, are associated with significant cognitive impairment among the elderly.
In any case, it is important for older adults to be aware of the relationship between folic acid and vitamin B12, because they are at greater risk of having a B12 deficiency. For this reason, a physician may wish to check the vitamin B12 status of patients 50 years of age or older before prescribing them a supplement that contains folic acid.

Overdose Risks
The risk of toxicity from folic acid is low, because folate is a water-soluble vitamin and is regularly removed from the body through urine. The Institute of Medicine has established a tolerable upper intake level (UL) for folate of 1 mg for adult men and women, and a UL of 800 µg for pregnant and lactating (breast-feeding) women less than 18 years of age. Supplemental folic acid should not exceed the UL to prevent folic acid from masking symptoms of vitamin B12 deficiency.
Research suggests high levels of folic acid can interfere with some antimalarial treatments.
A 10,000-patient study at Tufts University in 2007 concluded excess folic acid worsens the effects of B12 deficiency and, in fact, may affect the absorption of B12.
A study at the University of Adelaide concluded the intake of folic acid supplements during late pregnancy increases the risk of babies developing childhood asthma by 30%, although researchers emphasized that their finding did not contradict recommendations to supplement folic acid in first trimester, when no additional risk was found.

Elderly population
There are benefits and risks of food folic acid fortification for elderly populations. Elevated exposure to folic acid due to fortification can improve folate and homocysteine levels, but can also mask symptoms of vitamin B12 deficiency. A study where 747 subjects aged 67 to 96 years were measured for B vitamin and homocysteine status showed diets with folic acid fortification of 140 µg/100 g of grain product decreased homocysteine level and heart disease risk. However, Canada's food supply is fortified with 150 µg/100 g of grain, and much of the elderly population also takes a supplement that includes a folic acid component of 400 µg. Therefore, it is important not to consume quantities over the recommended DRI.

Folate deficiency
Folate deficiency may lead to glossitis, diarrhea, depression, confusion, anemia, and fetal neural tube defects and brain defects (during pregnancy). Folate deficiency is accelerated by alcohol consumption Folate deficiency is diagnosed by analyzing CBC and plasma vitamin B12 and folate levels. CBC may indicate megaloblastic anemia but this could also be a sign of vitamin B12 deficiency. A serum folate of 3 µg/L or lower indicates deficiency. Serum folate level reflects folate status but erythrocyte folate level better reflects tissue stores after intake. An erythrocyte folate level of 140 µg/L or lower indicates inadequate folate status. Increased homocysteine level suggests tissue folate deficiency but homocysteine is also affected by vitamin B12 and vitamin B6, renal function, and genetics. One way to differentiate between folate deficiency from vitamin B12 deficiency is by testing for methylmalonic acid levels. Normal MMA levels indicate folate deficiency and elevated MMA levels indicate vitamin B12 deficiency. Folate deficiency is treated with supplemental oral folate of 400 to 1000 µg per day. This treatment is very successful in replenishing tissues, even if deficiency was caused by malabsorption. Patients with megaloblastic anemia need to be tested for vitamin B12 deficiency before folate treatment, because if the patient has vitamin B12 deficiency, folate supplementation can remove the anemia, but can also worsen neurologic problems. Morbidly obese patients with BMIs of greater than 50 are more likely to develop folate deficiency. Patients with celiac disease have a higher chance of developing folate deficiency. Cobalamin deficiency may lead to folate deficiency, which, in turn, increases homocysteine levels and finally may result in the development of cardiovascular disease or birth defects.

Iron-folic acid supplementation risk for children
Some studies show iron-folic acid supplementation in children under 5 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.