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From Wikipedia, the free encyclopedia

Folic acid
Skeletal formula
Ball-and-stick model of folic acid.png
Clinical data
Pronunciation/ˈflɪk,ˈfɒlɪk/
Other namesFA, N-(4-{[(2-amino-4-oxo-1,4-dihydropteridin-6-yl)methyl]amino}benzoyl)-L-glutamic acid, pteroyl-L-glutamic acid, folacin, vitamin B9,[1] vitamin Bc,[2] vitamin M,[3] pteroyl-L-glutamate
AHFS/Drugs.comMonograph
MedlinePlusa682591
Pregnancy
category
  • US: A (No risk in human studies)
Routes of
administration
By mouth, IM, IV, sub-Q
ATC code
Legal status
Legal status
Pharmacokinetic data
Bioavailability50–100%[4]
MetabolismLiver[4]
ExcretionUrine[4]
Identifiers
CAS Number
PubChem CID
IUPHAR/BPS
DrugBank
ChemSpider
UNII
KEGG
ChEBI
ChEMBL
CompTox Dashboard (EPA)
ECHA InfoCard100.000.381 Edit this at Wikidata
Chemical and physical data
FormulaC19H19N7O6
Molar mass441.404 g·mol−1
3D model (JSmol)
Density1.6±0.1[6] g/cm3
Melting point250 °C (482 °F) (decomposition)
Solubility in water1.6 mg/L (25 °C) mg/mL (20 °C)

Folate, also known as vitamin B9 and folacin,[7] is one of the B vitamins.[4] Manufactured folic acid, which is converted into folate by the body, is used as a dietary supplement and in food fortification as it is more stable during processing and storage.[8] Folate is essential for the body to make DNA and RNA and metabolise amino acids, which are required for cell division.[1][9] As humans cannot make folate, it is required from the diet, making it an essential vitamin.[10] It occurs naturally in many foods.[7][1] The recommended adult daily intake of folate in the U.S. is 400 micrograms from foods or dietary supplements.[1]

Folate in the form of folic acid is used to treat anemia caused by folate deficiency.[4] Folic acid is also used as a supplement by women during pregnancy to reduce the risk of neural tube defects (NTDs) in the baby.[4][11] Low levels in early pregnancy are believed to be the cause of more than half of babies born with NTDs.[1] More than 80 countries use either mandatory or voluntary fortification of certain foods with folic acid as a measure to decrease the rate of NTDs.[12] Long-term supplementation with relatively large amounts of folic acid is associated with small reduction in the risk of stroke[13] and an increased risk of prostate cancer.[14] There are concerns that large amounts of supplemental folic acid can hide vitamin B12 deficiency.[1]

Not consuming enough folate can lead to folate deficiency.[1] This may result in a type of anemia in which red blood cells become abnormally large.[1] Symptoms may include feeling tired, heart palpitations, shortness of breath, open sores on the tongue, and changes in the color of the skin or hair.[1] Folate deficiency in children may develop within a month of poor dietary intake.[15] In adults, normal total body folate is between 10 and 30 mg with blood levels of greater than 7 nmol/L (3 ng/mL).[1]

Folic acid was discovered between 1931 and 1943.[16] It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system.[17] The wholesale cost of supplements in the developing world is between US$0.001 and 0.005 per dose as of 2014.[18] The term "folic" is from the Latin word folium (which means leaf) because it was found in dark-green leafy vegetables.[19]

YouTube Encyclopedic

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  • ✪ Folate deficiency - causes, symptoms, diagnosis, treatment, pathology
  • ✪ Folate deficiency
  • ✪ Folate Deficiency Vs Vitamin B12 Deficiency
  • ✪ Folate & B12 Deficiency, Megaloblastic Anemia Hypersegmented Macrocytic Methylmalonic
  • ✪ Folic acid (Vitamin B9)- One carbon metabolism, Megaloblastic anemia and Case discussion

Transcription

Learning medicine is hard work! Osmosis makes it easy. It takes your lectures and notes to create a personalized study plan with exclusive videos, practice questions and flashcards, and so much more. Try it free today! Folate deficiency is a clinical condition that occurs because of low level of folate or folic acid in the body. This can lead to a variety of problems ranging from anemia in individuals from all age groups to neural tube malformation in fetuses. Folate, also known as vitamin B9, mainly comes from eating leafy greens and citrus fruits like oranges and lemons and, nowadays, many countries fortify foods like grains and cereals with folate. Now, folic acid present in these food items is generally in polyglutamate form, which are basically chains of an amino acid called glutamic acid. Because of the carboxyl group present in its structure, the chain is negatively charged making it polar and soluble in water, which is a polar molecule but not soluble in lipids which are nonpolar molecules. So the polyglutamate residues of folic acid are almost non-absorbable from the GI tract, where all the cells are surfaced with lipid cell membranes. So, to make them absorbable, when polyglutamate residues reach a portion of the small intestine called the jejunum, special enzymes present at the jejunal mucosa cut down the polyglutamate residues into monoglutamate. Monoglutamate is smaller, and is less negatively charged, so these monoglutamate residues of folic acids can pass through the cell membrane and enter the jejunal cells, where they are converted into tetrahydrofolic acid or in short THF by the enzyme tetrahydrofolate reductase. These THFs then get methylated into a more stable form called methyl-THF. Once formed, the methyl-THF then leaves the jejunal cell and enters the bloodstream. Some of it goes to the liver and get stored for a short period of 2-3 months, while most of it is used up for metabolic activity inside various cells around the body. Folic acid is used to synthesize DNA precursors, which is essential for DNA replication and cell division. On target cells, there’s a specialized membrane protein called Folic Acid Transporter or FAT, which moves the circulating methyl-THF inside the cell. Once inside, methyl-THF transfers its methyl group to vitamin B12, ultimately making methylcobalamin and free THF in the process. THF then gets an extra “methylene” group from serine, an amino acid found within the cells. THF quickly transfers the methylene to a nucleotide called deoxyuridine monophosphate, or d-UMP for short. As a result, d-UMP becomes d-TMP or deoxythymidine monophosphate, which can then be converted to thymidine, one of the nucleotides used to build DNA. Going back, the methylcobalamin that was formed along with THF transfers its methyl group to homocysteine and converts it into an essential amino acid called methionine, thus lowering the levels of homocysteine in the body, too much of which can be harmful. Besides this, folic acid also plays a very important role during fetal development. Specifically, it’s needed for the closure of the anterior neuropore of the neural tube during the 23rd day, and posterior neuropore during the 26th day of gestation. This is a crucial step in the development of the central nervous system. So in short, the consequences of folic acid deficiency are impaired cell division, too much homocysteine in the body, and neural tube defects in fetuses. When cell division grinds to a halt, rapidly dividing cells in the bone marrow, such as red and white blood cells, as well as platelet precursors, are affected. Inside the bone marrow, red blood cell precursors are normally big and plump, and they undergo a series of cell divisions, which results in smaller mature RBCs. Now with folate deficiency, at first, the bone marrow pumps out larger, but still mature RBCs called macrocytes. These RBCs are destroyed in the spleen, which causes a decrease in the total RBC count, or anemia. In response, the bone marrow compensates by releasing megaloblasts, which are abnormally developed RBC precursors, into the blood - and the final result is macrocytic, megaloblastic anemia. Folate deficiency also affects white blood cell production - so the bone marrow starts releasing large, immature neutrophils. Immature neutrophils are also hypersegmented, which means their nuclei have 6 or more lobes. Finally, severe folate deficiency may also decrease bone marrow production of platelet precursors, which are called megakaryocytes. So when all 3 blood cell lines are affected, this results in pancytopenia - which is when red blood cell, white blood cell and platelet count is low and this happens only in cases of severe folate deficiencies. Other rapidly dividing cells are mucosal epithelial cells, especially those of the tongue mucosa. Have you ever noticed how fast your tongue heals if you accidentally bite it? That’s because old epithelial cells are replaced with new ones in the blink of an eye! Okay, it’s not literally that fast , but it is pretty quick. In folate deficiency, old epithelial cells aren’t replaced, and this slows down the healing of normal wear and tear of the tongue, which ultimately leads to inflammation of the tongue, known as glossitis. Next, when homocysteine builds up in the body, some of it is excreted in the urine leading to homocystinuria. It also builds up in the blood, where it binds to the endothelial cells lining blood vessels, causing them to secrete molecules called proinflammatory cytokines. These attract immune cells like leukocytes to the area and cause inflammation, which leads to atherosclerosis, or plaque buildup inside the arteries. This narrows the arteries and could lead to ischemia of the tissue supplied by them. Homocysteine also binds to platelets and makes them stick together to make blood clots. All of this increases the risk of ischemic heart disease and stroke. Lastly, when pregnant women suffer from folate deficiency around the time of conception, embryos may not properly develop the neural tube due to impairment in the closure of anterior and posterior neuropore of the neural tube, which may lead to neural tube defects. So, when the anterior neuropore doesn’t close properly, the baby may be born with anencephaly or absence of a major portion of the brain and the skull. Similarly, if the posterior neuropore doesn’t close well, the baby may be born with spina bifida or incomplete closure of the vertebrae and membranes of the spinal cord. Ok, now, folate deficiency can result because of increased demand, decreased dietary intake or impaired absorption. Normally, we have up to 2 to 3 months supply of folate in the body, but this can get used up even quicker during pregnancy due to the increased need for it. So generally, people who get folate deficiency are either pregnant, or have had an extremely restricted diet for longer than six weeks. Besides this, excessive alcohol consumption and medications like phenytoin, trimethoprim, sulfasalazine and methotrexate often interfere with folic acid absorption from the jejunum. Folate deficiency most commonly causes signs and symptoms of anemia like shortness of breath, pallor, and easy fatigability; as well as soreness of the tongue due to glossitis. In some cases, they may also present with signs and symptoms of ischemic heart disease, like chest pain, or signs of stroke, like slurred speech and paralysis. Lastly, long-standing cases of severe folate deficiency may also present with features of pancytopenia like anemia due to low RBC count, recurrent infections due to low WBC count and bleeding tendencies due to low platelet count. Diagnosis of folate deficiency relies on a peripheral blood smear, which shows large red blood cells. On a blood sample, a Mean Corpuscular Volume or MCV, larger than 100 fL suggests macrocytosis. Homocysteine level is also elevated. A bone marrow study can also be done to look for the megaloblastic changes in RBC precursors at various stages of differentiation. After confirming that there is folate deficiency, the cause should be found. First, low dietary intake should be ruled out, and a pregnancy test should be done. Then, the history can help identify other causes, like medications or excessive alcohol consumption. When the cause of folate deficiency is dietary, it’s treated with oral folate supplements. When the problem is absorption related, it has to be managed by stopping the offending medications or limiting alcohol consumption. Lastly, in pregnant people, folate deficiency can be avoided by providing folate supplements or fortifying foods like grains and cereals with folate. All right, as a quick recap, folate deficiency is a clinical condition caused by low levels of folate in the body, which impairs cell division and causes excess levels of homocysteine in the body. This can lead to neural tube defects in fetuses, macrocytic megaloblastic anemia causing pallor, fatigability and shortness of breath; glossitis or inflammation of tongue leading to swelling and soreness, difficulty in talking and swallowing. Diagnosis of folate deficiency can be made by doing a peripheral blood smear, measuring the MCV and serum levels of vitamin folate, as well as serum homocysteine levels. Folate deficiency is treated with folate supplements, folate-fortified food items, by stopping the offending medications or limiting alcohol consumption.

Contents

Definition

Chemical structure of the folate family
Chemical structure of the folate family

"Folate" 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, and folacin.[7][20][21][22] Other names include vitamin B9,[1] vitamin Bc,[2] and vitamin M.[3]

Chemically, folates consist of three distinct chemical moieties linked together. A pterin (2-amino-4-hydroxy-pteridine) heterocyclic ring is linked by a methylene bridge to a p-aminobenzoyl group that in turn is bonded through an amide linkage to either glutamic acid or poly-glutamate. One-carbon units in a variety of oxidation states may be attached to the N5 nitrogen atom of the pteridine ring and/or the N10 nitrogen atom of the p-aminobenzoyl group.[23]

Health effects

Folate is especially important during periods of frequent cell division and growth, such as infancy and pregnancy. Folate deficiency hinders DNA synthesis and cell division, affecting hematopoietic cells and neoplasms the most because of their greater frequency of 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).

Folate deficiency

Folate deficiency can be caused by unhealthy diets that do not include enough vegetables and other folate-rich foods; diseases in which folates are not well absorbed in the digestive system (such as Crohn's disease or celiac disease); some genetic disorders that affect levels of folate; and certain medicines (such as phenytoin, sulfasalazine, or trimethoprim-sulfamethoxazole).[24] Folate deficiency is accelerated by alcohol consumption, possibly by interference with folate transport.[25]

Folate deficiency may lead to glossitis, diarrhea, depression, confusion, anemia, and fetal neural tube and brain defects.[26] Other symptoms include fatigue, gray hair, mouth sores, poor growth, and swollen tongue.[24] Folate deficiency is diagnosed by analyzing a complete blood count (CBC) and plasma vitamin B12 and folate levels. A serum folate of 3 μg/L or lower indicates deficiency.[26] 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. Serum folate reacts more rapidly to folate intake than erythrocyte folate.[27]

Since folate deficiency limits cell division, erythropoiesis (production of red blood cells) is hindered. This 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.[28] Both adults and children need folate to make normal red and white blood cells and prevent anemia, which causes fatigue, weakness, and inability to concentrate.[29][30]

Increased homocysteine levels suggest 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 and vitamin B12 deficiency is by testing for methylmalonic acid (MMA) levels. Normal MMA levels indicate folate deficiency and elevated MMA levels indicate vitamin B12 deficiency.[26] Folate deficiency is treated with supplemental oral folic acid of 400 to 1000 μg per day. This treatment is very successful in replenishing tissues, even if deficiency was caused by malabsorption. People with megaloblastic anemia need to be tested for vitamin B12 deficiency before treatment with folic acid, because if the person has vitamin B12 deficiency, folic acid supplementation can remove the anemia, but can also worsen neurologic problems.[26] Cobalamin deficiency may lead to folate deficiency, which, in turn, increases homocysteine levels and may result in the development of cardiovascular disease or birth defects.[31]

Birth defects

Deficiency of folate in pregnant women has been implicated in neural tube defects (NTDs), with an estimate of 300,000 cases worldwide prior to the implementation in many countries of mandatory food fortification.[32] NTDs occur early in pregnancy (first month), therefore women must have abundant folate upon conception and for this reason there is a recommendation that any woman planning to become pregnant consume a folate-containing dietary supplement before and during pregnancy.[33] Compliance with this recommendation is not complete, and many women become pregnant without this being a planned pregnancy, or may not realize that they are pregnant until well into the first trimester, which is the critical period for reducing risk of NTDs. Countries have implemented either mandatory or voluntary food fortification of wheat flour and other grains,[34] or else have no such program and depend on public health and healthcare practitioner advice to women of childbearing age. A meta-analysis of global birth prevalence of spina bifida showed that when mandatory fortification was compared to countries with voluntary fortification or no fortification program, there was a 30% reduction in live births with spina bifida.[35] Some countries reported a greater than 50% reduction.[36] The United States Preventive Services Task Force recommends folic acid as the supplement or fortification ingredient, as forms of folate other than folic acid have not been studied.[21]

A meta-analysis of folate supplementation during pregnancy reported a 28% lower relative risk of newborn congenital heart defects.[37] Prenatal supplementation with folic acid did not appear to reduce the risk of preterm births.[38][39] One systematic review indicated no effect of folic acid on mortality, growth, body composition, respiratory, or cognitive outcomes of children from birth to 9 years old.[40] There was no relation between maternal folic acid supplementation and an increased risk for childhood asthma.[41]

Fertility

Folate contributes to spermatogenesis.[42] In women, folate is important for oocyte quality and maturation, implantation, placentation, fetal growth and organ development.[42]

Heart disease

One meta-analysis reported that multi-year folic acid supplementation, in amounts in most of the included clinical trials at higher than the UL of 1,000 μg/day, reduced the relative risk of cardiovascular disease by a modest 4%.[13] Two older meta-analyses, which would not have incorporated results from newer clinical trials, reported no changes to the risk of cardiovascular disease.[43][44]

Stroke

The absolute risk of stroke with supplementation decreases from 4.4% to 3.8% (a 10% decrease in relative risk).[13] Two other meta-analyses reported a similar decrease in relative risk.[45][46] Two of these three were limited to people with pre-existing cardiovascular disease or coronary heart disease.[13][45] The beneficial result may be associated with lowering circulating homocysteine concentration, as stratified analysis showed that risk was reduced more when there was a larger decrease in homocysteine.[13][45] The effect was also larger for the studies that were conducted in countries that did not have mandatory grain folic acid fortification.[45][46] The beneficial effect was larger in the subset of trials that used a lower folic acid supplement compared to higher.[45][46]

Cancer

Chronically insufficient intake of folate may increase the risk of colorectal, breast, ovarian, pancreas, brain, lung, cervical, and prostate cancers.[7][47][48]

Early after fortification programs were implemented, high intakes were theorized to accelerate the growth of preneoplastic lesions that could lead to cancer, specifically colon cancer.[49][50] Subsequent meta-analyses of the effects of low versus high dietary folate, elevated serum folate, and supplemental folate in the form of folic acid have reported at times conflicting results. Comparing low to high dietary folate showed a modest but statistically significant reduced risk of colon cancer.[51] For prostate cancer risk, comparing low to high dietary folate showed no effect,[52][53] but the same two studies reported a significant increased risk for prostate cancer correlating to elevated serum folate.[52][53] Two reviews of trials that involved folic acid dietary supplements reported, respectively, a statistically significant 24% increase in prostate cancer risk[14] and a not significant 17% increase in prostate cancer risk.[54] Supplementation with folic acid at 1,000 to 2,500 μg/day - the amounts used in many of the supplement trials[14][54] - would result in higher concentrations of serum folate than what is achieved from diets high in food-derived folate. The second study reported no significant increase or decrease in total cancer incidence, colorectal cancer, other gastrointestinal cancer, genitourinary cancer, lung cancer or hematological malignancies in people who were consuming folic acid supplements.[54] A third supplementation meta-analysis limited to reporting only on colorectal cancer incidence showed that folic acid treatment was not associated with colorectal cancer risk.[55]

Anti-folate chemotherapy

Folate is important for cells and tissues that divide rapidly.[56] Cancer cells divide rapidly, and drugs that interfere with folate metabolism are used to treat cancer. The antifolate drug methotrexate is often used to treat cancer because it inhibits the production of the active form of THF from the inactive dihydrofolate (DHF).[57] However, methotrexate can be toxic,[58][59][60] producing side effects such as inflammation in the digestive tract that make eating normally more difficult. Bone marrow depression (inducing leukopenia and thrombocytopenia) and acute kidney and liver 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.[61] Folic acid supplements have little established role in cancer chemotherapy.[62][63] The supplement of folinic acid in people undergoing methotrexate treatment is to give cells dividing less rapidly enough folate to maintain normal cell functions. The amount of folate given is depleted by rapidly dividing cells (cancer) quickly, so does not negate the effects of methotrexate.

Neurological disorders

Conversion of homocysteine to methionine requires folate and vitamin B12. Elevated plasma homocysteine and low folate are associated with cognitive impairment, dementia and Alzheimer's disease.[64][65] Supplementing the diet with folic acid and vitamin B12 lowers plasma homocysteine.[65] However, several reviews reported that supplementation with folic acid alone or in combination with other B vitamins did not prevent development of cognitive impairment nor slow cognitive decline.[66][65][67]

The relative risk of autism spectrum disorders was reduced by 23% when the maternal diet was supplemented during pregnancy. Subset analysis confirmed this among Asian, European and American populations.[68]

Some evidence links a shortage of folate with clinical depression.[69] Limited evidence from randomized controlled trials showed using folic acid in addition to selective serotonin reuptake inhibitors (SSRIs) may have benefits.[70] Research found a link between depression and low levels of folate.[71][72] The exact mechanisms involved in the development of schizophrenia and depression are not entirely clear, but the bioactive folate, methyltetrahydrofolate (5-MTHF), a direct target of methyl donors such as S-adenosyl methionine (SAMe), recycles the inactive dihydrobiopterin (BH2) into tetrahydrobiopterin (BH4), the necessary cofactor in various steps of monoamine synthesis, including that of dopamine. BH4 serves a regulatory role in monoamine neurotransmission and is required to mediate the actions of most antidepressants. 5-MTHF also plays both direct & indirect roles in DNA methylation, NO2 synthesis, and one-carbon metabolism.[73]

Folic acid, B12 and iron

A complex interaction occurs between folic acid, vitamin B12, and iron. A deficiency of one may be "masked" by excess of another, so when taken as dietary supplements, the three need to be in balance.[74][75][76]

Malaria

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.[77]

Metabolism

The biological activity of folate in the body depends upon dihydrofolate reductase action in the liver which converts folate into tetrahydrofolate (THF). This action is rate-limiting in humans leading to elevated blood concentrations of unmetabolized folic acid when consumption from dietary supplements and fortified foods nears or exceeds the U.S. Tolerable Upper Intake Level of 1,000 μg per day.[9][78]

Biosynthesis

Animals, including humans, cannot synthesize folate and therefore must obtain folate from their diet. All plants and fungi and certain protozoa, bacteria, and archaea can synthesize folate de novo through variations on the same biosynthetic pathway.[79] The folate molecule is synthesized from pterin pyrophosphate and para-aminobenzoic acid through the action of dihydrofolate synthase. Pterin is in turn derived in a series of enzymatically catalyzed steps from guanosine triphosphate (GTP), while para-aminobenzoic acid (vitamin B10) is a product of the shikimate pathway.[79]

Bioactivation

Biotransformation of folic acid into folinic acids where R = para-aminobenzoate-glutamate.[80]
Biotransformation of folic acid into folinic acids where R = para-aminobenzoate-glutamate.[80]

All of the biological functions of folic acid are performed by THF and its methylated derivatives. Hence folic acid must first be reduced to THF. This four electron reduction proceeds in two chemical steps both catalyzed by the same enzyme, dihydrofolate reductase.[80] Folic acid is first reduced to dihydrofolate and then to tetrahydrofolate. Each step consumes one molecule of NADPH (biosynthetically derived from vitamin B3) and produces one molecule of NADP.[9][81] Mechanistically, hydride is transferred from NADPH to the C6 position of the pteridine ring.[82]

A one-carbon (1C) methyl group is added to tetrahydrofolate through the action of serine hydroxymethyltransferase (SHMT) to yield 5,10-methylenetetrahydrofolate (5,10-CH2-THF). This reaction also consumes serine and pyridoxal phosphate (PLP; vitamin B6) and produces glycine and pyridoxal.[80] A second enzyme, methylenetetrahydrofolate dehydrogenase (MTHFD2)[83] oxidizes 5,10-methylenetetrahydrofolate to an iminium cation which in turn is hydrolyzed to produce 5-formy-THF and 10-formyl-THF.[80] This series of reactions using the β-carbon atom of serine as the carbon source provide the largest part of the one-carbon units available to the cell.[84]

Alternative carbon sources include formate which by the catalytic action of formate–tetrahydrofolate ligase add a 1C unit to THF to yield 10-formyl-THF. Glycine, histidine, and sarcosine can also directly contribute to the THF-bound 1C pool.[85]

Drug interference

A number of drugs interfere with the biosynthesis of THF from folic acid. Among them are the antifolate dihydrofolate reductase inhibitors such as the antimicrobial, trimethoprim, the antiprotozoal, pyrimethamine and the chemotherapy drug methotrexate,[86][87] and the sulfonamides (competitive inhibitors of 4-aminobenzoic acid in the reactions of dihydropteroate synthetase).[88]

Valproic acid, one of the most commonly prescribed epilepsy treatment drugs, also used to treat certain psychological conditions such as bipolar disorder, is a known inhibitor of folic acid, and as such, has been shown to cause birth defects, including neural tube defects, plus increased risk for children having cognitive impairment and autism. There is evidence that folate consumption is protective.[89][90][91]

Function

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.[92] These reactions are collectively known as folate-mediated one-carbon metabolism.[9][93]

DNA synthesis

Folate derivatives participate in the biosynthesis of both purines and pyrimidines. Formyl folate is required for two of the steps in the biosynthesis of inosine monophosphate, the precursor to GMP and AMP. Methylenetetrahydrofolate donates the C1 center required for the biosynthesis of dTMP (2′-deoxythymidine-5′-phosphate) from dUMP (2′-deoxyuridine-5′-phosphate).

Formyl folate is required for two of the steps in the biosynthesis of inosine monophosphate, the precursor to GMP and AMP. Methylenetetrahydrofolate donates the C1 center required for the biosynthesis of dTMP (2′-deoxythymidine-5′-phosphate) from dUMP (2′-deoxyuridine-5′-phosphate). The conversion is catalyzed by thymidylate synthase.[9]

Vitamin B12 activation

Simplified schematic diagram of the folate methionine cycle[94]
Simplified schematic diagram of the folate methionine cycle[94]

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.[95] Thus, a deficiency in B12 can cause accumulation of methyl-THF, mimicking folate deficiency.

Diet

The United States Department of Agriculture, Agricultural Research Service, maintains a food composition database from which folate content in hundreds of foods can be searched as shown in the table.[96] The Food Fortification Initiative lists all countries in the world that conduct fortification programs,[97] and within each country, what nutrients are added to which foods, and whether those programs are voluntary or mandatory. In the US, mandatory fortification of enriched breads, cereals, flours, corn meal, pastas, rice, and other grain products began in January 1998. As of December 21, 2018, 81 countries required food fortification with one or more vitamins.[34] The most commonly fortified vitamin – as used in 62 countries – is folate; the most commonly fortified food is wheat flour, followed by maize flour and rice. From country to country, added folic acid amounts range from 0.4 to 5.1 μg/100 g, but the great majority are in a more narrow range of 1.5 to 2.5 μg/100 g.[34] Folate naturally found in food is susceptible to destruction from high heat cooking, especially in the presence of acidic foods and sauces. It is soluble in water, and so may be lost from foods boiled in water.[98] For foods that are normally consumed cooked, values in the table are for folate naturally occurring in cooked foods.

Plant sources[96] Amount as
Folate
(μg / 100g)
Peanuts 246
Sunflower seed kernels 238
Lentils 181
Chickpeas 172
Asparagus 149
Spinach 146
Lettuce 136
Peanuts (oil-roasted) 125
Soybeans 111
Broccoli 108
Walnuts 98
Plant sources[96] Amount as
Folate
(μg / 100g)
Peanut butter 92
Hazelnuts 88
Avocados 81
Beets 80
Kale 65
Bread (not fortified) 65
Cabbage 46
Red bell peppers 46
Cauliflower 44
Tofu 29
Potatoes 28
Animal sources[96] Amount as
Folate
(μg / 100g)
Chicken liver 578
Calf liver 331
Cheese 20–60
Chicken eggs 44
Salmon 35
Chicken 12
Beef 12
Pork 8
Yogurt 8–11
Milk, whole 5
Butter, salted 3

Dietary recommendations

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. One 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.[1]

National Institutes of Health (U.S.) nutritional recommendations[1]
(µg DFE per day for RDA, µg folic acid for UL)
Age Infants Children and adults Pregnant women Lactating women
(AI) (UL) (RDA) (UL) (RDA) (UL) (RDA) (UL)
0–6 months 65 None set
7–12 months 80 None set
1–3 years 150 300
4–8 years 200 400  –
9–13 years 300 600
14–18 400 800 600 800 500 800
19+ 400 1000 600 1000 500 1000

The U.S. Institute of Medicine defines Estimated Average Requirements (EARs), Recommended Dietary Allowances (RDAs), Adequate Intakes (AIs) and Tolerable upper intake levels (ULs) at Dietary Reference Intakes (DRIs).[1][26] The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL defined the same as in United States. For women and men over age 18 the PRI is set at 330 μg/day. PRI for pregnancy is 600 μg/day, for lactation 500 μg/day. For children ages 1–17 years the PRIs increase with age from 120 to 270 μg/day. These values differ somewhat from the U.S. RDAs.[99] The United Kingdom's Dietary Reference Value for folate, set by the Committee on Medical Aspects of Food and Nutrition Policy in 1991, is 200 μg/day for adults.[100]

Safety

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. One potential issue associated with high doses of folic acid is that it has a masking effect on the diagnosis of pernicious anaemia due to vitamin B12 deficiency, and may even precipitate or exacerbate neuropathy in vitamin B12-deficient individuals. This evidence justified development of a UL for folate.[26] In general, ULs are set for vitamins and minerals when evidence is sufficient. The adult UL of 1,000 μg for folate (and lower for children) refers specifically to folic acid used as a supplement, as no health risks have been associated with high intake of folate from food sources. The EFSA reviewed the safety question and agreed with United States that the UL be set at 1,000 μg.[101] The Japan National Institute of Health and Nutrition set the adult UL at 1,300 or 1,400 μg depending on age.[102]

Reviews of clinical trials that called for long-term consumption of folic acid in amounts exceeding the UL have raised concerns. One theory is that consumption of large amounts of folic acid leads to detectable amounts of unmetabolized folic acid circulating in blood because the enzyme dihydrofolate reductase that converts folic acid to the biologically active forms is rate limiting. Evidence of a negative health effect of folic acid in blood is not consistent, and folic acid has no known cofactor function that would increase the likelihood of a causal role for free FA in disease development.[103] However, low vitamin B12 status in combination with high folic acid intake, in addition to the previously mentioned neuropathy risk, appeared to increase the risk of cognitive impairment in the elderly.[104] Long-term use of folic acid dietary supplements in excess of 1,000 μg/day has been linked to an increase in prostate cancer risk.[14]

Food labeling

For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For folate labeling purposes 100% of the Daily Value was 400 μg. As of the 27 May 2016 update, it was kept unchanged at 400 μg. [105] A table of the old and new adult Daily Values is provided at Reference Daily Intake. The original deadline to be in compliance was 28 July 2018, but on 29 September 2017 the FDA released a proposed rule that extended the deadline to 1 January 2020 for large companies and 1 January 2021 for small companies.[106] European Union regulations require that labels declare energy, protein, fat, saturated fat, carbohydrates, sugars, and salt. Voluntary nutrients may be shown if present in significant amounts. Instead of Daily Values, amounts are shown as percent of Reference Intakes (RIs). For folate, 100% RI was set at 200 μg in 2011.[107]

Food fortification

Folic acid fortification is a process where synthetic folic acid is added to wheat flour or other foods with the intention of promoting public health through increasing blood folate levels in the populace. It is used as it more stable during processing and storage.[8][104] After the discovery of the link between insufficient folic acid and neural tube defects, governments and health organizations worldwide made recommendations concerning folic acid supplementation for women intending to become pregnant. Because the neural tube closes in the first four weeks of gestation, often before many women even know they are pregnant, many countries in time decided to implement mandatory food fortification programs. A meta-analysis of global birth prevalence of spina bifida showed that when mandatory fortification was compared to countries with voluntary fortification or no fortification program, there was a 30% reduction in live births with spina bifida,[35] with some countries reporting a greater than 50% reduction.[36]

Folic acid is added to grain products in more than 80 countries, either as required or voluntary fortification,[12][34] and these fortified products make up a significant source of the population's folate intake.[108] Fortification is controversial, with issues having been raised concerning individual liberty,[104] as well as the theorized health concerns described in the Safety section. In the U.S., there is concern that the federal government mandates fortification but does not provide monitoring of potential undesirable effects of fortification.[104] The Food Fortification Initiative lists all countries in the world that conduct fortification programs,[97] and within each country, what nutrients are added to which foods. As of December 21, 2018, 81 countries required food fortification with one or more vitamins.[34] The most commonly mandatory fortified vitamin – in 62 countries – is folate; the most commonly fortified food is wheat flour.[34]

Australia and New Zealand

Australia and New Zealand jointly agreed to wheat flour fortification through the Food Standards Australia New Zealand in 2007. The requirement was set at 135 µg of folate per 100 g of bread. Australia implemented the program in 2009.[109] New Zealand was also planning to fortify bread (excluding organic and unleavened varieties) starting in 2009, but then opted to wait until more research was done. The Association of Bakers and the Green Party had opposed mandatory fortification, describing it as "mass medication."[110][111] Food Safety Minister Kate Wilkinson reviewed the decision to fortify in July 2009, citing as reasons to oppose claims for links between over consumption of folate with increased risk of cancer.[112] In 2012 the delayed mandatory fortification program was revoked and replaced by a voluntary program, with the hope of achieving a 50% bread fortification target.[113]

Canada

Canadian public health efforts focused on promoting awareness of the importance of folic acid supplementation for all women of childbearing age and decreasing socio-economic inequalities by providing practical folic acid support to vulnerable groups of women.[114] Folic acid food fortification became mandatory in 1998, with the fortification of 150 µg of folic acid per 100 grams of enriched flour and uncooked cereal grains.[50] The results of folic acid fortification on the rate of neural tube defects in Canada have been positive, showing a 46% reduction in prevalence of NTDs; the magnitude of reduction was proportional to the prefortification rate of NTDs, essentially removing geographical variations in rates of NTDs seen in Canada before fortification.[115]

United Kingdom

While the Food Standards Agency recommended folic acid fortification,[116][117][118] and wheat flour is fortified with iron,[119] folic acid fortification of wheat flour is allowed voluntarily rather than required. A 2018 review by authors based in the United Kingdom strongly recommended that mandatory fortification be reconsidered as a means of reducing the risk of neural tube defects.[12]

United States

In the United States and many other countries, wheat flour is fortified with folic acid, some countries also fortify maize flour and rice.[34]
In the United States and many other countries, wheat flour is fortified with folic acid, some countries also fortify maize flour and rice.[34]

In 1996, the United States Food and Drug Administration (FDA) published regulations requiring the addition of folic acid to enriched breads, cereals, flours, corn meals, pastas, rice, and other grain products.[120] This ruling took effect on 1 January 1998, and was specifically targeted to reduce the risk of neural tube birth defects in newborns.[121] There were concerns expressed that the amount of folate added was insufficient.[122]

The fortification program was expected to raise a person's folic acid intake level by 70–130 µg/day;[123] however, an increase of almost double that amount was actually observed.[124] This could be from the fact that many foods are fortified by 160–175% over the required amount.[124] Much of the elder population take supplements that add 400 µg to their daily folic acid intake. This is a concern because 70–80% of the population have detectable levels of unmetabolized folic acid in their blood, a consequence of folic acid supplementation and fortification.[49] However, at blood concentrations achieved via food fortification, folic acid has no known cofactor function that would increase the likelihood of a causal role for free FA in disease development.[103]

The U.S. National Center for Health Statistics conducts biannual National Health and Nutrition Examination Survey (NHANES) to assess the health and nutritional status of adults and children in the United States. Some results are reported as What We Eat In America. The 2013–2014 survey reported that for adults ages 20 years and older, men consumed on average of 249 μg/day folate from food plus 207 μg/day of folic acid from consumption of fortified foods, for a combined total of 601 μg/day of dietary folate equivalents (DFEs; because each microgram of folic acid counts as 1.7 μg of food folate). For women, the values are 199, 153 and 459 μg/day, respectively. This means that fortification led to a bigger increase in folic acid intake than first projected, and that more than half the adults are consuming more than the RDA of 400 μg (as DFEs). Even so, fewer than half of pregnant women are exceeding the pregnancy RDA of 600 μg/day.[125]

Before folic acid fortification, about 4,100 pregnancies were affected by a neural tube defect each year in the United States. The Centers for Disease Control and Prevention reported in 2015 that since the addition of folic acid in grain-based foods as mandated by the FDA, the rate of neural tube defects dropped by 35%. This translates to an annual saving in total direct costs of approximately $508 million for the NTD-affected births that were prevented.[126][127]

In 2016, folate was the 96th most prescribed medication in the United States, with more than 8 million prescriptions.[128]

History

In the 1920s, scientists believed folate deficiency and anemia were the same condition.[129] 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.[16][130] In the late 1930s, folate was identified as the corrective substance in brewer's yeast. It was first isolated via extraction from spinach leaves by Herschel K. Mitchell, Esmond E. Snell, and Roger J. Williams in 1941.[131] The term "folic" is from the Latin word folium (which means leaf) because it was found in dark-green leafy vegetables.[19] 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.[95] 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, NY.[132][133] This research subsequently led to the synthesis of the antifolate aminopterin, which was used to treat childhood leukemia by Sidney Farber in 1948.[95][134]

In the 1950s and 1960s, scientists began to discover the biochemical mechanisms of action for folate.[129] In 1960, researchers linked folate deficiency to risk of neural tube defects.[129] In the late 1990s, the U.S. and Canadian governments decided that despite public education programs and the availability of folic acid supplements, there was still a challenge for women of child-bearing age to meet the daily folate recommendations, which is when those two countries implemented folate fortification programs.[121] As of December 2018, 62 countries mandated food fortification with folic acid.[34]

Other animals

Veterinarians may test cats and dogs if a risk of folate deficiency is indicated. Cats with exocrine pancreatic insufficiency, moreso than dogs, may have low serum folate. In dog breeds at risk for cleft lip and cleft palate dietary folic acid supplementation significantly decreased incidence.[135]

See also

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Biochemistry links
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