Folic Acid

Folic acid crystals

Folic acid (chemical name: pteroylglutamic acid) belongs to the group of B vitamins. It is also known as folacin, vitamin Bc, vitamin B9 and Lactobacillus casei factor, although these names are now considered obsolete.

The term "folates" is used to designate all members of the same family of compounds in which pteroic acid is bound to one or more molecules of L-glutamate.

Figure 1: Structure of folic acid (pteroyl glutamic acid)

Folates are found in a wide variety of foods. Richest sources are liver, dark green leafy vegetables, beans, wheat germ and yeast. Other sources are egg yolk, milk and dairy products, beets, orange juice and whole-wheat bread.

Most dietary folates exist in the polyglutamate form, which is converted in the wall of the small intestine to the monoglutamate form before it is absorbed into the bloodstream. Only about half of the folate ingested in the diet is actually absorbed. Under normal circumstances, folates synthesized by intestinal bacteria do not contribute significantly to folate nutrition in humans, because bacterial folate synthesis is usually restricted to the large intestine (colon), whereas absorption occurs mainly in the upper part of the small intestine (jejunum).

Folic acid is widely distributed in tissues, most of it as polyglutamate derivatives. The main storage organ is the liver, which contains about half of the body's stores. The actual amount of folate stored in the body has not been accurately determined, but has been estimated to be around 7.5 mg in the healthy adult male. other studies have found more than this amount in the liver alone. Normal plasma levels are around 5-20 µg/L; red blood cells contain some thirty times more. Daily excretion of folates, mainly through the urine, is normally less than 1% of the body's stores.

Methyltetrahydrofolate, which is the predominant folate in the plasma, can be measured by a microbiological assay using Lactobacillus casei as test organism.

Radioassays based on competitive protein binding are simpler to perform and are not affected by antibiotics, which give false low values in microbiological assays.

Plasma folate levels are not a reliable indicator of folate deficiency, as they react swiftly to fluctuations in dietary intake. A more accurate index of folate status is provided by levels in the red blood cells.

Other indicators of folate deficiency are the folate clearance test and the deoxyuridine suppression test.

Most forms of folate in food are unstable. Fresh leafy vegetables stored at room temperature may lose up to 70% of their folate activity in three days. Considerable losses also occur by extraction into the cooking water (up to 95%) and by heating.

Several chemotherapeutic agents (e.g. methotrexate, trimethoprim, pyrimethamine) inhibit the enzyme dihydrofolate reductase, which is necessary for the metabolism of folates.

Many drugs may interfere with the absorption, utilisation and storage of folates. These include oral contraceptives, alcohol, cholestyramine (a drug used to lower blood cholesterol), antiepileptic agents such as barbiturates and diphenylhydantoin, as well as sulfasalazine, which is a sulfonamide used in the treatment of ulcerative colitis. Drugs that reduce the acidity in the intestine, such as antacids and modern anti-ulcer drugs, have also been reported to interfere with the absorption of folic acid.

Proper folate utilisation depends on an adequate supply of other vitamins of the B group and vitamin C, which are involved in the chemical reactions needed for folate metabolism. Vitamin C may also provide the reducing conditions needed to preserve folates in the diet and a diet deficient in folates is also likely to be deficient in vitamin C.

Tetrahydrofolic acid, which is the active form of folate in the body, acts as a coenzyme in numerous essential metabolic reactions. It plays an important role in the metabolism of amino acids, the constituents of proteins. It is also involved in the synthesis of the nucleic acids, the molecules that carry the genetic information in cells, as well as in the formation of blood cells and some of the constituents of nervous tissue. Folic acid is therefore essential for proper growth and for optimal functioning of the nervous system and the bone marrow.

Folate deficiency is one of the commonest vitamin deficiencies. It can result from inadequate intake, defective absorption, abnormal metabolism or increased requirements. Diagnosis of a subclinical deficiency depends on demonstrating reduced tissue levels or other biochemical evidence, as hematological manifestations are usually absent and plasma levels are not a reliable indicator. A folate deficiency state may appear in 1-4 weeks, depending on dietary habits and body stores of the vitamin.

Early symptoms of folate deficiency are non-specific and may include tiredness, irritability and loss of appetite.

Severe folate deficiency almost always leads, within a short period of time, to a megaloblastic anaemia, a condition in which the bone marrow produces giant, immature red blood cells. Clinical symptoms vary and relate to the severity of the anaemia and the acuteness of onset. If left untreated, megaloblastic anaemia may be fatal.

In acute deficiency (e.g. after administration of folate antagonists) there may be loss of appetite, abdominal pain, sickness and diarrhoea. Painful ulcers may develop in the mouth and pharynx; skin changes and hair loss may occur.

Common symptoms of chronic folate deficiency are tiredness and loss of energy and drive. A sore mouth and tongue may occur. Deficiency during pregnancy may result in premature birth and/or malformation of the offspring. In children, growth may be retarded and puberty delayed. Folate deficiency has also been associated with neurological disorders such as dementia and depression.

Folate deficiency is very common in many parts of the world and is part of the general problem of undernutrition. In developed countries, nutritional folate deficiency may be encountered above all in economically underprivileged groups (e.g. the elderly). Reduced folate intake is also often seen in people on special diets (e.g. weight-reducing diets).

Disorders of the stomach (e.g. atrophic gastritis) and small intestine (e.g. celiac disease, sprue, Crohn's disease) may lead to folate deficiency as a result of malabsorption. In conditions with a high rate of cell turnover (e.g. cancer, certain anaemias and skin disorders), folate requirements are increased. This is also the case during pregnancy and lactation, due to rapid tissue growth during pregnancy and to losses through the milk during lactation.

People undergoing drug treatment, e.g. for epilepsy, cancer or an infection, are at high risk of developing a folate deficiency, as well as women on oral contraceptives and patients with renal failure who require regular haemodialysis. Acute folate deficiencies have been reported to occur within a relatively short time in patients under intensive care, especially those on total parenteral nutrition.

Recommendations for daily intake of folates have been formulated in 27 countries, whereby many of them follow the recommendations of the Food and Nutrition Board of the US National Research Council. In its latest guide (1989), this Organisation recommends a daily intake of 20-35 µg of dietary folate for infants, 50-150 µg for children, 180 µg for adult females and 200 µg for adult males. To cover increased needs during pregnancy and lactation, 400 µg per day and 260-280 µg per day, respectively, are recommended. Some specialists, including a group under the auspices of the World Health Organisation, recommend an intake of 600 µg daily during lactation.

Folic acid is available as oral preparations, alone or in combination with other vitamins or minerals (e.g. iron), and as an aqueous solution for injection. As the acid is only poorly soluble in water, folate salts are used to prepare liquid dosage forms. Folinic acid (also known as leucovorin or citrovorum factor) is a derivative of folic acid administered by intramuscular injection to circumvent the action of inhibitors of dihydrofolate reductase, such as methotrexate. Otherwise it is not indicated for the prevention or treatment of folic acid deficiency.

In situations where there is a high risk of folate deficiency, oral folic acid supplementation is recommended, usually in a multivitamin preparation containing 400-500 µg of folic acid. Multivitamin supplementation during pregnancy with daily doses of up to 5 mg folic acid has been shown to reduce the incidence of malformations in the offspring, especially neural tube defects (malformations of the brain and spinal cord). Such supplementation is also effective in pregnant women receiving anticonvulsant drugs for epilepsy.

Abnormal tissue development (dysplasia), characteristic of some precancerous conditions, can also be favourably affected by folate supplementation. In various studies, involving oral-contraceptive users with dysplasia in the uterine cervix or smokers with dysplasia of the bronchi, three to four months of treatment with 10 mg folic acid daily significantly reduced the number of abnormal cells.

In acute cases of megaloblastic anaemia, treatment often has to be started before a diagnosis of the cause has been made. To avoid complications that may arise by treating a B12 deficiency with folic acid in such circumstances (see below), both folic acid and vitamin B12 need to be administered until a specific diagnosis is available.

Oral folic acid is not toxic for man. Even with daily doses as high as 15 mg (about 40 times the RDA) there have been no substantiated reports of toxicity, and a daily supplement of 10 mg has been taken for five years without adverse effect. It has been claimed that high doses of folic acid may counteract the effect of antiepileptic medication and so increase the frequency of seizures in susceptible patients. It has also been reported that high-dose folic acid may interfere with zinc absorption.

A high intake of folic acid can mask a vitamin B12 deficiency. It should therefore not be used indiscriminately in patients with anaemia because of the risk of damage to the nervous system due to the B12 deficiency.

Folic acid is manufactured on a large scale by chemical synthesis. Various processes are known. Most synthesised folic acid is used in animal feed.

1931: Wills in India observes the effect of liver and yeast extracts on tropical macrocytic anaemia and concludes that this disorder must be due to a dietary deficiency. She recognizes that yeast contains a curative agent equal in potency to that of liver

1938: Day and coworkers find an antianaemia factor for monkeys in yeast and designated it "vitamin M". Around the same time, Stokstad and Manning discover a growth factor for chicks, which they called "Factor U"

1939: Hogan and Parrott identify an antianaemia factor for chicks in liver extracts, which they name "Vitamin Bc"

1940: Growth factors for Lactobacillus casei and Streptococcus lactis are found. Snell and Peterson coin the term "norite-eluate factor"

1941: Mitchell and colleagues suggest the name "folic acid" (folium, Latin for leaf) for the factor responsible for growth stimulation of Streptococcus is that they isolate from spinach and which they suspect to have vitamin-like properties for animals

1945: Angier and co-workers report the synthesis of a compound identical to the L. casei factor isolated from liver. They later describe the chemical structures of the basic and related compounds.

1945: Spies demonstrates that folic acid cures megaloblastic anaemia of pregnancy

1962: Herbert consumes a folate-deficient diet for several months and records the development of deficiency symptoms. His findings set the criteria for the diagnosis of folate deficiency. In the same year, Herbert estimates the folic acid requirements for adults, which still serve as a basis for many RDAs

1991: Wald establishes that folic acid supplementation reduced risk of neural tube defects by 70% among women who had already given birth to a child with this birth defect

1992: Butterworth finds that higher than normal serum levels of folic acid are associated with decreased risk of cervical cancer in women infected with human papillomavirus. Also, Czeizel demonstrates that first-time occurrence of neural tube defects may be largely eliminated with a multivitamin containing folic acid taken in the perioconceptional period

1993: The US Public Health Service recommends that all women of childbearing potential consume 0.4 mg (400 µg) of folate daily in order to reduce the risk of foetal malformations such as spina bifida and other neural tube defects