Vitamin B9 (Folate)

Folate is the general name for a group of related compounds that share the activity of vitamin B9, built from a pterin ring joined through a bridge to para-aminobenzoic acid and one or more glutamate units ^[2]. The naturally occurring folates in food are chemically less stable than folic acid, the fully oxidized synthetic form manufactured for supplements and fortification; the body reduces both to active forms such as 5-methyltetrahydrofolate ^[2]. Because these forms differ in stability and in how readily they are absorbed and used, folate, folic acid, and 5-methyltetrahydrofolate are not interchangeable in every respect ^[2].

Folate's biological role is as a carrier of one-carbon groups, which it shuttles within a network of reactions collectively called one-carbon metabolism ^[1]. These reactions build the purine and pyrimidine bases of DNA and RNA and, together with vitamin B12, regenerate methionine from homocysteine, supplying methyl groups for the methylation of DNA, proteins, and lipids ^[1]. Because rapidly dividing cells need a steady supply of nucleotides, tissues such as bone marrow and the developing embryo are especially dependent on folate ^[1].

A shortage of folate impairs DNA synthesis and produces megaloblastic anemia, in which red blood cells are large and immature, with symptoms such as fatigue and breathlessness ^[1]. Folate deficiency around conception is an important preventable cause of neural tube defects, malformations of the brain and spine including spina bifida and anencephaly that arise when the neural tube fails to close in early pregnancy, often before pregnancy is recognized ^[3][4]. Because folate and vitamin B12 metabolism are linked, high folic acid intake can also mask the anemia of B12 deficiency while nerve damage progresses ^[2].

The vitamin traces to work in the 1930s by Lucy Wills, who found that a factor in yeast could correct a form of anemia in pregnancy, and folate was later isolated from leafy greens, giving rise to its name from the Latin word for leaf ^[3]. Recognition that folic acid before and during early pregnancy prevents many neural tube defects led numerous countries to mandate fortification of wheat flour and other staples, which has measurably reduced the prevalence of these defects ^[3][4]. Even so, mandatory fortification currently reaches only a fraction of preventable cases worldwide, and coverage remains uneven ^[3].

Folic acid is included on the World Health Organization's list of essential medicines and is used to treat and prevent deficiency and to reduce the risk of neural tube defects ^[3]. It is available over the counter and by prescription, in stand-alone tablets, prenatal and multivitamin products, and fortified foods, and related forms such as folinic acid and 5-methyltetrahydrofolate are also used ^[2]. An upper limit on supplemental folic acid is set in part because very high intakes may obscure vitamin B12 deficiency and because the health effects of large amounts of unmetabolized folic acid remain debated ^[2][4].

Folate is very safe, but high folic acid intake can mask a vitamin B12 deficiency by correcting the anemia while nerve damage continues, so B12 status matters. Otherwise side effects are minimal. Using methylfolate sidesteps some conversion concerns but can feel activating for some people.

Two folate interactions matter more than the rest.

The first is diagnostic. Folic acid corrects the megaloblastic anemia of vitamin B12 deficiency while doing nothing for the neurological damage, so supplementation can mask a B12 deficiency until the neuropathy is advanced and irreversible. That is why B12 status is checked before folate is used against an unexplained macrocytic anemia.

The second is with phenytoin, and it runs in both directions. Phenytoin depletes folate over months; folate repletion then accelerates phenytoin metabolism, and a documented case showed serum phenytoin falling into the subtherapeutic range with a breakthrough seizure after supplementation began. Phenobarbital and primidone behave similarly.

Folate also collides with antifolate drugs by design. It antagonizes methotrexate at the enzyme level, which is exploited deliberately to reduce toxicity in rheumatoid arthritis but is unwanted during antineoplastic use, and it can blunt pyrimethamine against toxoplasmosis and malaria. Running the other way, trimethoprim, sulfasalazine, triamterene and cholestyramine all lower folate status by inhibiting its reductase, its absorption, or both.

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