5-methyl tetrahydrofolate (5-MTHF)

5‑methyl tetrahydrofolate (5‑MTHF) is the naturally occurring, reduced form of folate and represents the predominant circulating form of vitamin B9 in human plasma. Unlike folic acid, which must undergo multiple enzymatic conversions to become bioactive, 5‑MTHF is immediately active upon absorption and participates directly in cellular methylation reactions and one‑carbon metabolism, including the synthesis of methionine from homocysteine and the production of nucleotides for DNA and RNA. Folate was first identified in the 1930s and later established as an essential nutrient for preventing megaloblastic anemia. Over time, the biochemistry of folate metabolism was elucidated, revealing the critical role of reduced folate forms such as 5‑MTHF in human physiology. While the term folate covers a family of related compounds found in foods, 5‑MTHF denotes the specific methylated tetrahydrofolate form that serves as the active coenzyme in methylation cycles and nucleotide synthesis. Because folate cannot be synthesized by humans, dietary sources or supplements are essential. In plasma, 5‑MTHF accounts for the majority of folate species, and its levels reflect folate status and availability for metabolic processes. Its active role in homocysteine regulation is especially important, as elevated homocysteine is a risk factor for cardiovascular disease and other conditions. Although 5‑MTHF is included under the broader category of folate in dietary recommendations, its specific function and metabolism distinguish it from both food folates and synthetic folic acid, particularly in individuals with genetic variations such as MTHFR polymorphisms that impair conversion of folic acid.

5‑MTHF plays indispensable roles in human health at the biochemical, cellular, and systemic levels. At the molecular level, 5‑MTHF acts as a methyl donor in the conversion of homocysteine to methionine, a reaction that also supports S‑adenosylmethionine (SAMe) synthesis, the universal methyl donor for methylation reactions involving DNA, RNA, neurotransmitters, and phospholipids. This methylation capacity underpins gene expression, neural function, and detoxification pathways. Adequate folate status, reflected by sufficient 5‑MTHF levels, is essential for DNA synthesis and repair due to its involvement in thymidylate and purine synthesis, which are critical during rapid cell division and growth. A deficiency impairs these pathways, causing megaloblastic anemia and compromised tissue regeneration. Clinical evidence supports the role of folate (folic acid and 5‑MTHF) in reducing the risk of neural tube defects (NTDs) when taken before and during early pregnancy, with supplementation decreasing NTD incidence significantly in randomized studies of folic acid, and 5‑MTHF serving as the active bioavailable form in plasma. In cardiovascular health, 5‑MTHF contributes to lowering homocysteine levels, which are associated with endothelial dysfunction and cardiovascular risk when elevated; while large trials have shown modest risk reductions with folate supplementation, the direct effect of 5‑MTHF remains an area of active investigation. Beyond structural functions, 5‑MTHF is involved in neurotransmitter synthesis, potentially influencing mood and cognitive health. Some clinical studies have found that methylfolate supplementation can augment antidepressant response in individuals with major depressive disorder, particularly those with MTHFR polymorphisms, by enhancing monoamine synthesis. Emerging research also explores roles in reproductive health, where folate status influences oocyte quality and spermatogenesis, and metabolic regulation, where folate perturbations intersect with methylation and epigenetic patterns. Despite these benefits, the evidence base specifically for 5‑MTHF rather than folic acid remains limited in certain areas, and more high‑quality randomized controlled trials are needed.

Because 5‑MTHF is the active form of folate, requirements are expressed in terms of dietary folate equivalents (DFEs), a measure that accounts for differences in bioavailability among food folates, folic acid, and 5‑MTHF. The NIH dietary reference intakes for folate recommend 400 µg DFE per day for adults, rising to 600 µg DFE during pregnancy to support fetal development and to 500 µg DFE for lactation. For infants, an adequate intake is set at 65–80 µg DFE, with increasing requirements throughout childhood and adolescence. DFEs consider that synthetic forms such as folic acid and 5‑MTHF are more bioavailable than natural food folates, but specific conversion factors for 5‑MTHF have not been formally established. Age, sex, physiological status (e.g., pregnancy), and genetic factors like MTHFR polymorphisms influence folate needs and effective utilization of 5‑MTHF. Individuals with impaired folic acid conversion may benefit from direct 5‑MTHF intake to achieve equivalent folate status. While the RDA aims to prevent deficiency and support normal metabolic function, clinicians may adjust targets based on clinical context, blood folate levels, and homocysteine measurements. Optimal status is reflected by plasma folate concentrations in the range considered sufficient by clinical laboratories, though these ranges vary by assay and population. Dietary patterns rich in folate‑containing foods such as leafy greens, legumes, and liver, combined with fortified foods, typically meet or exceed the RDA when consumed regularly.

Deficiency of folate, including 5‑MTHF, manifests clinically as impaired DNA synthesis and methylation, leading to hematological and systemic symptoms. Megaloblastic anemia is a hallmark, characterized by large, immature red blood cells, fatigue, pallor, shortness of breath, and heart palpitations due to ineffective erythropoiesis. Neurological and cognitive symptoms such as irritability, headaches, and forgetfulness can occur, particularly when homocysteine levels rise, though these are nonspecific. Glossitis (smooth, reddened tongue), loss of appetite, and gastrointestinal disturbances may accompany deficiency. In pregnancy, inadequate folate status increases the risk of neural tube defects due to disrupted neural tube closure during early embryogenesis. Subclinical deficiency may present with elevated homocysteine, which has been associated with vascular endothelial dysfunction. Individuals with genetic variants affecting MTHF reductase conversion (e.g., MTHFR C677T) may show functional deficiency despite adequate folate intake, as less folate is converted to the active 5‑MTHF form. Cerebral folate deficiency syndromes, a rare genetic disorder affecting folate transport into the central nervous system, result in low cerebrospinal fluid 5‑MTHF despite normal plasma levels, leading to developmental delays, motor dysfunction, and seizures, highlighting the critical role of 5‑MTHF in neural function. Because folate stores are limited, deficiency can develop within months of inadequate intake or increased demand, such as in pregnancy or chronic illness.

Foods rich in naturally occurring folates provide precursors that are converted into 5‑MTHF in the body. While exact 5‑MTHF content varies by food and preparation, leafy greens, legumes, and certain animal organs are excellent sources of folate. Spinach tops the list of folate‑rich foods, providing a substantial amount per serving. Lentils and other legumes supply both folate and fiber, contributing to cardiovascular and digestive health. Asparagus is notable for its folate density among vegetables, while liver, especially beef or chicken liver, offers very high folate concentrations per gram. Other vegetables such as broccoli and Brussels sprouts support folate intake, as do fruits like oranges and avocado, which supply moderate levels along with vitamins and healthy fats. Fortified grains and cereals contribute folic acid, which is converted to 5‑MTHF in metabolism, and can be important in meeting total folate needs. Cooking methods influence folate retention; light steaming preserves more folate than boiling, which can leach water‑soluble vitamins. Fermented foods may also enhance folate availability through microbial synthesis. A balanced diet that regularly includes a variety of these foods supports adequate 5‑MTHF status and associated metabolic functions.

5‑MTHF is absorbed in the small intestine via active transport mechanisms. Because it is already in the reduced, methylated form, 5‑MTHF bypasses several enzymatic conversion steps required by folic acid, which must be reduced and methylated in the liver. As a result, 5‑MTHF exhibits high bioavailability, and supplemental 5‑MTHF may raise plasma folate more effectively than folic acid in certain individuals. Factors enhancing absorption include co‑ingestion with foods containing B‑vitamin cofactors such as B12 and B2, which participate in folate metabolism. Inhibitors of folate absorption include alcohol, which can impair transport mechanisms, and certain medications that interfere with folate pathways. The food matrix also affects bioavailability; natural folates bound within complex food structures require deconjugation prior to absorption, making their availability somewhat lower than supplemental forms. Genetic variations, such as MTHFR polymorphisms, influence the efficiency of converting folic acid to 5‑MTHF, underscoring the advantage of consuming 5‑MTHF directly for individuals with compromised folate metabolism.

Supplementation with 5‑MTHF may be beneficial for people who have difficulty converting folic acid to its active form due to MTHFR genetic variants, those with elevated homocysteine, and women who are pregnant or planning pregnancy. Clinical practice often uses folic acid supplements to reduce neural tube defect risk, but 5‑MTHF provides the active form directly and may offer advantages in individuals with metabolic impairments. Typical supplemental doses range from 400 to 800 µg daily, aligned with folate recommendations, though higher therapeutic doses may be used under medical supervision. When choosing supplements, forms such as calcium‑L‑5‑MTHF or glucosamine salts (e.g., Quatrefolic) improve stability and absorption. Supplements are generally taken with meals to enhance uptake and are often combined with vitamin B12 to support methylation pathways synergistically. Consultation with a healthcare provider is essential, particularly for pregnant women, those with medical conditions, or individuals taking medications that affect folate metabolism.

5‑MTHF itself has not been assigned a separate tolerable upper intake level, but high intakes of synthetic folic acid, which is converted to 5‑MTHF, have an established UL of 1,000 µg per day to avoid masking vitamin B12 deficiency and potential adverse effects. Excessive intake of synthetic folic acid can lead to unmetabolized folic acid circulating in the blood, which has raised concerns about potential immunological and neurological effects, though evidence remains limited. Because 5‑MTHF bypasses conversion steps, it does not contribute to unmetabolized folic acid buildup, but extremely high supplemental doses should still be approached with caution. Symptoms of excessive folate intake may include gastrointestinal discomfort and sleep disturbances, though true toxicity is rare with food sources. Individuals with certain conditions or medication regimens should consult health professionals before high‑dose supplementation.

5‑MTHF and folate metabolism interact with certain medications. Antifolate chemotherapy agents such as methotrexate inhibit dihydrofolate reductase, affecting folate pathways; supplemental methylfolate may reduce some adverse effects but can also interfere with therapeutic efficacy. Certain antiepileptic drugs like phenytoin and phenobarbital can impair folate status, necessitating monitoring and potential supplementation. Capecitabine, a chemotherapeutic prodrug converted to 5‑fluorouracil, may have increased adverse effects in combination with high folate levels, as folate enhances thymidylate synthase inhibition. Folate supplementation can also mask hematologic signs of vitamin B12 deficiency, particularly in older adults, delaying diagnosis and treatment of neurological complications.

Common Forms: Calcium L‑5‑MTHF, Quatrefolic (glucosamine salt), L‑5‑MTHF capsules or tablets

Typical Doses: 400–800 µg daily, higher under medical supervision

When to Take: With meals to enhance absorption

Best Form: Calcium L‑5‑MTHF or glucosamine salt

⚠️ Interactions: Methotrexate, Capecitabine, Antiepileptic drugs