What Is Folinic Acid

Folinic acid (also known as the pharmaceutical leucovorin or 5-formyl tetrahydrofolic acid) is a natural, active form of folate. It is one enzymatic step away from becoming methylfolate, the main form of folate used by the body. As a folate coenzyme, folinic acid is involved in DNA synthesis and repair, the conversion of homocysteine to methionine, SAMe production, neurotransmitter synthesis and many other methylation processes.

Folinic acid is able to enter folate cycles without the enzyme dihydrofolate reductase. It can be converted into multiple folate cofactors that enter the folate cycle and eventually contribute to methylation (if it undergoes further conversion to become methylfolate). One of the main biochemical roles of folate cofactors derived from folinic acid is nucleotide synthesis.

• Cancer treatment potentiator
  As a pharmaceutical agent, leucovorin calcium is used to intensify the effects of the chemotherapy drug fluorouracil (5‑FU) by blocking the enzyme required for cell metastasis. Leucovorin increases the effectiveness of 5-FU by stabilizing its inhibition of thymidylate synthase, an enzyme required for DNA synthesis. This combination can improve response rates and help slow disease progression in patients with advanced colon cancer.
  
  
• Leucovorin rescue
  Leucovorin is also used as a rescue therapy following high-dose methotrexate treatment. Because it is already a reduced folate, it bypasses DHFR inhibition and helps reduce toxicities in healthy tissues without substantially reducing methotrexate's anticancer effects when administered appropriately.¹
  
  
• Megaloblastic anemia
  Leucovorin may be used to treat megaloblastic anemia caused by folate deficiency. Before treatment, vitamin B12 deficiency should be excluded or treated because leucovorin may correct the megaloblastic anemia while allowing the neurological manifestations of untreated vitamin B12 deficiency to persist or worsen.
  
  
• Restore folate levels
  Folinic acid is a reduced form of folate that bypasses the DHFR enzyme and supports normal folate metabolism. Although common variants in genes such as MTHFR, MTR and MTRR may influence folate metabolism, current evidence does not support using these variants alone to recommend folinic acid or determine dosage.²
  
  
• Cerebral folate deficiency (CFD)
  The FDA has approved leucovorin calcium for patients with cerebral folate deficiency (CFD), a neurological condition that affects folate transport into the brain. One cause is a pathogenic variant in the FOLR1 gene, which encodes folate receptor alpha, a protein that transports folate into the brain. This differs from DHFR, which converts folic acid into reduced folate forms, and MTHFR, which converts folate into 5-methyltetrahydrofolate (5-MTHF) for use in methylation and other metabolic processes. Leucovorin may be prescribed for adults and children with a confirmed FOLR1 pathogenic variant.
  
  
• Autism spectrum disorder
  Research has shown that folinic acid may improve symptoms in some children with autism, particularly those with impairments in folate transport.
  A 2023 study found that high-dose folinic acid supplementation over 12 weeks improved social interaction skills in children with ASD.³ Previous studies have also shown that high-dose folinic acid can significantly improve the severity of autism symptoms and the use of expressive language.⁴

Folinic acid supplements are available over the counter (OTC) as capsules or tablets. 

They provide a reduced form of folate that bypasses the DHFR enzyme and supports normal folate metabolism. Some healthcare practitioners may recommend folinic acid instead of folic acid in specific clinical situations. Unlike methylfolate, however, folinic acid still requires conversion through the MTHFR enzyme to form 5-methyltetrahydrofolate (5-MTHF). Although people with MTHFR variants may choose either folinic acid or methylfolate, current evidence does not support recommending one over the other solely on the basis of common MTHFR variants.

Folinic acid has generally been well tolerated in published clinical studies. Most available safety information, however, comes from prescription leucovorin used to treat medical conditions rather than low-dose dietary folinic acid supplements.

Anyone who experiences signs of an allergic reaction, such as a rash, swelling or difficulty breathing, should discontinue use and consult a healthcare professional.

Most clinically significant interactions involving folinic acid have been reported with prescription leucovorin, which is used at therapeutic doses under medical supervision. These interactions should not be assumed to occur with low-dose dietary folinic acid supplements, as there is limited evidence that such supplements cause clinically significant drug interactions. Nevertheless, individuals taking prescription medications - particularly antiepileptic drugs, chemotherapy agents (such as methotrexate or fluorouracil), or folate-antagonist medications (such as pyrimethamine or trimethoprim) - should consult a healthcare professional before using a folinic acid supplement.

Natural foods contain a combination of folate types, but there is no study or reliable data source quantifying the particular form, “folinic acid”, to be found in specific foods. This should not be confused with folic acid, which is a synthetic form of folate that has been added to various fortified foods such as breads, cereals, juices and snacks. 

Plant foods and organ meats are usually the best sources of B vitamins, including folates. Some natural folate sources include: 

• Beef liver
• Eggs
• Legumes (beans, peas, lentils)
• Leafy greens (spinach, kale, lettuce)
• Asparagus
• Beets

Folic acid is a synthetic (laboratory-made) form of folate. It is often used in supplements and fortified foods as it is cheap to produce and highly stable. Before it can participate in folate metabolism, it must first be reduced by the enzyme dihydrofolate reductase (DHFR) to form tetrahydrofolate (THF). THF is then converted into other biologically active folate forms through pathways involving enzymes such as serine hydroxymethyltransferase (SHMT) and methylenetetrahydrofolate reductase (MTHFR).

FOLR1 is different. Rather than converting folate into another form, it encodes folate receptor alpha, a transport protein that helps move folate into the brain and certain other tissues.

Folinic acid (5-formyltetrahydrofolate) is already a reduced form of folate. Unlike folic acid, it bypasses the initial DHFR-dependent reduction step and enters the body's reduced folate pool. However, folinic acid does not bypass the MTHFR enzyme, and conversion to 5-methyltetrahydrofolate (5-MTHF) still requires MTHFR activity.

Folinic acid may be preferred when a reduced, non-methylated form of folate is desired, such as by individuals who wish to avoid synthetic folic acid and/or who report poor tolerance to L-methylfolate. However, this approach is supported primarily by biochemical rationale and clinical experience rather than direct comparative clinical trials. Current evidence does not support recommending folinic acid over folic acid or L-methylfolate based on common MTHFR variants alone. High-dose prescription leucovorin is used under specialist medical supervision for certain rare disorders affecting folate transport or metabolism, such as cerebral folate transport deficiency associated with confirmed pathogenic FOLR1 variants. These therapeutic uses should not be extrapolated to routine dietary supplementation.

Folate is the umbrella term for the naturally occurring forms of vitamin B9 present in foods and the body. These include folinic acid, L-methylfolate (5-methyltetrahydrofolate; 5-MTHF) and other reduced tetrahydrofolate derivatives that participate in folate metabolism.

Unlike synthetic folic acid, naturally occurring folates are already reduced forms of vitamin B9. Rather than requiring the initial DHFR-dependent reduction step needed for folic acid, they are interconverted into different folate cofactors according to the body's metabolic needs.

Folinic acid is one of these naturally occurring reduced folate forms. It enters the body's reduced folate pool and can be converted into multiple intracellular folate cofactors that support DNA synthesis, nucleotide production, red blood cell production and folate-dependent one-carbon metabolism. Because many folate-dependent functions do not require L-methylfolate, folinic acid can support several important metabolic pathways without first being converted to L-methylfolate. However, conversion to L-methylfolate for the remethylation of homocysteine to methionine still requires MTHFR activity.

Folinic acid and L-methylfolate (5-methyltetrahydrofolate; 5-MTHF) are both reduced, biologically active forms of folate, but they serve different roles within folate metabolism.

Folinic acid is a non-methylated, reduced (but still biologically active) form of folate that enters the body's reduced folate pool and can be converted into multiple intracellular folate cofactors. These cofactors support DNA synthesis and repair, nucleotide production, red blood cell production, and other folate-dependent one-carbon metabolic pathways.

L-methylfolate is a methylated, reduced form of folate. It is the major circulating form of folate and the form required for the remethylation of homocysteine to methionine. Through this pathway, it supports methylation reactions involved in the production of S-adenosylmethionine (SAMe), the body's primary methyl donor, which contributes to the synthesis of neurotransmitters and many other biologically important methylated compounds.

Rather than one form being universally better than the other, folinic acid and L-methylfolate have distinct biochemical functions. The most appropriate choice depends on an individual's nutritional needs, clinical circumstances and the advice of a qualified healthcare professional.