What Does Folic Acid Do? How Folate Works in Your Body

Folic acid gives your body folate (vitamin B9), which it uses to build DNA and RNA, divide cells, make healthy red blood cells, and convert homocysteine into methionine.¹ These jobs matter most where cells multiply quickly, such as in a developing embryo and in red blood cell production, which is why folate is so important before and during early pregnancy.¹²

This page explains each of those jobs in plain English, how your body turns folic acid into a usable form, and what happens when folate runs low.

Folate's core job: moving one-carbon units

Most of what folate does comes down to one task. Folate coenzymes carry small chemical pieces called one-carbon units from one molecule to another.¹

Those one-carbon units are building materials. The body uses them to make DNA and RNA and to process amino acids, the building blocks of protein.¹ Without enough folate, those assembly lines slow down.

Making DNA and dividing cells

New cells need new DNA, and folate supplies parts needed to make it, including through a step called thymidylate synthesis.¹ When that step falters, DNA formation is disrupted.¹

This is why folate matters most in tissues where cells divide fast. The two clearest examples are a developing embryo and red blood cell production.¹ CDC describes folic acid as supporting the production of new cells, including blood, brain, skin, hair, and nail cells.²

Building healthy red blood cells

Red blood cell production is one of the places where folate matters most, because those cells divide rapidly.¹ Vitamin B12 and folate are both needed to make healthy red blood cells.³

When folate runs short, the marrow cannot copy DNA properly. The result is large, immature red blood cells and a condition called megaloblastic anemia.¹ Its symptoms can include fatigue, weakness, difficulty concentrating, irritability, headache, palpitations, and shortness of breath.¹

Because vitamin B12 deficiency causes a very similar anemia, unexplained anemia should be checked for B12 deficiency before anyone treats it with folic acid.⁴¹ Large amounts of folate can correct the anemia of B12 deficiency without treating the nerve damage.⁴¹ Our guide to folic acid and vitamin B12 explains how the two work together.

Recycling homocysteine and supporting methylation

Folate-dependent reactions convert homocysteine into methionine, which is then used to make S-adenosylmethionine, a major methyl donor used throughout the body.¹

When folate is low, homocysteine can rise.¹ A high homocysteine result is nonspecific, though, because B12 deficiency, kidney problems, and other factors also raise it.⁵¹

Folic acid does lower homocysteine. But trials generally do not show that this reduces heart attacks, cardiovascular deaths, or total cardiovascular events, so homocysteine lowering is not the same as heart protection (evidence grade: Mixed).⁶¹

Protecting early pregnancy

In early pregnancy, the embryo grows by rapid cell division, which depends heavily on folate.¹ Neural tube defects are major birth defects of the brain and spine.⁷ The neural tube normally closes about 26 to 28 days after fertilization, often before pregnancy is recognized.⁸⁹

When the neural tube does not close correctly, the result is a neural tube defect (NTD) such as spina bifida, anencephaly, or encephalocele.¹⁰ Folic acid taken before conception and in early pregnancy prevents many NTDs, and USPSTF judges this benefit substantial with high certainty (evidence grade: Established).¹⁰²

CDC recommends 400 mcg of folic acid daily for everyone who could become pregnant, and USPSTF recommends 400 to 800 mcg starting at least one month before conception.²¹⁰ Folic acid is the only form of folate shown in studies to help prevent NTDs.¹¹ Read more about these conditions in our guide to neural tube defects.

Folate's job does not end after the first trimester. It remains necessary throughout pregnancy for the growth of maternal and fetal tissues, and the RDA during pregnancy is 600 mcg DFE a day.¹²¹ WHO recommends daily iron plus 400 mcg of folic acid throughout pregnancy in relevant populations, though individual prenatal plans should follow the treating clinician.¹²¹³

How your body absorbs and uses folic acid

Folic acid and food folate take slightly different routes into the body.

1. Food folate must first be broken down into a simpler single-glutamate form before the gut can absorb it.¹
2. Folic acid is absorbed and then reduced by an enzyme called dihydrofolate reductase into active tetrahydrofolate forms. The activity of this enzyme varies from person to person.¹
3. 5-MTHF is the predominant folate form circulating in blood plasma.¹

Source: NIH Office of Dietary Supplements.¹

The body holds an estimated 15 to 30 mg of folate in total, with roughly half in the liver and the rest in blood and other tissues.¹ Gut bacteria also make some folate, but how much that contributes to the body's overall status is uncertain.¹

How folate status is measured

Because folate is spread across blood and tissues, no single number tells the whole story. Serum folate reflects recent intake and is usually the first test; red blood cell folate reflects longer-term status.⁵¹

NIH's population discussion treats serum folate above 3 ng/mL and red blood cell folate above 140 ng/mL as adequate, but lab methods and reference ranges differ, so results should be read against the lab's own range.¹ A high result often reflects recent fortified food or supplements and is not a diagnosis by itself.¹⁴¹ Our guide to the folate blood test explains how results are interpreted.

What about MTHFR?

MTHFR is another enzyme in the folate pathway. Common MTHFR gene variants change its activity, but they do not make a person unable to process folic acid.¹⁵ People with the 677TT genotype have, on average, blood folate about 16% lower than people with the 677CC genotype at the same intake, and CDC still recommends folic acid for them.¹⁵ See folic acid and MTHFR for details.

What happens when folate runs low?

Low folate shows up first where cells divide fastest. The main effects are:¹

• Megaloblastic anemia: fatigue, weakness, poor concentration, irritability, headache, palpitations, and shortness of breath.
• Mouth and gut: a sore or ulcerated tongue and mouth, and digestive symptoms.
• Skin, hair, and nails: changes in pigment.
• Blood chemistry: raised homocysteine.
• Pregnancy: higher risk of neural tube defects.

These signs overlap with B12 deficiency, iron deficiency, thyroid disease, and other problems, so symptoms alone cannot confirm low folate.¹⁶³ A clinician typically checks folate alongside a complete blood count and vitamin B12 levels.¹⁴¹⁶ For causes and testing, see folic acid deficiency.

What folic acid does not do

Folate is essential, but the list of things it is proven to do is narrower than many product labels suggest.¹

For the medical uses doctors actually prescribe it for, see what folic acid is used for.

This article is for general education and is not medical advice. Doses and needs differ from person to person, so talk with your clinician or pharmacist about your situation. Read our editorial policy and medical disclaimer.