Folic acid is the synthetic form of folate, a water-soluble vitamin also known as vitamin B9.

It does not occur naturally in food but is frequently added to supplements and refined grain products, such as bread and cereals.

Since folic acid is different from natural folate, it must be converted into an active form before your body can use it.

Your genetics affect how quickly this conversion takes place, so folic acid is a hot topic in current nutrition research.

This article discusses folic acid, its benefits, sources, your risk of deficiency and more.

Folic acid is a stable, artificial form of vitamin B9.

It’s not found naturally in food but often added to processed foods and used in multivitamin-mineral supplements.

However, folic acid must be converted into active vitamin B9, known as 5-MTHF, before your body can use it.

This is a four-step process that requires several enzymes, including one called MTHFR.

Some people have genetic mutations that make their MTHFR enzymes less effective at converting folic acid to 5-MTHF.

This can lead to a build-up of folic acid in the blood, which has been linked to negative health outcomes in some people, including poor immunity, reduced brain function and increased growth of pre-existing cancers (1, 2, 3, 4).

While more research is needed, people with MTHFR mutations may want to avoid consuming large amounts of folic acid and choose supplements that contain active 5-MTHF instead.

Summary Folic acid is a synthetic form of vitamin B9 that is added to some foods and supplements. It must be converted to an active form before your body can use it, but some genetic mutations can slow this process.

Since folic acid is a synthetic form of vitamin B9, it’s not required in your diet.

Instead, you can consume folate – the natural form of B9 – through foods.

However, research suggests that most people don’t eat enough naturally folate-rich foods to meet their needs (5).

Because of this, many countries, including the US and Canada, now add folic acid to refined grain products, such as white flour, bread and cereals.

In the US, this practice has increased folate consumption by 28% and reduced the rate of folate deficiency — measured by the amount of folate in red blood cells — from 39% to less than 4% of the population (6).

Since most people consume both natural and synthetic forms of folate, requirements are described as “dietary folate equivalents” (DFEs):

  • 1 mcg of natural folate from food = 1 DFE
  • 1 mcg of synthetic folic acid taken with food = 1.7 DFE
  • 1 mcg of synthetic folic acid taken on an empty stomach = 2 DFE

Measuring intake this way helps account for the fact that folic acid is nearly twice as absorbable as natural folate from foods.

The reference daily intake (RDI) of folate is 400 mcg DFE for adults and 600 mcg DFE during pregnancy (7).

Summary Synthetic folic acid is not a required nutrient. However, it’s added to refined grain products in many countries. The average adult needs 400 mcg of dietary folate equivalents per day, while pregnant women require 600 mcg per day.

Adequate vitamin B9 levels have been linked to various health benefits, including:

Prevention of Neural Tube Defects

Low folate levels during the early weeks of pregnancy have been linked to neural tube defects in infants, such as malformations of the brain, spine and/or spinal cord (8, 9).

In contrast, children of women who supplement with folate before and throughout pregnancy are born with significantly lower rates of these defects (10).

Since up to 90% of women don’t have adequate folate levels for maximum protection against neural tube defects, it’s recommended that all women of reproductive age take at least 400 mcg of supplemental folate per day (6, 11).

While many prenatal supplements contain folic acid, it may be beneficial to look for varieties that contain methylated folate instead (12).

This is because methylfolate, also known as 5-MTHF or vitamin B9, is the active form of the vitamin that your body can use without needing to convert it from one form to another.

Cancer Prevention

High intakes of folate may protect against certain cancers, including those of the breast, gut, lung and pancreas (13, 14, 15).

This is likely because of folate’s role in gene expression — controlling when genes are turned on or off.

Some researchers believe that low folate levels can cause this process to go awry, increasing your risk of abnormal cell growth and cancer (16).

Low folate levels also contribute to the formation of unstable and easily breakable DNA which may increase cancer risk (17, 18).

However, in people with pre-existing cancers or tumors, there is some evidence that high folate intakes may promote tumor growth (19, 20).

Folic acid supplements — but not natural food folate — have also been linked to increased occurrence of some types of cancer (21, 22, 23).

More research is needed to understand how supplemental folic acid may impact cancer risk long term.

Reduced Homocysteine Levels

Adequate folate helps reduce levels of homocysteine, an inflammatory molecule linked to the development of heart disease (24).

Folate helps lower homocysteine levels because it’s required in the process of converting homocysteine into another molecule called methionine.

Without enough folate, this conversion slows down and homocysteine levels rise.

While studies have found that daily folic acid supplements can reduce homocysteine levels by up to 25%, this reduction has not corresponded with lower rates of heart disease (25).

The reasons for this are unclear, but it suggests that other important factors besides homocysteine influence the development of heart disease.

Summary Adequate vitamin B9 intake has been linked to positive health outcomes, such as a reduced risk of neural tube birth defects, cancer prevention and lower homocysteine levels. However, some of these effects are not yet fully understood or confirmed.

Since folic acid is synthetic, it does not occur naturally in foods.

Still, it’s commonly added to refined grain products and used in supplements.

In many countries, including the US and Canada, folic acid is added to all refined grain products (9).

Foods that are often fortified or enriched with folic acid include:

Natural food sources of folate include:

  • Liver: 123% of the RDI per 3 ounces (84 grams) (26)
  • Edamame: 121% of the RDI per cup (155 grams) (27)
  • Lentils: 90% of the RDI per cup (198 grams) (28)
  • Beans: 74% of the RDI per cup (171 grams) (29)
  • Asparagus: 68% of the RDI per cup (180 grams) (30)
  • Okra: 68% of the RDI per cup (184 grams) (31)
  • Spinach: 66% of the RDI per cooked cup (180 grams) (32)
  • Artichokes: 50% of the RDI per cooked cup (168 grams) (33)
  • Collard greens: 44% of the RDI per cooked cup (190 grams) (34)
  • Turnip greens: 42% of the RDI per cooked cup (144 grams) (35)
  • Broccoli: 42% of the RDI per cooked cup (156 grams) (36)
Summary Folic acid does not occur naturally in food but is often added to refined grain products. Natural food sources of folate include liver, legumes and green vegetables.

Folate deficiency is rare in countries with mandatory folic acid fortification and affects less than 4% of the US population (6).

However, people with inflammatory bowel diseases, those who smoke cigarettes or people who drink excessive alcohol are more likely to be deficient (37, 38, 39, 40).

Signs and symptoms of folate deficiency include (41):

  • Elevated homocysteine levels in the blood
  • Megaloblastic anemia, a type of anemia with enlarged red blood cells
  • Fatigue
  • Weakness
  • Irritability
  • Shortness of breath

Folate deficiency can be tested by measuring the amount of folate stored within your red blood cells or circulating in your blood (42).

Summary Folate deficiency is relatively rare in countries with mandatory folic acid fortification, including the US. Signs include high levels of homocysteine and megaloblastic anemia, accompanied by weakness and shortness of breath.

There are some side effects and precautions to be aware of when consuming folic acid (9):

  • Masked vitamin B12 deficiency: Megaloblastic anemia can be a sign of both folate and vitamin B12 deficiency. Supplementing with folate could correct the anemia without addressing B12 deficiency, possibly leading to nerve damage.
  • Risk of cancer promotion: Some research has found a link between folic acid consumption and the development of some cancers. Folic acid may also promote the growth of pre-existing tumors, but more research is needed.
  • Circulating free folic acid: Since the conversion of folic acid to its active form is a slow process, free folic acid can build up within your bloodstream. This has been linked to poor immunity and reduced brain function.

Additionally, since some people have MTHFR genetic mutations that reduce their ability to convert folic acid into its active form, supplements containing 5-MTHF are often preferred.

That’s because taking supplements that contain the active form of vitamin B9 ensures that the folate can be used by your body, regardless of genetics.

Summary Folic acid may not be a good fit for people with MTHFR genetic mutations or pre-existing cancers. It can also mask a B12 deficiency if levels are not checked prior to supplementing.

Folic acid, a synthetic form of vitamin B9, is often added to refined grain products and supplements to help prevent deficiency.

The RDI for adults is 400 mcg DFEs per day.

Adequate B9 levels are linked to lower homocysteine levels and a reduced risk of neural tube birth defects and certain cancers.

However, it’s important to note that folic acid is not the same as natural folate from food and requires conversion to the active form 5-MTHF before your body can use it.

Because of this, people with MTHFR genetic mutations may want to choose supplements that contain 5-MTHF instead.