Do You Really Need to Avoid Folic Acid if You Have MTHFR?

Search for information about MTHFR online and you'll quickly encounter two very different messages. One group screams if you have an MTHFR variant, avoid folic acid and use methylfolate instead. The other group insists that people with MTHFR variants can process folic acid, so there is no reason to avoid it.

As is often the case in nutrition, the science is more nuanced than either statement suggests. Just to disclose early that my bias after many years of working with people with MTHFR variants, is to avoid folic acid completely, but I want to present both sides of the story.

Common MTHFR variants do not make someone incapable of metabolizing folic acid. Human intervention studies clearly demonstrate that folic acid can raise folate status and lower homocysteine even in people carrying the MTHFR C677T variant. At the same time, folic acid is not metabolically identical to the folates found naturally in food or to supplemental 5-methyltetrahydrofolate (5-MTHF). There are legitimate scientific questions about how much folic acid humans can efficiently process, what happens when intake exceeds that capacity, and whether some people may respond differently.

Folic acid Has One Important Advantage: We Know it Works

Folic acid is the synthetic, oxidized form of vitamin B9 that has been used in supplements and fortified foods since its discovery in the 1960s. Its greatest public-health success is the prevention of neural-tube defects (NTDs). In the landmark Medical Research Council randomized trial, folic acid supplementation produced approximately a 72% reduction in recurrent neural-tube defects among women at high risk. A much larger intervention involving more than 247,000 pregnancies in China subsequently found that 400 micrograms of folic acid around conception substantially reduced neural-tube defects in both high- and low-risk geographic regions. These results are tremendous and highly impactful.

There is no way to overstate the magnitude of these studies, or the positive impact folic acid has had on undernourished populations for prevention of neural tube defects.

It also explains why public-health agencies emphasize folic acid rather than methylfolate: we have direct human outcome data demonstrating that folic acid prevents neural-tube defects.

We do not currently have equivalent large clinical trials showing that 5-MTHF prevents NTDs to the same degree. That doesn't mean that methyl folate doesn't prevent NTDs, it just means that it hasn't been researched to the same degree and in the same number of people.

“Folic Acid Works” Doesn't Answer Every Question

Before folic acid can participate fully in cellular folate metabolism, it must first be reduced by an enzyme called dihydrofolate reductase, or DHFR, ultimately producing tetrahydrofolate derivatives. Unfortunately, human DHFR is surprisingly slow. Bailey and Ayling examined human liver samples and found that DHFR activity was, on average, less than 2% of that measured in rat liver under their experimental conditions. They also observed almost a fivefold difference in activity among the human samples, meaning some people's DHFR is up to five times slower than average. After being converted by the DHFR enzyme, then folic acid must still go through all the steps of folate processing which ends with the MTHFR enzyme.

This suggests that different people have very different capacities to begin to metabolize folic acid. And when intake exceeds the body's immediate capacity to convert folic acid, some circulates as unmetabolized folic acid (UMFA). In fact, research has shown that a single intake of 200 mcg or more of folic acid almost universally produces measurable levels of UMFA. Human studies confirm that UMFA can be detected in circulation and that its concentration increases with folic-acid exposure. In a 2026 randomized dose-response trial, increasing folic acid fortification produced corresponding increases in circulating UMFA as well as increases in beneficial 5-MTHF.

This illustrates the complexity beautifully: folic acid can simultaneously improve folate status and increase unmetabolized folic acid. The presence of UMFA itself does not prove harm, but there is a growing body of evidence that UMFA may lead to some unwanted consequences.

Pseudo-MTHFR Deficiency - The Most Important Answer to The Question "Do You Really Need to Avoid Folic Acid if You Have MTHFR?"

In 2015, Christensen and colleagues fed mice a diet containing approximately ten times their recommended folic-acid intake. The researchers found that high levels of UMFA reduced liver MTHFR protein and activity, and resulted in lower levels of 5-MTHF, or active folate. They also observed reduced S-adenosylmethionine (SAM), changes in choline metabolism and evidence of liver injury. Also, these effects were particularly pronounced in mice already carrying MTHFR gene variances with reduced MTHFR activity.

The researchers described this as a pseudo-MTHFR deficiency caused by high folic-acid exposure. This is a mouse study, and no true research studies on this subject have been done in humans, but there is one published case report that also demonstrates this effect in humans. A woman with no MTHFR gene variances was given 5 mg folic acid per day for infertility, which is a common treatment, but her homocysteine rose to dangerous levels. She was taken off of folic acid and given a five day course of 5-LMTHF, or active folate. After those five days her homocysteine was retested and had returned to the optimal range. These researchers also described this as "pseudo-MTHFR"

If a person with wild type or "normal" genes can get pseudo-MTHFR from high doses of folic acid, then it stands to reason that the effects would be more pronounced in a person who already has actual MTHFR, but again there is no research study to prove that, it is simply a logical deduction.

Interestingly, another study involving high-dose folic acid and male reproductive biology found altered sperm DNA methylation after prolonged high-dose exposure, with MTHFR genotype appearing to influence susceptibility. Much of the mechanistic work again came from animal models, but the observations add to the argument that high folic-acid intake is not metabolically neutral, and is not a good idea for everyone.

Unmetabolized Folic Acid, or UMFA

Another intriguing human study examined 105 postmenopausal women. Detectable unmetabolized folic acid was associated with approximately 23% lower natural-killer-cell cytotoxicity. Lower NK cell cytotoxicity means immune system compromise, with one of our first defences, the natural killer cell, being 23% less effective.

There is also evidence that high folate status may behave differently depending on vitamin B12 status. In an analysis of older adults from NHANES, Morris and colleagues found that people with low B12 status and very high serum folate (including UMFA) were more likely to have anemia and cognitive impairment. Interestingly, among people with adequate B12, high folate was associated with less cognitive impairment. This does not prove folic acid caused the problems, the study was observational. But it does tell us definitively that nutrients don't operate in isolate, and driving one nutrient up without considering others is not a good idea.

Does Methylfolate Offer an Advantage?

5-MTHF bypasses several conversion steps required by folic acid and is already a reduced folate form, instantly useable by your body. In a randomized controlled trial of women of childbearing age, low-dose 5-MTHF and folic acid produced similar improvements in plasma and red-blood-cell folate over 24 weeks. This seems intuitive, but because the body of research concerning 5-MTHF is tiny compared to that of folic acid, it does actually matter.

A small 2026 feasibility trial comparing prenatal vitamins containing 5-MTHF versus folic acid also found that UMFA decreased in the 5-MTHF group and increased in the folic-acid group. However, the trial contained only 22 reproductive couples and encountered stability problems with the 5-MTHF formulation, so it cannot establish superiority or differences in pregnancy outcomes.

In simple terms, we know methylfolate is a reasonable folate source, but we should be just as careful about claiming that it is universally superior as we are about claiming folic acid is universally harmless.

Should People with MTHFR Avoid Folic Acid?

Do you really need to avoid folic acid if you have MTHFR? My clinical bias is toward avoiding folic acid in all forms and focusing on food sources of natural folate, and 5-MTHF or folinic acid supplementation where needed, but even I recognize that this is oversimplified. This is my broad recommendation because it is difficult or impossible for many people to test the efficiency of their DHFR, to test their levels of UMFA and to gain the same level of information as people in research studies have access to. If those tests were readily available, I could make more personalized recommendations.

Folic acid clearly can be metabolized by people with MTHFR variants. In randomized human studies, it lowers homocysteine even among individuals with the 677TT genotype.

But that doesn't mean every dose, in every person, under every circumstance is metabolically equivalent.

There are legitimate unresolved questions involving:

• total folic-acid exposure from fortified foods plus supplements,

• individual variation in DHFR activity,

• accumulation of unmetabolized folic acid,

• vitamin B12 status,

• MTHFR and other folate-related genetic variants,

• very high supplemental doses,

• and long-term effects on methylation and other aspects of methylation.

In spite of my bias toward 5-MTHF, I have to acknowledge that the avalanche of research we have on folic acid will continue to force public health organizations such as the CDC or WHO to favor folic acid above any other form of folate for decades to come, simply because it will take decades for the research on methyl folate to even begin to approach the level of research on folic acid. Public health organizations require "proof" and proof does not come from clinicians like myself making observations from their patient population, proof comes from large randomized controlled trials and other research studies.

Having said that, remember that a research study shows the average results across a large group of people, not the individual results in your body. Since your results are what you are looking for when you start a supplement, trust your body, test your levels, and be sure to work with a practitioner you trust and respect.

References

MRC Vitamin Study Research Group. Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. Lancet. 1991;338(8760):131-137. PMID: 1677062.

Berry RJ, Li Z, Erickson JD, et al. Prevention of neural-tube defects with folic acid in China. N Engl J Med.1999;341(20):1485-1490. doi:10.1056/NEJM199911113412001. PMID: 10559448.

Bailey SW, Ayling JE. The extremely slow and variable activity of dihydrofolate reductase in human liver and its implications for high folic acid intake. Proc Natl Acad Sci U S A. 2009;106(36):15424-15429. doi:10.1073/pnas.0902072106. PMID: 19706381.

Pfeiffer CM, Fazili Z, Arnold CD, et al. Serum folate forms show dose-dependent increases in nonpregnant Ethiopian women of reproductive age participating in a randomized controlled trial with double-fortified salt containing iodine and folic acid. Curr Dev Nutr. 2026;10(7):109398. doi:10.1016/j.cdnut.2026.109398. PMID: 42437195.

Christensen KE, Mikael LG, Leung KY, et al. High folic acid consumption leads to pseudo-MTHFR deficiency, altered lipid metabolism, and liver injury in mice. Am J Clin Nutr. 2015;101(3):646-658. doi:10.3945/ajcn.114.086603. PMID: 25733650.

Aarabi M, Christensen KE, Chan D, et al. Testicular MTHFR deficiency may explain sperm DNA hypomethylation associated with high dose folic acid supplementation. Hum Mol Genet. 2018;27(7):1123-1135. doi:10.1093/hmg/ddy021. PMID: 29360980.

Morris MS, Jacques PF, Rosenberg IH, Selhub J. Folate and vitamin B-12 status in relation to anemia, macrocytosis, and cognitive impairment in older Americans in the age of folic acid fortification. Am J Clin Nutr.2007;85(1):193-200. doi:10.1093/ajcn/85.1.193.

Troen AM, Mitchell B, Sorensen B, et al. Unmetabolized folic acid in plasma is associated with reduced natural killer cell cytotoxicity among postmenopausal women. J Nutr. 2006;136(1):189-194. doi:10.1093/jn/136.1.189. PMID: 16365081.

Venn BJ, Green TJ, Moser R, McKenzie JE, Skeaff CM, Mann J. Increases in blood folate indices are similar in women of childbearing age supplemented with [6S]-5-methyltetrahydrofolate and folic acid. J Nutr.2002;132(11):3353-3355. doi:10.1093/jn/132.11.3353. PMID: 12421850.

Ledowsky C, Scarf V, Rogers K, Steel A. Feasibility of a randomized clinical trial comparing 5-methyltetrahydrofolate and folic acid prenatal multivitamins in couples with recurrent pregnancy loss. Nutr Res.2026;146:68-81. doi:10.1016/j.nutres.2025.12.008. PMID: 41544303.

Cornet D, Clement A, Clement P, Menezo Y. High doses of folic acid induce a pseudo-methylenetetrahydrofolate syndrome. SAGE Open Med Case Rep. 2019 May 17;7:2050313X19850435. doi: 10.1177/2050313X19850435. PMID: 31205715; PMCID: PMC6537060.