There is a lot of buzz online about MTHFR variants and the need to avoid folic acid. I’ve seen recommendations ranging from avoiding all processed foods that are fortified with folic acid — to recommendations that people with MTHFR variants need to take extra folic acid.
I’ve dug into the topic to see what is in the research studies about folic acid. Is it so evil that everyone should go out of their way to avoid it? Or is it a wonderful benefit for women trying to conceive?
What is folic acid?
Folic acid is a synthetic form of folate (vitamin B9) that is temperature and pH stable, allowing it to easily be added to processed foods and multivitamins. The chemical name for folic acid is pteroylmonoglutamic acid or PteGlu.
Natural folates from foods differ a bit from synthetic folic acid. They contain more than one glutamic acid and are called pteroyl polyglutamic acid. They are less heat stable and cooking or processing can decrease the amount of natural folate in foods.[ref]
The body converts folates (folic acid and natural folates from plants) into tetrahydrofolate. This conversion is a little different for natural folate vs. folic acid.
A 2014 review in Preventive Nutrition and Food Science explained the breakdown of folic acid:
- First folic acid must be reduced to dihydrofolate (using the DHFR enzyme).
- Then the DHFR enzyme is needed again for the conversion of dihydrofolate into tetrahydrofolate (the active version used by the body).
Tetrahydrofolate is then used (along with the MTHFR enzyme) for a critical step within the methylation pathway. This vital pathway is responsible for methyl groups that are needed in DNA synthesis, detoxification reactions, the creation of certain neurotransmitters, and more.
How is folic acid different from methylfolate?
While folic acid has to go through a couple of reactions to become the active form of folate used by the body, methylfolate (L-5-methyltetrahydrofolate) skips all those steps and can be used immediately. Methylfolate is available in supplemental forms; it isn’t added to fortified foods, in part, because it is quite a bit more expensive than folic acid.
What happens to folic acid that isn’t used by the body?
Often when reading about folic acid you will see mentioned that it is a water-soluble vitamin that isn’t toxic. This may lead some to assume that extra folic acid won’t hurt you. But this may not be a correct assumption.
The folic acid that isn’t used by the body can often be found in the bloodstream as unmetabolized folic acid. There are concerns that the excess folic acid could inhibit the DHFR enzyme from converting dihydrofolate into tetrahydrofolate when the body is needing it. Both reactions use the DHFR enzyme, but most of the time the enzyme is preferentially used for creating the active form (tetrahydrofolate) when needed. Some studies indicate that too much unmetabolized folic acid, though, inhibits DHFR from completing that second reaction. This could leave cells lacking in tetrahydrofolate when high levels of folic acid are in circulation.[ref]
There are also concerns that excess folic acid intake could downregulate folate transporters in the kidneys and intestines. [ref]
How much folic acid does it take to get unmetabolized folic acid in the bloodstream?
- One study shows that the level is around 400μg for the average adult. [ref]
- Another study was completed in the UK, which had not mandated fortification of flour, so the participants had no unmetabolized folic acid in their bloodstream prior to beginning the study. The study found that around 200 μg doses would show up in the bloodstream. The timing was also important and repeated exposures close together had a cumulative effect.[ref]
- Finally, a 2012 study looking at the effects of supplementing with either 1.1mg or 5 mg of folic acid found that there is a great variation in people’s capacity for metabolizing folic acid.[ref]
How much folic acid are people eating?
In the US, flour is fortified at 140 mcg/100g. But since I have no idea how many grams of flour are in most foods, let’s take a look at servings of some popular foods [ref]:
- A bowl of cheerios gives you 50% of the daily value for folic acid (200mcg?) [website]
- Two slices of bread for a sandwich gives you 70 mcg.
- A bagel contains 119 mcg.
- 1 cup of macaroni gives you 179 mcg.
- 1 cup of enriched white rice gives you 200 mcg.
This is just folic acid — many foods contain natural folate as well.
A study of over 2,700 US children, adolescents, and adults found that unmetabolized folic acid was found in almost all blood samples. [ref]
Does folic acid cause cancer?
This is a question that researchers have been looking into for a few decades. It’s a controversial topic without totally clear answers.
It has been theorized that mandated folic acid fortification in 1998 caused an increase in the number of colon cancer cases in the US. The sharp increase in colon cancers from 1998-2002 came after a long period when colon cancer rates had been declining. [article] [ref]
Recent studies have shown that folic acid in higher amounts, such as 1mg/day, are linked to breast cancer, prostate cancer, and colon cancer. [ref]
High amounts of unmetabolized folic acid are also associated with decreased natural killer cells (a cytokine that is part of the body’s defense against cancer).[ref]
One commonly used cancer-fighting drug is methotrexate, which blocks the action of an enzyme (DHFR) that is part of the folate pathway. This essentially inhibits the synthesis of DNA, RNA, and more in cancer cells — and healthy cells.
Here are a few of the studies on cancer and folic acid:
- A study of 848 women with breast cancer and a control group of 28,345 women without cancer found that premenopausal women with higher plasma folate level were at a higher risk of breast cancer. The study was conducted to determine if B12 and folate prevented breast cancers; researchers noted that the results were ‘unexpected’. [ref]
- Another study found that women who had higher plasma folate levels (top third) were at double the odds of ERβ− breast cancer.[ref]
- An animal study investigated folic acid’s effect on breast cancer. Animals with mammary tumors received a diet containing higher amounts of folic acid (2.5 to 5 times the recommended amount) for 12 weeks. Compared with the animals fed a control diet, the folic acid fed animals had significantly larger tumors. [ref]
- A 2009 randomized placebo-controlled trial for colorectal adenomas found that folic acid supplementation (1 mg/day) more than doubled the risk of prostate cancer. “Among the 643 men who were randomly assigned to placebo or supplementation with folic acid, the estimated probability of being diagnosed with prostate cancer over a 10-year period was 9.7% (95% confidence interval [CI] = 6.5% to 14.5%) in the folic acid group and 3.3% (95% CI = 1.7% to 6.4%) in the placebo group ” [ref]
Don’t freak out just stop eating folate all together... folate in the right amounts has been repeatedly shown to reduce the risk of getting cancer. The key here seems to be that adding folic acid when there is cancer already present will increase the growth of cancer.
Does folic acid cause autism?
This is another controversial topic without a clear answer.
A meta-analysis that combined together data from a bunch of studies showed that the overall effect of supplementing with folic acid while pregnant is that it reduces the risk of autism.[ref]
Other studies, though, raise some serious questions about folic acid on an individual level. A study of 1,391 mothers in Boston found that high maternal folate and/or B12 levels significantly increased the risk of autism in their children. Overall, though, women who used multivitamins were at a lower risk. The researchers did factor in MTHFR genetic variants and found that they were not a risk factor. [ref]
Mouse studies also raise questions about the impact of higher levels of folic acid during pregnancy. The results showed that high folic acid during pregnancy leads the offspring to have disturbed choline/methyl metabolism, memory impairment, and embryonic growth delays. [ref]
History of how Folic Acid came to be supplemented in the US
Widespread folic acid fortification began in the US in 1996 and then became mandatory in 1998. It is currently added to all “enriched bread, flour, cornmeal, rice, pasta, and other grain products”. [ref] This mandate was made to reduce the risk of neural tube defects, which happened at the rate of 2,500 babies with NTD/year in the US in 1992. A 2015 CDC article claims: “The birth prevalence of NTDs (proportion of babies in the population born with an NTD) has decreased by 35% in the United States since folic acid fortification was required in 1998.” [ref]
The history of the FDA decision to mandate fortification with folic acid is interesting to read. It states that:
“Folic acid fortification, for all practical purposes, was not even on the regulatory horizon when it was first included in the 1990 Nutrition Labeling and Education Act (NLEA). FDA scientists at the time felt that the charge to consider the link between folic acid and neural tube defects literally came “out of the blue.” Later, they concluded it probably started with a science workshop hosted by CDC in the late 80s in which unpublished data on folic acid and neural tube defects were presented and seized upon by the supplement industry.” The article goes on to explain that a British study published in 1991 showed that women who already had a child with neural tube defects reduced their risk of having a second child with NDT when they supplemented with 4 mg of folic acid per day. This study was in contrast to the FDA contracted report in 1991 which stated that studies had failed to show the connection between folic acid and NTD.
By August of 1992, the FDA changed its stance on folic acid. The article goes on to say:
“In August and September 1992, FDA reviewed results obtained from two additional, unpublished studies, and worked closely with authors and journals publishing them to ensure that the results were made public early. The first, a Hungarian research study, showed a reduced risk of ntd’s in women consuming .8 mg. of folic acid as part of a multivitamin/mineral supplement. The study had been conducted with a sample of women in the general population without a previous history of an ntd pregnancy. The second study was a case control study of women in the general population of Boston, Toronto, and Philadelphia and was referred to as the “Werler study.” This study suggested that .4 mg. of folic acid daily from multivitamin/mineral supplements was associated with a reduced risk of ntd. It also suggested that a diet adequate in folate with more than .25 mg. daily was protective. The new preponderance of scientific evidence on folic acid created a platform from which the Public Health Service (PHS) spoke on September 14, 1992. PHS formally recognized the link between folic acid intake and ntd’s as a compelling public health issue. It recommended that all women of childbearing age should have adequate folate intakes (.4 mg. daily) throughout their childbearing years, but warned that the total intakes should not exceed 1 mg.”
The Werler study that the FDA used for their recommendation looked at 436 cases of NTD with a control group of 2,615 babies that had other types of birth defects. They based the conclusion on the reported use of daily multivitamins that also contained folic acid. I admit that I am confused by this study, specifically by why the control group was only babies with birth defects and how the conclusion was reached that it was the folic acid in the multivitamin that lowered the risk of NTD.
Please don’t get me wrong… I am not really questioning a link between NTD and folate levels since there is other evidence for this. The CDC estimates that folic acid fortification in the US prevents 1,300+ cases of NTD each year.[ref] (You can decide if it is a good trade-off for possibly increased cancer risk.)
Rather, I’m surprised at how little data the FDA used in making the decision to force the fortification of folic acid on the public. I had rather naively assumed that there were lots of good studies showing without a doubt that 400 μg/d of folic acid was exactly what we all needed.
Genetic Variants Related to Folic Acid Metabolism
DHFR – Dihydrofolate reductase
The DHFR gene codes for the enzyme that converts dihydrofolate (from folic acid or folate) into tetrahydrofolate. It also converts folic acid into dihydrofolate.
Check your genetic data for rs70991108: (23andMe v5 only):
- I/I orTGGCGCGTCCCGCCCAGGT: normal DHFR variant
- D/I: carrier of one copy of a deletion in part of the DHFR gene
- D/D or -/-: carrier of two copies of a deletion in part of the DHFR gene, more unmetabolized folic acid[ref]
The 19bp deletion is quite common and found in almost half the population. Studies are mixed as to the effect of the deletion:[ref]
- One study showed that people with two copies of the deletion were twice as likely to have unmetabolized folic acid when intake was over 500mcg/day.[ref]
- The deletion is also linked to an increased risk of breast cancer in multivitamin users[ref], which may point to it having an effect with higher folic acid supplementation.
- In the elderly, those with two copies of the deletion and higher folate levels had significantly worse memory score[ref], which again points to a detrimental effect from folic acid for those with the deletion.
Check your genetic data for rs1677693 (23andMe v4; AncestryDNA):
- T/T: decreased risk of colon cancer only for people not taking folic acid supplements[ref]
- G/T: decreased risk of colon cancer only for people not taking folic acid supplements
- G/G: normal (most common)
The study referenced above looked at colon cancer cases in the US, Canada, and Australia. The rs1677693 T-allele was associated with a decreased risk of colon cancer — but only in people not taking vitamins with folic acid in them. But note that these people were probably all getting some folic acid through fortified foods.
Check your genetic data for rs1643649 (23andMe v4 only): (the minor allele is C)
- A study showed the minor allele increased susceptibility to spina bifida. [ref]
Check your genetic data for rs1650697 (23andMe v4, v5; AncestryDNA):
MTHFR is a well researched genetic variant in the folate pathway. It converts tetrahydrofolate into methyltetrahydrofolate for use in the methylation cycle.
Check your genetic data for rs1801133 (23andMe v4, v5; AncestryDNA):
- G/G: normal (wildtype)
- A/G: one copy of C677T allele (heterozygous), MTHFR efficiency reduced by 40%
- A/A: two copies of C677T (homozygous), MTHFR efficiency reduced by 70 – 80%
Check your genetic data for rs1801131 (23andMe v4, v5: AncestryDNA):
- T/T: normal (wildtype)
- G/T: one copy of A1298C allele (heterozygous), MTHFR efficiency slightly reduced
- G/G: two copies of A1298C (homozygous), MTHFR efficiency reduced
While at first glance it may seem that those with MTHFR 677 and 1298 variants may gain some kind of benefit from folic acid as long as their DHFR gene is working well, this may not be the case. The studies on high levels of unmetabolized folic acid decreasing the conversion to tetrahydrofolate would indicate that people with MTHFR variants would be at an increased risk for not having enough active folate for the methylation cycle.
Another recent study showed that, regardless of MTHFR status, [6S]-5-MTHF (methyl folate) had higher bioavailability than folic acid.[ref]
I’m not a doctor, so don’t take this as medical advice… Instead, this is a suggestion for a common sense application of the research.
Figure out how much folic acid you get in a day:
I think that caution is warranted for higher doses of folic acid, especially in older people or anyone at a higher risk for cancer.
The question is whether a “higher dose” is 200 mcg or 400 mcg since the studies referenced above showed different answers for the point at which unmetabolized folic acid is found. My guess is that for anyone with a DHFR variant that is on the lower side.
Check your multivitamins, prepackaged shakes, vitamin drinks, etc to see how much you are getting.
Should everyone stop eating foods fortified with folic acid?
Not necessarily. You can drive yourself a bit nuts trying to eliminate this or that from your diet. But I think people need to realize how much folic acid they are getting in fortified foods and how that can stack up over a day. Try tracking your intake for a week to see how much folic acid you consume. If you are consuming more than you should, then switch out some of the higher foods in your diet, such as white rice or cereal, for options that don’t have folic acid added. You can find unfortified rice in some of the imported varieties, such as imported Indian basmati rice.
What about prenatal vitamins?
The research is pretty clear that women trying to conceive and during the first trimester need plenty of folate, but this doesn’t mean that folic acid is your only option. There are many options now for prenatal vitamins with methylfolate rather than folic acid.
What about MTHFR?
Limiting folic acid consumption to be less than the level for unmetabolized folic acid seems prudent for anyone with an MTHFR variant. If you don’t get enough folate through your diet, methylfolate is a better option for supplementation than folic acid.
Foods high in folate:
Including foods that are high in natural folate is important for overall health. Higher folate foods include legumes, green leafy vegetables, asparagus, avocado, and liver. Stacking a few of those together in your diet can provide the 400 mcg of folate recommended each day — without needing to add foods fortified with folic acid.
Article originally published in June 2016. Updated and republished in April 2019.