What MTHFR actually does
MTHFR (methylenetetrahydrofolate reductase) is an enzyme that converts folate (vitamin B9) into its active form, methylfolate (5-MTHF). Methylfolate is what your body actually uses — the folate in food and basic supplements needs to be converted first.
This matters because methylfolate feeds into a process called the methylation cycle, which your body uses for:
- Breaking down homocysteine (an amino acid that's harmful at high levels)
- Making neurotransmitters like serotonin, dopamine, and norepinephrine
- DNA repair and gene expression regulation
- Detoxification processes
- Building and maintaining cells
When the MTHFR enzyme works less efficiently, less methylfolate is produced. This can lead to higher homocysteine levels and reduced methylation capacity — but whether it actually causes problems depends on your diet, your other genes, and your overall health.
The two variants everyone talks about
C677T (rs1801133)
This is the more impactful of the two MTHFR variants. It changes an amino acid in the enzyme, making it less heat-stable and less active.
- CC (normal) — full enzyme activity. About 40-50% of most populations.
- CT (heterozygous) — approximately 35% reduced enzyme activity. About 35-40% of most populations. Usually not a problem on its own.
- TT (homozygous) — approximately 70% reduced enzyme activity. About 10-15% of most populations. This is the one that can actually matter, especially if your diet is low in folate.
A1298C (rs1801131)
This variant has a milder effect on enzyme activity.
- AA (normal) — full activity at this position.
- AC (heterozygous) — mildly reduced activity.
- CC (homozygous) — moderately reduced activity, but typically less impactful than C677T TT.
Compound heterozygous (C677T + A1298C)
Having one copy of each variant (CT for C677T and AC for A1298C) is sometimes called "compound heterozygous." The combined effect is roughly similar to being homozygous C677T, though individual responses vary.
The hype vs. the reality
If you've spent any time in wellness spaces, you've probably seen MTHFR blamed for everything from chronic fatigue to infertility to depression to autoimmune disease. Here's a reality check:
What the evidence supports
- Elevated homocysteine — homozygous C677T (TT) is clearly associated with higher homocysteine levels, especially when folate intake is low. Elevated homocysteine is a cardiovascular risk factor.
- Neural tube defect risk — women who are homozygous C677T have a modestly increased risk of having a baby with neural tube defects, which is why adequate folate before and during pregnancy is so important.
- Reduced folate conversion — if you're TT and taking folic acid (the synthetic form), your body has a harder time converting it. Methylfolate (the pre-converted form) bypasses this issue.
What the evidence does NOT support
- MTHFR as a root cause of chronic disease — having an MTHFR variant doesn't cause disease by itself. It's one factor among many.
- Everyone with MTHFR variants needs treatment — most people with heterozygous variants (CT or AC) function perfectly fine, especially with a decent diet.
- Mega-dosing methylfolate — taking very high doses of methylfolate can cause anxiety, irritability, and insomnia in some people, particularly those with certain COMT variants. More is not always better.
Related methylation genes
MTHFR doesn't work alone. Several other genes affect the same pathways:
COMT (rs4680)
COMT (catechol-O-methyltransferase) breaks down dopamine, norepinephrine, and estrogen. The Val158Met variant (rs4680) creates two common patterns:
- Val/Val (GG) — fast COMT activity. Breaks down neurotransmitters quickly. Often described as "warrior" — calmer under stress but may need more stimulation baseline.
- Met/Met (AA) — slow COMT activity. Neurotransmitters stick around longer. Often described as "worrier" — better cognitive performance at baseline but more sensitive to stress. These people may also be more sensitive to methyl donors and should be cautious with high-dose methylfolate.
- Val/Met (AG) — intermediate activity.
MTRR (rs1801394)
MTRR (methionine synthase reductase) helps regenerate the active form of vitamin B12, which works alongside methylfolate in the methylation cycle. The A66G variant reduces MTRR efficiency. If you have both MTHFR and MTRR variants, the combined effect on methylation may be more significant.
MTR (rs1805087)
MTR (methionine synthase) uses B12 to convert homocysteine back to methionine. The A2756G variant can increase activity, pulling more B12 into the cycle and potentially increasing B12 requirements.
BHMT
BHMT (betaine-homocysteine methyltransferase) provides an alternative pathway for converting homocysteine to methionine using betaine (trimethylglycine) instead of methylfolate. This is a backup pathway that becomes more important when MTHFR function is reduced.
What to actually do about your MTHFR results
If you're homozygous C677T (TT)
- Get your homocysteine checked. A simple blood test. If it's elevated (above 10-12 μmol/L), addressing it makes sense. If it's normal, you may not need to change anything.
- Consider methylfolate instead of folic acid. Since your MTHFR enzyme is less efficient at converting folic acid, taking pre-converted methylfolate (400-800 mcg daily) is a reasonable step. This is especially important if you're pregnant or planning to be.
- Check B12 levels. Methylation needs both methylfolate and B12 (as methylcobalamin). If B12 is low, supplementing folate alone won't fix the issue.
- Eat folate-rich foods. Leafy greens, legumes, and liver are good natural sources of folate in forms your body can use more easily than synthetic folic acid.
If you're heterozygous C677T (CT) or any A1298C variant
You probably don't need to do anything specific unless blood tests show elevated homocysteine or low folate/B12. Eating a diet with adequate vegetables and not having B vitamin deficiencies is usually sufficient. You can take methylfolate instead of folic acid if you want, but it's not urgent.
What to avoid
- Don't mega-dose methyl donors without monitoring. High doses of methylfolate, SAMe, or methyl-B12 can cause anxiety, irritability, racing thoughts, and insomnia in sensitive individuals (especially slow COMT people).
- Don't assume MTHFR explains everything. If you have fatigue, depression, or brain fog, MTHFR variants are one small piece of a much larger puzzle. Get a proper medical workup.
- Don't avoid folic acid in fortified foods entirely. For most people, even with MTHFR variants, the folic acid in fortified flour and cereals isn't causing harm. The concern is mainly about supplemental folic acid at higher doses.
Lab tests worth getting
- Serum homocysteine — the single most useful test to see if your MTHFR variants are actually causing a problem. Normal is roughly 5-15 μmol/L, with optimal often considered below 10.
- Serum folate — checks your overall folate status.
- Serum B12 — methylation needs B12 too. Low B12 causes problems regardless of MTHFR status.
- Methylmalonic acid (MMA) — a more sensitive marker for functional B12 deficiency than serum B12 alone.
Frequently asked questions
How common are MTHFR variants?
Extremely common. About 40% of people are heterozygous C677T and 10-15% are homozygous. It varies by ethnicity — higher prevalence in Mediterranean, Hispanic, and East Asian populations. Being a "carrier" of an MTHFR variant is not a disease.
Should I avoid folic acid completely?
No. The concern about folic acid is mainly theoretical for most people. If you want to be cautious, take methylfolate as your supplement and don't worry about the small amounts in fortified foods.
Can MTHFR variants cause miscarriage?
The relationship between MTHFR and recurrent pregnancy loss is controversial. Some studies show a weak association with homozygous C677T, but major medical organizations (including ACOG) do not recommend MTHFR testing as part of recurrent pregnancy loss workup. Elevated homocysteine is a more relevant test.
My practitioner put me on 15mg of methylfolate. Is that safe?
That's a very high dose — roughly 20x the recommended dietary allowance. Some practitioners prescribe it for specific conditions, but it can cause significant side effects in some people. If you're experiencing anxiety, insomnia, or irritability, the dose may be too high. Discuss with your prescriber.
Does this affect my children?
Your children will inherit one copy of your MTHFR gene. If you're homozygous C677T (TT), all your children will be at least heterozygous (CT). Whether this matters for them depends on what they inherit from the other parent and their own diet and health.