Few genetic variants have generated as much consumer-health buzz — or as much overreach — as MTHFR. Search the term and you'll find everything from accurate research summaries to claims that MTHFR variants cause autism, Alzheimer's, miscarriage, and cancer.
The truth is more measured. MTHFR variants are real, common, and clinically relevant in specific contexts. Outside those contexts, the evidence is weak.
This article walks through what MTHFR does, what C677T and A1298C variants actually change, and where the evidence is strong vs hyped.
What MTHFR does
MTHFR encodes the enzyme methylenetetrahydrofolate reductase, which converts 5,10-methylenetetrahydrofolate into 5-methyltetrahydrofolate (5-MTHF) — the active form of folate. 5-MTHF donates a methyl group to homocysteine, converting it back to methionine.
The downstream methionine cycle is essential for DNA methylation, neurotransmitter synthesis, and amino-acid metabolism. When MTHFR activity drops, homocysteine can accumulate.
The two main variants
C677T (rs1801133, also called Ala222Val): replaces alanine with valine at position 222. Reduces enzyme activity by:
- ~35% in heterozygotes (CT)
- ~70% in homozygotes (TT)
A1298C (rs1801131, Glu429Ala): a separate variant in the same gene. Reduces activity less than C677T, mostly in homozygotes.
About 30-40% of people of European ancestry carry at least one C677T allele; ~10-15% are homozygous TT. Frequencies differ across populations.
Where the evidence is strong
Homocysteine elevation in homozygotes: TT homozygotes have measurably higher homocysteine, especially when folate intake is low. Folic acid fortification of grain products in the US (mandated 1998) substantially blunted this effect at the population level — homocysteine levels dropped after fortification, and clinical relevance has shrunk.
Cardiovascular risk via homocysteine: Elevated homocysteine is associated with cardiovascular disease, but the causal link is contested. Mendelian randomization studies suggest the association is real but the effect size is small. MTHFR TT alone is not a cardiovascular risk factor in folate-replete populations.
Recurrent pregnancy loss: Modest association in some studies, but the ASRM (American Society for Reproductive Medicine) and ACOG (American College of Obstetricians and Gynecologists) do not recommend MTHFR testing in workup for recurrent pregnancy loss as of 2025 guidelines. The evidence is mixed and inconsistent.
Methotrexate response: MTHFR variants influence methotrexate metabolism. CPIC has guidance for this drug-gene pair (Level B evidence).
Where the evidence is weak
Despite popular claims, the evidence does NOT strongly support MTHFR variants as primary causes of:
- Autism spectrum disorder
- Alzheimer's disease
- Depression
- Most cancers (breast, colon, etc.) — meta-analyses are inconsistent and effect sizes small
- "Detoxification" deficits
Weak association in some studies + strong popular advocacy does not equal causal link.
What about "methylated folate" supplements?
The argument for taking 5-MTHF (the bioavailable form, sold as L-methylfolate, Metafolin, or Quatrefolic) instead of folic acid: people with MTHFR TT have reduced ability to convert folic acid → 5-MTHF, so 5-MTHF supplementation bypasses the bottleneck.
Evidence: in folate-deficient TT homozygotes, 5-MTHF does normalize blood folate faster. In folate-replete populations (i.e., most US adults eating fortified grains), the difference at typical supplement doses is small.
If you take folate, especially during preconception/pregnancy, taking 5-MTHF is reasonable and well-tolerated. It's not a magic intervention.
What about high-dose folate?
High-dose folate (>5mg/day) without B12 status assessment can mask vitamin B12 deficiency by correcting the megaloblastic anemia while the neurologic damage progresses. Don't take high-dose folate without checking B12 first. This is the most clinically relevant safety note for MTHFR-positive folate supplementation.
How Atlagene reports MTHFR
Atlagene checks MTHFR rs1801133 (C677T), rs1801131 (A1298C), and the compound heterozygote case (one copy of each). For TT homozygotes or compound heterozygotes, the report includes:
- Phenotype description (~70% reduced activity for TT)
- Homocysteine implications
- 5-MTHF supplement guidance
- B12 status check recommendation
- Methotrexate flag (if relevant)
We do not report MTHFR as causing autism, Alzheimer's, or pregnancy loss — the evidence doesn't support those claims. See our methodology for evidence grading.
What to do with an MTHFR finding
- Don't panic. ~12% of people are TT homozygotes; another ~25% are heterozygotes. It's common.
- Get homocysteine checked if you haven't.
- Get B12 status checked, especially if vegan/vegetarian or over 50.
- If supplementing folate, consider 5-MTHF over folic acid — small but plausible advantage.
- If pregnant or planning pregnancy, follow ACOG's standard prenatal folate guidance (most prenatal vitamins include 5-MTHF anyway).
- Don't seek MTHFR testing through a clinic for general workup of unexplained symptoms — most clinical guidelines say it's not indicated.
MTHFR is one of 700+ variants Atlagene analyzes. See the full registry or upload your DNA file.