MTHFR: One of the Most Searched Genetic Variants
If you have ever searched for information about your 23andMe results, you have likely encountered claims about MTHFR mutations. Supplement companies, alternative health practitioners, and wellness blogs have made MTHFR one of the most discussed genes on the internet, linking it to dozens of health conditions and promoting methylfolate supplements as a solution.
The reality is more nuanced. MTHFR variants are extremely common, their health effects are modest, and much of the online discourse overstates their clinical significance. This guide separates what research actually shows from what the supplement industry claims.
The Two Key MTHFR Variants
C677T (rs1801133)
The C677T variant is a missense change that substitutes alanine with valine at position 222 of the MTHFR enzyme. This reduces enzyme thermostability and activity:
- CC genotype — Normal enzyme activity (reference).
- CT genotype — Approximately 65% of normal activity. Present in 40–50% of many populations.
- TT genotype — Approximately 30% of normal activity. Present in 10–25% of populations depending on ethnicity.
A1298C (rs1801131)
The A1298C variant substitutes glutamate with alanine at position 429. Its effect on enzyme activity is milder than C677T:
- AA genotype — Normal activity (reference).
- AC genotype — Slightly reduced activity.
- CC genotype — Moderately reduced activity, though less impactful than C677T TT.
Being compound heterozygous (CT at C677T and AC at A1298C) results in a moderate reduction in enzyme activity, roughly comparable to being homozygous TT at C677T alone.
What Research Actually Shows
Homocysteine Elevation
The most well-established effect of MTHFR C677T TT is a mild elevation in blood homocysteine levels, typically 2–3 micromol/L above average. This effect is most pronounced when folate intake is low. With adequate dietary folate, homocysteine levels in TT individuals often remain within the normal range.
Neural Tube Defect Risk
The strongest clinical association is between maternal MTHFR C677T TT genotype and increased risk of neural tube defects (NTDs) in offspring, particularly when combined with low folate status. This is why folate supplementation before and during pregnancy is universally recommended — it reduces NTD risk regardless of MTHFR genotype. Standard folic acid supplementation is effective even for TT genotype carriers.
Cardiovascular Risk
Elevated homocysteine is an independent but modest risk factor for cardiovascular disease. Meta-analyses show that MTHFR C677T TT genotype is associated with a small increase in stroke risk (roughly 1.2–1.4 fold). However, large randomized trials of homocysteine lowering with B vitamins have not shown consistent cardiovascular benefit, suggesting that the association may not be directly causal.
What the Supplement Industry Claims vs. Evidence
Many online sources link MTHFR variants to an expansive list of conditions including anxiety, depression, chronic fatigue, fibromyalgia, autism, migraines, recurrent miscarriage, and autoimmune diseases. They then recommend methylfolate (5-MTHF) supplements as a solution. Here is what the evidence actually supports:
- Depression — Some studies show a weak association between C677T TT and depression risk, but effect sizes are small and inconsistent across populations. Methylfolate has been studied as an adjunctive treatment, with mixed results.
- Recurrent miscarriage — The association is weak and not consistent enough for medical societies to recommend routine testing.
- Autism — No convincing evidence links MTHFR variants to autism risk.
- Chronic fatigue / fibromyalgia — Not supported by controlled studies.
The American College of Medical Genetics (ACMG) and the American Heart Association do not recommend routine MTHFR genotyping. The ACMG specifically advises against testing because the variants are common and the clinical utility is limited.
Looking Up Your MTHFR Genotype
If you have raw data from 23andMe or AncestryDNA, checking your MTHFR status is straightforward. Look up these two rsIDs:
- rs1801133 — MTHFR C677T. Look for your genotype: CC, CT, or TT.
- rs1801131 — MTHFR A1298C. Look for your genotype: AA, AC, or CC.
Use the free SNP Lookup tool to search either rsID and see trait associations, population frequencies, and confidence levels for the research behind each annotation.
For a broader pharmacogenomic picture including how MTHFR status may interact with methotrexate response, check the Pharmacogenomics Checker or explore your full genome through the Genomics Dashboard.