MTHFR: The Folate and Methylation Gene

What MTHFR Does

MTHFR (methylenetetrahydrofolate reductase) encodes one of the most important enzymes in folate metabolism. The MTHFR enzyme converts 5,10-methylenetetrahydrofolate into 5-methyltetrahydrofolate (5-MTHF) — the primary circulating form of folate in your blood and the form your body uses for methylation reactions.

Methylation is a fundamental biochemical process involved in DNA repair, neurotransmitter production, detoxification, gene expression regulation, and homocysteine metabolism. The MTHFR enzyme sits at a critical junction in the folate cycle, making it one of the most studied genes in nutrigenomics. Understanding MTHFR is also relevant to anyone exploring pharmacogenomics from raw DNA data, since the same consumer DNA files that reveal MTHFR status also contain pharmacogenomic variants.

Key Variants

Two MTHFR variants have well-established effects on enzyme activity:

• C677T (rs1801133): The most studied nutrigenomic variant. A C→T change at position 677 produces an alanine-to-valine substitution, reducing thermostability and activity. This is the variant with the largest effect on MTHFR function.
• A1298C (rs1801131): A secondary variant with a milder effect. An A→C change at position 1298 produces a glutamate-to-alanine substitution. This variant reduces enzyme activity less dramatically than C677T but may have additive effects when combined with C677T (compound heterozygosity).

Genotype Breakdown

The table below summarises what each C677T and A1298C genotype means for enzyme function, folate conversion, and common lab findings. Population frequencies are approximate and vary by ethnicity.

Enzyme activity percentages are approximate estimates from in-vitro studies. Actual metabolic impact depends on dietary folate, B12 status, and other factors.

MTHFR and Homocysteine

The best-established clinical consequence of reduced MTHFR activity is its effect on homocysteine levels. 5-MTHF donates a methyl group to homocysteine, converting it to methionine — which feeds into the SAM (S-adenosylmethionine) cycle, your body's primary methyl donor pathway. When MTHFR activity is reduced, less 5-MTHF is available for this conversion, and homocysteine can accumulate.

Key points about homocysteine:

• C677T TT carriers have the strongest association with elevated homocysteine, particularly when folate intake is low.
• Elevated homocysteine is a cardiovascular risk marker, though the causal relationship between homocysteine lowering and reduced cardiovascular events remains debated in clinical literature.
• A routine blood test can measure homocysteine — this is far more clinically actionable than genotype alone. For a full comparison of genetic vs blood-based approaches, see our methylation testing overview.
• Adequate folate and vitamin B12 intake normalises homocysteine in many C677T TT carriers, reinforcing the importance of diet before supplementation.

What This Means for You

MTHFR variants affect the efficiency of folate metabolism, which has downstream effects on several processes:

• Methylation capacity: Reduced 5-MTHF production may affect the SAM cycle, influencing DNA methylation, neurotransmitter synthesis, and detoxification pathways.
• Folate form considerations: Because the MTHFR enzyme converts folate into its active form, people with significantly reduced activity (particularly TT homozygotes) may benefit more from methylfolate (5-MTHF) than from folic acid. For guidance on which supplements to consider — and which to approach with caution — see our evidence-based MTHFR supplement guide and supplements to approach with caution.
• Pregnancy considerations: Folate is critical during pregnancy for neural tube development. While MTHFR C677T TT has been associated with slightly elevated risk in some studies, adequate folate intake is the protective factor for all genotypes. Our MTHFR and pregnancy guide covers what the evidence actually says.

Having an MTHFR variant is not a disease. It is a common genetic variation that affects one metabolic pathway. Its practical impact depends heavily on your dietary folate intake and overall nutritional status. For practical guidance, see our MTHFR C677T: What It Means and What to Do guide.

MTHFR and Nutrition

Diet is the most practical lever for managing MTHFR variant effects. Natural food folate is already partially in the form that bypasses the MTHFR-dependent step, making it an efficient source regardless of genotype.

High-folate foods: Dark leafy greens, legumes (lentils, chickpeas, black beans), asparagus, Brussels sprouts, broccoli, avocado, and citrus fruits. A diet rich in these foods can support adequate folate status for most people, including those with MTHFR variants.

Methylfolate vs. folic acid: Folic acid (found in fortified foods and most supplements) requires enzymatic conversion — including the MTHFR step — to become active. Methylfolate (5-MTHF) is already in the active form. For people with significantly reduced MTHFR activity, methylfolate may be more bioavailable, though folic acid fortification has been effective at reducing neural tube defects across all MTHFR genotypes. The field of nutrigenomics aims to personalise these recommendations based on individual genetic profiles.

DecodeMyBio's Nutrition & Methylation Report includes MTHFR analysis with food-first strategies, evidence ratings, and context for discussing results with your healthcare provider. For a comparison of clinician-ordered genetic panels and their costs, see our GeneSight cost, insurance & alternatives guide.

How MTHFR Is Tested

MTHFR genotyping can come from several sources, each with different strengths:

• Clinical genetic testing: Ordered by a physician, typically as part of a workup for elevated homocysteine or recurrent pregnancy loss evaluation. Results are interpreted by a healthcare provider in the context of lab work. For a comparison of clinical testing services and costs, see our pharmacogenomic testing comparison.
• Consumer DNA raw data: Both key MTHFR variants — C677T (rs1801133) and A1298C (rs1801131) — are included in 23andMe, AncestryDNA, MyHeritage, and FamilyTreeDNA arrays. Unlike pharmacogenomic genes like CYP2D6 (where consumer arrays miss structural variants), MTHFR genotyping from consumer data is straightforward — both variants are single-nucleotide changes that arrays detect reliably. See what to do with your 23andMe raw data or our step-by-step upload guide for a full walkthrough.
• Blood biomarkers: Homocysteine, serum folate, and vitamin B12 levels provide functional context that genotyping alone cannot. A high homocysteine level in a C677T TT carrier is more clinically meaningful than the genotype alone. Our methylation testing guide compares genetic and blood-based approaches in detail.

Limitations

MTHFR genotype information has important limitations that are essential to understand:

• Not diagnostic: MTHFR genotype alone does not diagnose any condition. Many people with TT genotypes have normal homocysteine and no clinical issues.
• Diet is the primary modifier: The clinical impact of MTHFR variants depends heavily on dietary folate and B12 intake. Genotype without nutritional context is incomplete.
• Overhyped in wellness circles: MTHFR variants have been associated with dozens of conditions in low-quality studies. Rigorous evidence supports effects on homocysteine and neural tube defect risk; most other associations are weak or unreplicated.
• One gene among many: Folate metabolism involves multiple genes beyond MTHFR. Other genes in the one-carbon metabolism pathway (MTR, MTRR, COMT) also contribute, and polygenic effects are not captured by looking at MTHFR alone.
• Requires clinical interpretation: Supplement or treatment decisions should be made with a healthcare provider, not based solely on genotype. Lab work (homocysteine, folate, B12) provides the actionable clinical data. For more context, see our limitations page.

Frequently Asked Questions

Related Genes and Reports

MTHFR is one of several genes that DecodeMyBio analyses from consumer DNA raw data. Related resources:

• CYP2D6 — Drug metabolism for antidepressants, opioids, and tamoxifen
• CYP2C19 — Clopidogrel, SSRIs, and proton pump inhibitors
• CYP2C9 — Warfarin and NSAID metabolism
• VKORC1 — Warfarin sensitivity
• Nutrition & Methylation Report — MTHFR analysis with food-first strategies
• Psychiatric Medication Report — CYP2D6, CYP2C19, and other pharmacogenes
• DNA testing for medication response — Overview of pharmacogenomic vs nutrigenomic testing