# MTHFR Variants (C677T & A1298C): A Practical Guide
URL: https://genohealth.app/pathways/mthfr-variants
Type: Pathway
Evidence: Strong evidence
Last reviewed: 2026-07-01

## Intro
MTHFR (methylenetetrahydrofolate reductase) is the enzyme that converts dietary folate into 5-MTHF — the active form your body uses to lower homocysteine and run the methylation cycle. Two common variants, C677T and A1298C, reduce enzyme activity. They're not a disease, but they change which folate form works best for you.

## Summary
What the MTHFR C677T and A1298C variants actually do, who should care, and how to choose between folic acid, methylfolate and folinic acid.

## Key facts
- C677T homozygous (TT): ~10–15% of population · ~70% reduced activity
- C677T heterozygous (CT): ~30–40% of population · ~35% reduced activity
- A1298C homozygous (CC): ~7–12% of population · mild reduction
- Best folate form if TT/CC: 5-MTHF (methylfolate) or folinic acid

## What each variant does
- C677T (rs1801133): the most studied variant. TT genotype reduces MTHFR enzyme activity by ~70%; CT by ~35%.
- A1298C (rs1801131): milder functional impact than C677T. Homozygotes see a modest reduction; heterozygotes are typically clinically silent.
- Compound heterozygous (C677T + A1298C on opposite alleles): behaves similarly to a single homozygote — worth flagging with your clinician if homocysteine is elevated.
- Variants do NOT cause disease on their own. They change the folate form your body handles best.

## How to test for MTHFR
- Direct DNA test: 23andMe, AncestryDNA and most consumer chips genotype rs1801133 (C677T) and rs1801131 (A1298C). Upload the raw file to GENO to see your calls in plain English.
- Clinical PCR panel: available through most labs (LabCorp, Quest) if your clinician orders it — ~$100–200 out of pocket.
- Functional check: pair genotype with a fasting homocysteine test. Homocysteine < 9 µmol/L means the pathway is working regardless of genotype.

## Folic acid vs methylfolate vs folinic acid
- Folic acid: synthetic, cheap, in fortified flour and most multivitamins. Requires MTHFR conversion — inefficient if you're TT/CC. High doses (> 400 mcg/day) can build up as unmetabolised folic acid in TT carriers.
- Methylfolate (L-5-MTHF, Quatrefolic, Metafolin): already in active form. Bypasses MTHFR. Best default for TT/CC carriers — but can feel stimulating in slow COMT / methyl-sensitive individuals.
- Folinic acid (calcium folinate, leucovorin): one step upstream of 5-MTHF. Gentler for methyl-sensitive users (slow COMT, HNMT variants) and often the smarter choice if methylfolate causes anxiety, insomnia or irritability.
- Food folate: leafy greens, legumes, liver. Always the first-line recommendation — no genotype makes food folate a bad idea.

## When MTHFR actually matters clinically
- Elevated fasting homocysteine (> 10 µmol/L) — modifiable cardiovascular and cognitive risk marker.
- Pregnancy planning: TT carriers benefit from active folate in prenatal formulas to reduce neural-tube-defect risk.
- Recurrent pregnancy loss or unexplained early miscarriage — discuss testing with your OB-GYN.
- Depression that hasn't responded to standard SSRIs — some clinicians trial 5-MTHF as an adjunct (evidence: moderate).
- Anti-folate medications (methotrexate, some anticonvulsants) — flag your genotype to your prescriber.

## When MTHFR doesn't matter as much as the internet says
- Normal homocysteine + folate-rich diet = no action needed, regardless of genotype.
- MTHFR alone does not cause autism, ADHD, chronic fatigue or 'toxin overload' — the popular claims outrun the evidence.
- You do not need expensive 'methylation protocols'. Most people improve with food, a B-complex with active B12, and less alcohol.

## Practical starting point
- 1. Test homocysteine (fasting).
- 2. Eat leafy greens, legumes and eggs most days.
- 3. If supplementing, choose a B-complex with 5-MTHF (400–800 mcg) + hydroxocobalamin or methylcobalamin B12.
- 4. If methylfolate feels stimulating, switch to folinic acid.
- 5. Recheck homocysteine in 8–12 weeks.

## FAQ
**Q: Is MTHFR a mutation or a variant?**
A: Technically a common polymorphism, not a mutation. Over half the population carries at least one copy of C677T or A1298C — it's normal human variation, not a disease.

**Q: Should I stop eating fortified foods if I have MTHFR?**
A: No. Fortified foods provide small doses (100–200 mcg per serving) that even reduced-activity MTHFR handles fine. The concern with unmetabolised folic acid applies to high-dose supplements (> 400 mcg/day), not fortified flour.

**Q: Methylfolate makes me anxious — is that normal?**
A: It happens, especially in slow COMT carriers. 5-MTHF speeds up the methylation cycle and can transiently raise dopamine/norepinephrine turnover. Switch to folinic acid, or take a smaller dose earlier in the day.

**Q: What's the difference between methylfolate and folinic acid?**
A: Folinic acid (5-formyl-THF) is one metabolic step before methylfolate (5-MTHF). Both bypass MTHFR. Folinic acid is generally gentler because your body decides how much to convert to the active methyl form.

**Q: Does GENO tell me which folate form to take?**
A: Yes. GENO reads your MTHFR and COMT genotype from your raw DNA file and personalizes supplement recommendations — swapping methylfolate for folinic acid, and methylcobalamin for hydroxocobalamin, when your genetics suggest you'll tolerate the gentler form better.

## Citations
- Frosst et al., Nat Genet 1995 (PMID 7647779): First characterization of the MTHFR C677T variant and its ~70% activity reduction in TT homozygotes.
- van der Put et al., Am J Hum Genet 1998 (PMID 9545395): A1298C variant described; milder functional impact than C677T.
- Wald et al., BMJ 2002 (PMID 12446535): Homocysteine is a causal, modifiable cardiovascular risk factor — folate lowers it.
- ACMG Practice Guideline, Genet Med 2013 (PMID 23306798): Routine MTHFR genotyping is NOT recommended as a stand-alone thrombophilia workup — pair with homocysteine.

Disclaimer: Informational only — not medical advice.