What does MTHFR in my 23andMe file mean?
Your file shows two common MTHFR variants, C677T (rs1801133) and A1298C (rs1801131). Carrying one copy of either is ordinary — about half of people in some populations do — and in a cited study even two copies of C677T raised homocysteine only when blood folate was low, which is why folate from food is the practical takeaway.
What to look for in your file
23andMe and AncestryDNA report both positions on the forward (plus) strand. MTHFR sits on the opposite strand, so the letters differ from the names used in research papers:
rs1801133 — C677T
Your file shows G and A: G = 677C (the original form), A = 677T.
| Your letters | Paper name | Associated with |
|---|---|---|
| G G | 677 CC | Typical MTHFR activity. |
| A G | 677 CT | One copy: mildly reduced enzyme activity (roughly 35% lower). Very common; small effect. |
| A A | 677 TT | Two copies: enzyme activity roughly 70% lower. Homocysteine was higher only in people with low folate. |
In 365 people from the NHLBI Family Heart Study, those with 677 TT had fasting homocysteine about 24% higher than CC — but only among people with lower plasma folate (under 15.4 nmol/L). Among people with folate at or above that level, there was no difference between genotypes (PMID 8616944). The activity figures come from the original description of the variant, cited in the Genomeats knowledge base (PMID 7647779).
rs1801131 — A1298C
Your file shows T and G: T = 1298A (the original form), G = 1298C.
| Your letters | Paper name | Associated with |
|---|---|---|
| T T | 1298 AA | Typical activity at this position. |
| G T | 1298 AC | One copy: mild effect on its own. |
| G G | 1298 CC | Mildly reduced activity; on its own, not linked to higher homocysteine. |
The study that described A1298C found it reduces MTHFR activity, more in two copies than one, but neither one nor two copies was associated with higher plasma homocysteine or lower plasma folate. Carrying one copy of each variant (677 CT plus 1298 AC, "compound heterozygous") was associated with lower enzyme activity, higher homocysteine and lower plasma folate than one copy of either alone (PMID 9545395).
How common is each result?
| Population group | 677T (A) letter | A G (estimated) | A A (estimated) | 1298C (G) letter |
|---|---|---|---|---|
| African | 9.0% | 16% | 1% | 15.1% |
| Admixed American | 47.4% | 50% | 22% | 15.1% |
| East Asian | 29.6% | 42% | 9% | 21.9% |
| European | 36.5% | 46% | 13% | 31.3% |
| South Asian | 11.9% | 21% | 1% | 41.7% |
Allele frequencies: 1000 Genomes phase 3 via Ensembl (rs1801133, rs1801131), read 2026-10-23. Genotype shares are Hardy–Weinberg estimates.
One copy of C677T is carried by roughly 4 to 5 in 10 people in the European, East Asian and Admixed American groups. A result that common is a normal variation, not a diagnosis or a disease marker.
What MTHFR results do not mean
- They do not diagnose any disease, and they do not tell you your homocysteine or folate level — only a blood test does.
- These are two positions in one gene; other MTHFR changes and other genes are not tested here.
- If you are pregnant, planning a pregnancy, or have been told your homocysteine is high, follow your clinician's advice; your raw-data result is not a substitute for it.
What to do with it, in food terms
- The cited evidence points to folate status as what matters: the homocysteine difference appeared only with low folate. So the useful step is the same for every genotype — eat folate-rich foods regularly.
- Leafy greens and legumes (lentils, chickpeas, beans) are good everyday sources of folate.
- This page does not recommend supplements or "methylation protocols". If you are considering a supplement, discuss it with a clinician or pharmacist.
References
- Jacques PF et al. Relation between folate status, a common mutation in methylenetetrahydrofolate reductase, and plasma homocysteine concentrations. Circulation 1996;93:7–9. PMID 8616944
- van der Put NM et al. A second common mutation in the methylenetetrahydrofolate reductase gene: an additional risk factor for neural-tube defects? Am J Hum Genet 1998;62:1044–51. PMID 9545395
- Cited in the Genomeats knowledge base for MTHFR C677T enzyme activity: PMID 7647779
- 1000 Genomes Project phase 3 allele frequencies, via Ensembl.
Genomeats reads both MTHFR markers on your phone — nothing is uploaded. Your MTHFR result is in the free overview; the full write-up is part of Genomeats Complete (one-time purchase). Coming soon to the App Store.