What MTHFR actually does
The MTHFR gene encodes an enzyme that converts folate (from food) and folic acid (the synthetic form in supplements) into 5-methyltetrahydrofolate (5-MTHF) — the active form that your cells actually use. That active form is essential for DNA synthesis, cellular division, and a process called methylation that regulates gene expression throughout the body.
When the MTHFR enzyme works less efficiently, two things happen: less active folate is available for the body's processes, and a compound called homocysteine can accumulate (because folate is needed to convert homocysteine to methionine). Elevated homocysteine is the downstream effect that's genuinely linked to health outcomes — not the gene variant itself.
The two MTHFR variants — and why only one really matters
There are two clinically relevant MTHFR variants: C677T and A1298C. C677T homozygous (two copies of the C677T variant) reduces enzyme activity by roughly 70% — this is the one with meaningful clinical implications. C677T heterozygous (one copy) reduces activity by around 35% and has minimal real-world impact for most people who eat a varied diet with adequate folate.
A1298C has a smaller functional effect and is rarely clinically significant on its own. The "compound heterozygous" status (one C677T + one A1298C) falls somewhere in between. The wellness industry has created significant anxiety around heterozygous MTHFR status that the clinical evidence simply does not support. If you have one copy of C677T, the honest answer is: eat leafy greens, take a prenatal with methylfolate if you're planning pregnancy, and move on with your life.
What actually needs testing (Botto & Yang, BMJ 2003): In women planning pregnancy, C677T homozygous status combined with low folate intake creates approximately double the risk of neural tube defect pregnancies compared to non-carriers. The intervention is straightforward: switch from folic acid to methylfolate (5-MTHF) in your prenatal supplement, as methylfolate bypasses the impaired MTHFR conversion step entirely. The neural tube closure window is the first 28 days after conception — often before a pregnancy is confirmed.
What to actually do with this information
The most clinically useful test is a homocysteine blood test, not the MTHFR gene panel. Homocysteine tells you whether impaired MTHFR function is actually causing a downstream problem — the gene variant alone doesn't tell you that, because dietary folate intake, B12 levels, and other factors all influence homocysteine independently. A normal homocysteine level in a woman with a C677T variant is reassuring. An elevated homocysteine in anyone — variant or not — is worth addressing.
- If planning pregnancy, switch to methylfolate: Prenatals containing 5-MTHF (methylfolate) rather than standard folic acid are appropriate for women with C677T homozygous status — and are a reasonable default for anyone who doesn't know their MTHFR status, since methylfolate works for everyone. Research used doses of 400-800 mcg; discuss specific supplementation with your OB-GYN.
- Ask for homocysteine testing, not just the gene panel: A homocysteine level above 10-12 μmol/L is worth addressing regardless of MTHFR status. B12, B6, and folate all reduce homocysteine — the approach is nutritional, not complicated.
- Don't over-supplement B vitamins: High-dose methylfolate supplementation without clinical indication is unnecessary and can mask B12 deficiency. Work with a clinician rather than self-prescribing based on a gene panel alone.
- Eat folate-rich foods regardless: Leafy greens, legumes, and asparagus provide natural folate (not folic acid) that doesn't require MTHFR conversion to be absorbed. Food-first is the most reliable approach for anyone with a mild variant.
MTHFR genetic testing is widely available but not routinely recommended by ACOG for preconception screening unless there's a history of neural tube defect pregnancy or recurrent miscarriage with elevated homocysteine. If you've had unexplained recurrent miscarriages, asking for homocysteine testing alongside a thrombophilia workup is reasonable. An OB-GYN or maternal-fetal medicine specialist is the right person to interpret these results in a preconception context.
- Botto LD, Yang Q (2000). 5,10-Methylenetetrahydrofolate reductase gene variants and congenital anomalies: a HuGE review. American Journal of Epidemiology, 151(9):862-877.
- Botto LD, Yang Q (2003). Methylenetetrahydrofolate reductase C677T variant and neural tube defects. BMJ, 326(7380):29-30.
- Vollset SE et al. (2000). Plasma total homocysteine and pregnancy complications: the Hordaland Homocysteine Study. American Journal of Clinical Nutrition, 71(4):962-968.
- ACOG Practice Bulletin #36 (2002, reaffirmed 2021). Inherited thrombophilias in pregnancy. American College of Obstetricians and Gynecologists.
- Liew SC, Gupta ED (2015). Methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism: epidemiology, metabolism, and associated diseases. European Journal of Medical Genetics, 58(1):1-10.