MTHFR Gene Variants Explained: What C677T and A1298C Mean for Your Health
Published 10/02/2026
If you've had a methylation test or spoken to a functional medicine practitioner, you've almost certainly heard about MTHFR. It's the most discussed gene in consumer genetics — and with good reason. Variants in MTHFR affect a process that touches cardiovascular health, mental health, pregnancy outcomes, and ageing. Here's what you actually need to know.
What is the MTHFR gene?
MTHFR (Methylenetetrahydrofolate Reductase) encodes an enzyme that performs one of the most critical steps in folate metabolism. Specifically, MTHFR converts dietary folate (or synthetic folic acid, after several conversion steps) into 5-methyltetrahydrofolate — abbreviated 5-MTHF — which is the active form the body uses in the methylation cycle.
Without adequate 5-MTHF, the methionine cycle slows. Homocysteine — which should be recycled back into methionine — accumulates in the blood. Elevated homocysteine is one of the most well-evidenced independent risk factors for cardiovascular disease.
Standard folic acid (the synthetic form in most supplements and fortified foods) requires conversion through several enzymatic steps before it becomes 5-MTHF. If your MTHFR enzyme is impaired by variants, this conversion is slowed or blocked — meaning standard folic acid supplementation may not adequately address your methylation needs. Methylfolate (5-MTHF) bypasses this step entirely.
The two main MTHFR variants
A cytosine-to-thymine substitution at position 677. One copy (heterozygous) reduces MTHFR activity by ~30–40%. Two copies (homozygous TT) reduce activity by ~60–70%. The most clinically significant MTHFR variant for cardiovascular risk.
An adenine-to-cytosine substitution at position 1298. Has a smaller impact on its own than C677T, but compound heterozygosity (one C677T + one A1298C) significantly reduces MTHFR activity — often more than a single homozygous C677T.
How common are MTHFR variants?
What MTHFR variants actually affect
Homocysteine and cardiovascular risk
This is the most clinically established consequence of MTHFR impairment. Elevated homocysteine damages the endothelial lining of blood vessels, promotes oxidative stress, and accelerates atherosclerosis. Homozygous C677T individuals have significantly higher average homocysteine levels than those with no variant, with population studies consistently linking this to increased risk of:
- Coronary artery disease
- Ischaemic stroke
- Deep vein thrombosis
- Peripheral artery disease
Neural tube defects and pregnancy
MTHFR variants were among the first genetic risk factors identified for neural tube defects (NTDs) such as spina bifida. The current guidance in most countries recommends folate supplementation before and during early pregnancy; for women with MTHFR variants, methylfolate (5-MTHF) supplementation is generally considered preferable to standard folic acid.
Mental health
5-MTHF is required for the synthesis of BH4 (tetrahydrobiopterin), a cofactor essential for producing serotonin, dopamine, and noradrenaline. Impaired MTHFR activity can reduce BH4 availability, contributing to neurotransmitter imbalances. Meta-analyses have found elevated rates of MTHFR C677T in people with depression, bipolar disorder, and schizophrenia — though the relationship is complex and multifactorial.
Cognitive function and dementia risk
Chronically elevated homocysteine is neurotoxic. Longitudinal studies have linked high homocysteine to accelerated cognitive decline and increased risk of Alzheimer's disease. Lowering homocysteine through B vitamin supplementation has been shown in some trials to slow brain atrophy in people with mild cognitive impairment.
What to do if you carry MTHFR variants
- Test your homocysteine
A simple blood test measures serum homocysteine. If you carry C677T (especially homozygous) or compound heterozygosity, checking your current homocysteine level tells you whether your variants are actually affecting your biochemistry right now.
- Switch to methylated B vitamins
Replace standard folic acid with 5-MTHF (methylfolate) and standard B12 with methylcobalamin. These are the active forms that bypass the MTHFR conversion step entirely. Doses should be guided by your results and a qualified practitioner.
- Assess your broader B vitamin status
MTHFR doesn't work in isolation. B6 and riboflavin (B2) are also required for the methylation cycle. B2 is particularly important: it's a cofactor for MTHFR itself, and supplementing riboflavin has been shown in trials to reduce homocysteine specifically in C677T homozygotes.
- Reduce lifestyle factors that elevate homocysteine
Alcohol, smoking, high red meat intake, and low dietary folate all raise homocysteine independently of genetics. These are modifiable — addressing them compounds the benefit of targeted supplementation.
- Consider a comprehensive methylation panel
MTHFR doesn't tell the whole story. Variants in MTRR, MTR, COMT, and other genes interact with MTHFR in the same pathways. A comprehensive test gives you the full picture needed for an effective intervention strategy.
MTHFR variants are common, consequential, and highly actionable. If you carry C677T homozygous or compound heterozygosity, the evidence for switching to methylated B vitamins and monitoring homocysteine is strong. The key is not to treat MTHFR in isolation — it's one node in a larger methylation network, and comprehensive genetic testing gives you the complete map needed to intervene effectively.
Stride Optimal Health covers all major methylation SNPs in a single test, with expert consultation to help you act on the results.