MTHFR
Methylenetetrahydrofolate Reductase (C677T Variant)
What it is
MTHFR (methylenetetrahydrofolate reductase) is an enzyme that catalyses a rate-limiting step in the folate metabolic cycle: the conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate (5-MTHF), the primary circulating form of folate. 5-MTHF is the methyl donor required for the remethylation of homocysteine to methionine — a reaction that also requires vitamin B12 as a cofactor. This remethylation step is the principal route for clearing homocysteine from the circulation. A common variant in the MTHFR gene, the C677T substitution, results in an alanine-to-valine change at position 222 of the protein. This produces a thermolabile enzyme with reduced activity: approximately 30% reduced activity in heterozygotes (CT genotype) and approximately 60% reduced activity in homozygotes (TT genotype). A second variant, A1298C, also reduces enzyme activity, with compound heterozygosity (carrying one C677T and one A1298C variant) producing an effect intermediate between 677CT and 677TT. The C677T variant has a population frequency of approximately 10–15% for the TT homozygous genotype in most populations of European and East Asian descent, with higher rates in Mediterranean populations and lower rates in African populations.
Why we measure it
The clinical significance of MTHFR variants is not independent of nutritional status — this is the essential point. An individual with the 677TT genotype who maintains adequate folate and B12 intake will typically have a normal homocysteine level, because the remaining enzyme activity, combined with adequate substrate, is sufficient to maintain normal methylation flux. But in the context of inadequate folate or B12 — which is common, given that deficiency in both is prevalent — 677TT individuals accumulate homocysteine more readily and to higher levels than people with normal enzyme activity. Elevated homocysteine is independently associated with cardiovascular disease, stroke, and cognitive decline, as detailed in the Homocysteine marker page. MTHFR status serves two functions in the protocol. First, it contextualises the Homocysteine result: a homocysteine of 11 μmol/L in a 677TT individual who is taking no supplements carries a different implication than the same number in a person with normal MTHFR function — the 677TT individual has less metabolic reserve and a steeper path to elevation if folate or B12 status declines. Second, it directly informs supplementation strategy. Standard folic acid (the synthetic oxidised form) requires conversion to 5-MTHF through a multi-step pathway that includes MTHFR. In 677TT individuals, this conversion is approximately 60% less efficient — meaning standard folic acid supplementation is less effective at raising 5-MTHF levels and normalising homocysteine than supplementation with the active 5-methylfolate form (5-MTHF, sold as methylfolate). This is a practical, actionable piece of information that shapes the supplementation recommendation for the rest of the individual's life.
Why once at enrollment
MTHFR genotype is determined at conception and does not change. A single test at enrollment produces a lifetime result. The nutritional and supplementation implications it generates are permanent.
What movement means
MTHFR testing produces a categorical genotype result for the C677T variant. The three genotypes differ in residual enzyme activity and their susceptibility to homocysteine accumulation under conditions of nutritional insufficiency. Where the A1298C variant is also tested, compound heterozygosity (677CT + 1298AC) is reported and carries intermediate functional impact.
677CC (Normal)
No C677T variant present
Normal MTHFR enzyme activity. No increased susceptibility to homocysteine accumulation from this variant. Standard Homocysteine monitoring within the Pulse protocol applies. Adequate dietary folate and B12 are sufficient without specific supplementation for this reason alone.
Frosst P et al., Nature Genetics, 1995 — doi:10.1038/ng0595-111
677CT (Heterozygous)
One C677T variant; ~30% reduced enzyme activity
Modest reduction in MTHFR activity. Homocysteine is likely within normal range with adequate folate and B12 intake, but will rise more readily under conditions of nutritional insufficiency. Ensuring adequate dietary folate and B12 is important; methylfolate supplementation is reasonable but not invariably necessary. Homocysteine should be tracked across the quarterly Pulse draws.
Frosst P et al., Nature Genetics, 1995 — doi:10.1038/ng0595-111
677TT (Homozygous)
Two C677T variants; ~60% reduced enzyme activity
The most clinically significant MTHFR genotype. Substantially reduced enzyme activity creates meaningful susceptibility to elevated homocysteine when folate or B12 is insufficient. Supplementation with active 5-methylfolate (5-MTHF) rather than standard folic acid is recommended — the conversion from folic acid to 5-MTHF is approximately 60% impaired at this genotype. Vitamin B12 adequacy is equally important. Homocysteine monitoring in the quarterly Pulse draws is particularly informative for this genotype.
Schwahn B, Rozen R, American Journal of Pharmacogenomics, 2001 — doi:10.2165/00129785-200101030-00004
In the protocol
References
- 1.
Frosst P, Blom HJ, Milos R, et al.. “A Candidate Genetic Risk Factor for Vascular Disease: A Common Mutation in Methylenetetrahydrofolate Reductase.” Nature Genetics. 1995.
- 2.
Schwahn B, Rozen R. “Polymorphisms in the Methylenetetrahydrofolate Reductase Gene: Clinical Consequences.” American Journal of Pharmacogenomics. 2001.
- 3.
Homocysteine Studies Collaboration. “Homocysteine and Risk of Ischemic Heart Disease and Stroke: A Meta-analysis.” JAMA. 2002.
- 4.
Liew SC, Gupta ED. “Methylenetetrahydrofolate Reductase (MTHFR) C677T Polymorphism: Epidemiology, Metabolism and the Associated Diseases.” European Journal of Medical Genetics. 2015.