Folate metabolism (MTHFR gene)

The MTHFR gene makes an enzyme that helps your body process folate (vitamin B9) and clear a substance called homocysteine. A common version of the gene lowers that enzyme's activity, though its health importance is usually mild and often overstated online.

What the C677T variant does

MTHFR converts folate into the form used to recycle homocysteine back into methionine. This variant makes the enzyme thermolabile, meaning less stable and less active, with two copies giving roughly 30% of typical activity and one copy an intermediate level.

The measurable consequence is modest: slightly higher blood homocysteine and slightly lower folate, most visible when folate intake is low. In populations with folic acid fortification, the difference between genotypes narrows considerably.

What the evidence actually supports, and what it does not

The link to homocysteine is real. In a study of 5,175 people, the variant was clearly associated with homocysteine levels, and ClinVar lists it with three-star review status under the thermolabile polymorphism and folate-sensitive neural tube defects.

The onward claims are where this variant went wrong. Higher homocysteine was hypothesised to cause clots, heart disease and recurrent pregnancy loss, and MTHFR testing spread widely on that basis. Meta-analyses then failed to support it: homocysteine levels turned out not to predict coronary heart disease as expected, and MTHFR genotype turned out not to predict venous thromboembolism.

Why professional bodies advise against testing for this

In 2013 the American College of Medical Genetics published a practice guideline stating that MTHFR polymorphism testing has minimal clinical utility, and that it should not be ordered as part of an evaluation for thrombophilia or recurrent pregnancy loss, nor for at-risk family members. Many clinical laboratories have since withdrawn the test.

That is an unusual thing to say on a page about a variant, and it is the most useful thing this page can tell you. A raised homocysteine level, if measured, is worth discussing; the genotype behind it usually is not.

Folate, pregnancy and methylfolate supplements

Folic acid supplementation before and during early pregnancy reduces neural tube defects, and that recommendation applies to everyone regardless of MTHFR genotype. Nothing about a C677T result changes the standard advice.

Methylfolate supplements are widely marketed to people with this variant on the reasoning that they bypass the enzyme. For people eating an ordinary diet there is no good evidence that they are superior to ordinary folic acid, and the marketing has consistently run ahead of the science.

How common it is

In 1000 Genomes reference data, about 11% of European-ancestry individuals carry two copies and 45% carry one, with the highest frequencies in admixed American samples at around 24% with two copies, and the lowest in African-ancestry samples at about 1%.

A variant this common, carried in double dose by roughly one in ten Europeans and one in four people in some populations, was never likely to be a hidden cause of disease.

What each rs1801133 genotype means

rs1801133 has three possible genotypes: CC, CT and TT.

Normal folate metabolism (rs1801133 CC). You have two standard copies of MTHFR, so the enzyme that processes folate (vitamin B9) works at full strength. Your body clears a substance called homocysteine normally, and this gene gives you no special reason to worry about folate. A balanced diet is plenty.

Slightly reduced folate metabolism (rs1801133 CT). You have one standard and one slower copy of MTHFR, so the enzyme runs at roughly two-thirds of its usual speed. For almost everyone this makes no real difference, and a normal diet with folate from greens, beans and grains covers it.

Reduced folate metabolism (rs1801133 TT). You have two copies of the slower MTHFR variant, so this enzyme runs at roughly 30% of its usual speed. That can let a substance called homocysteine build up a little, which is why some people make a point of getting enough folate from leafy greens, beans and fortified grains. For most people the effect is mild, and this gene's health impact is often exaggerated online.

Evidence & sources

Across global populations, about 11% of people carry two copies of this variant, and about 47% carry none.

This variant appears in 11 published genetic studies: PubMed 20031578 · PubMed 23696881 · PubMed 23754956 · PubMed 23824729 · PubMed 29953918 · PubMed 30339177 · PubMed 34707639 · PubMed 37479695 · PubMed 38626723 · PubMed 39024449 · PubMed 39414775.

Common questions

What does reduced folate metabolism mean? It means the MTHFR enzyme that converts dietary folate into its active form works more slowly than average. With one slower copy (CT) it runs at roughly two-thirds speed, and with two (TT) at roughly a third. In practice this only means your body is a little less efficient at processing folate, which a folate-rich diet easily makes up for.

Is the MTHFR C677T variant dangerous? For most people, no. The T version of MTHFR C677T lowers the enzyme's activity and can raise homocysteine slightly, but the health impact is usually mild and is often exaggerated online. Getting enough folate from leafy greens, beans and fortified grains covers it for the large majority of carriers, even those with two copies.

Should I take methylfolate if I have the MTHFR gene variant? Usually it is not necessary. A normal diet with folate from food is enough for most people with the CT or TT genotype, and ordinary folic acid is still processed effectively by the body. Some clinicians suggest methylfolate in specific situations, such as pregnancy planning with elevated homocysteine, but that is an individual medical decision. This result is educational and not a prescription, so discuss supplements with a doctor.

I have two copies of C677T. Do I need methylfolate? There is no good evidence that methylfolate is superior to ordinary folic acid for people with this variant who eat an ordinary diet. Standard folate advice, including folic acid before and during early pregnancy, applies regardless of genotype.

Does MTHFR cause miscarriage or blood clots? The evidence does not support it. Meta-analyses failed to confirm an association between MTHFR genotype and venous thromboembolism, and in 2013 the American College of Medical Genetics recommended against testing for it in evaluations of thrombophilia or recurrent pregnancy loss.

Should I get my homocysteine checked? That is a reasonable question for a doctor if there is a clinical reason, and it measures the thing itself rather than a genetic tendency toward it. Treating a raised homocysteine with B vitamins lowers the number, though trials have not shown that this reduces cardiovascular events.

Related

Adult lactose digestion · Beta-carotene to vitamin A conversion (BCO1) · Caffeine metabolism speed · Fat taste sensitivity (CD36) · LDL cholesterol · Sweet tooth / carbohydrate preference (FGF21) · Vitamin D status (low-D genetic burden)