The MTHFR gene provides instructions for making an enzyme that helps your body process folate, a B vitamin essential for building DNA, repairing cells, and converting the amino acid homocysteine into something useful. Certain variants of this gene are extremely common, and while they slightly reduce how efficiently this enzyme works, the real-world health impact is far smaller than much of the internet suggests.
What the MTHFR Gene Does
MTHFR stands for methylenetetrahydrofolate reductase, which is both the name of the gene and the enzyme it produces. This enzyme performs one specific job: it converts a form of folate called 5,10-methylenetetrahydrofolate into another form called 5-methyltetrahydrofolate. That second form is the one your body actually uses for a process called methylation, which is involved in DNA production, neurotransmitter regulation, and detoxification.
One of the enzyme’s most important roles is helping recycle homocysteine, an amino acid that circulates in your blood. When the MTHFR enzyme is working normally, it activates folate so your body can convert homocysteine into methionine, a harmless and useful amino acid. When homocysteine builds up instead, it can contribute to cardiovascular problems over time.
The Two Common Variants
Everyone has two copies of the MTHFR gene, one from each parent. Two well-known variants exist: C677T and A1298C. These are single-letter changes in the genetic code that slightly alter the shape and efficiency of the enzyme.
C677T is the more studied of the two. You can carry zero, one, or two copies of the variant. People with two copies (the TT genotype) have the most reduced enzyme activity, but even in this group, the practical effect on folate levels is modest. According to the CDC, people with the TT genotype who get the same amount of folic acid as those without the variant have blood folate levels only about 16% lower. The A1298C variant has been studied less, and current evidence does not show it significantly affects folate processing on its own.
These variants are not rare mutations. The C677T variant is distributed unevenly across populations, with the highest frequency in Mexican and Hispanic populations and the lowest in people of African descent. European populations fall somewhere in between, with a general south-to-north gradient across the continent. Because of how common these variants are, most people who carry them never experience any health consequences.
MTHFR and Homocysteine
The clearest biological effect of the C677T variant is a mild increase in homocysteine levels. Research published in the American Heart Association’s journal Circulation found that people with the TT genotype have homocysteine levels roughly 2.6 micromoles per liter higher than people without the variant. For context, normal homocysteine levels range from about 5 to 15 micromoles per liter, so this increase is noticeable but not dramatic.
The important caveat: this increase in homocysteine primarily shows up in people who aren’t getting enough folate. When folate intake is adequate, homocysteine levels tend to normalize regardless of genotype. In other words, the variant creates a vulnerability that adequate nutrition largely neutralizes.
Do You Need to Avoid Folic Acid?
One of the most persistent claims online is that people with MTHFR variants should avoid folic acid (the synthetic form of folate found in supplements and fortified foods) and take methylfolate instead. The CDC directly addresses this: people with MTHFR variants can process all types of folate, including folic acid. Folic acid intake matters more for your blood folate levels than your MTHFR genotype does.
Studies confirm that 400 micrograms of folic acid daily raises blood folate levels effectively regardless of which MTHFR variant you carry. The CDC explicitly states that common MTHFR variants like C677T are not a reason to avoid folic acid. This is particularly relevant during pregnancy, when adequate folate is critical for preventing neural tube defects. Standard folic acid supplementation remains the recommendation for all women of childbearing age, MTHFR status included.
Should You Get Tested?
MTHFR testing is available through labs and some direct-to-consumer genetic tests. The lab test uses a standard DNA analysis method, checks for both the C677T and A1298C variants, and typically returns results in 6 to 10 days. Many people discover their MTHFR status through consumer genomics services like 23andMe rather than through a doctor’s order.
However, the American College of Medical Genetics and Genomics has published a practice guideline stating there is a “lack of evidence for MTHFR polymorphism testing.” Most major medical organizations do not recommend routine MTHFR screening for the general population. The reasoning is straightforward: the variants are extremely common, the health effects are mild and diet-dependent, and the recommended action (getting enough folate) is the same advice that applies to everyone.
If you already know your MTHFR status, perhaps from a consumer DNA test, there’s no reason to panic about a positive result. If your homocysteine levels are normal and you eat a diet with adequate folate or take a standard supplement, the variant is unlikely to cause problems. For the small number of people who have persistently elevated homocysteine despite adequate folate intake, a doctor can evaluate whether other factors are at play and whether additional B vitamin supplementation would help.
What Actually Matters
The practical takeaway is simpler than most MTHFR content online would have you believe. The gene makes an enzyme that activates folate. Common variants slightly reduce that enzyme’s efficiency. The health consequence, mildly elevated homocysteine, primarily affects people who aren’t getting enough folate in the first place. Getting 400 micrograms of folic acid daily corrects the issue for virtually everyone, regardless of genotype.
Where MTHFR variants can become clinically relevant is when they combine with other genetic or nutritional factors. Someone with the TT genotype who also has a poor diet, smokes, or has kidney disease may accumulate homocysteine more readily. But the variant alone, in the context of a reasonably healthy diet, is not the health crisis it’s often portrayed as online.