Vitamin B9: folate, folic acid, the MTHFR mutation and precautions

"Vitamin B9" refers to a family of molecules. Folates are the forms naturally present in food. Folic acid is the synthetic form added to fortified foods and most supplements. 5-MTHF, or methylfolate, is another form sold as a supplement. Confusing them risks misreading a label or unknowingly stacking two products.

Dietary folate is found in leafy green vegetables, legumes, certain fruits, nuts, and liver. Folic acid is more stable and better absorbed: taken with a meal, about 85% of it is available to the body, according to the US NIH.

5-MTHF is marketed as an "already active" form. Its bioavailability is at least equivalent to that of folic acid, and it can be useful for certain people. But recommendations for preventing neural tube defects rest on folic acid, and the conversion factor of 5-MTHF into dietary folate equivalents hasn't been formally established.

This is an increasingly searched-for topic, often presented misleadingly by the marketing of certain supplements. The MTHFR gene encodes an enzyme involved in folate and homocysteine metabolism. A common variant (C677T) reduces this enzyme's activity and can slightly raise blood homocysteine levels in carriers.

The problem is how this information is exploited commercially. Expert analyses, notably a summary published by the Royal Australian College of General Practitioners, conclude that there are very few validated clinical indications for systematically testing this mutation, and that this test is not recommended as nonspecific screening in the asymptomatic general population. These variants have been weakly associated with multiple conditions (autism, heart problems, adverse pregnancy outcomes) in studies whose results vary widely by population studied, which doesn't support making it a reliable predictive tool.

Anses (the French food safety agency) cites legumes (lentils, chickpeas), leafy green vegetables (spinach, broccoli, Brussels sprouts, lettuce), and liver among the main sources. Actual content varies with the food, the serving, and the preparation; cooking and processing can reduce it.

Regularly adding a legume, a green vegetable, or a salad, and varying sources over the course of the week, helps meet needs. This approach doesn't replace supplementation recommended in a specific situation, such as planning a pregnancy.

Many countries (the United States, Canada, and many others) have mandated fortifying certain flours with folic acid since the late 1990s, precisely to reduce neural tube defects across the whole population, including in women whose pregnancy wasn't planned. The United Kingdom announced in 2021 similar mandatory fortification of non-wholemeal flours (250 µg of folic acid per 100 g of flour), with implementation planned for late 2026 and an expected reduction of about 20% in neural tube defect cases in the country.

Comparisons between countries with and without mandatory fortification show a reduction of up to about half the risk of neural tube defects in fortified countries. This is one of the best-documented examples of a large-scale nutritional public health measure with a measurable effect, which contrasts with many less solid claims about other vitamins.

The European Food Safety Authority (EFSA) sets a reference of 330 µg of dietary folate equivalents (DFE) per day for adults, 600 µg during pregnancy, and 500 µg during breastfeeding. These are population needs, not pill doses.

The "dietary folate equivalent" unit accounts for the better absorption of folic acid compared with dietary folate. A label can list µg of folic acid, µg of folate, or µg of equivalents: check the unit before adding amounts together.

The embryo's neural tube closes within the first few weeks, often before the pregnancy is known. Anses stresses that B9 must be present in sufficient amounts from the very first day of pregnancy, whether planned or not.

In the United States, authorities recommend 400 µg of folic acid per day for any woman who could become pregnant, in addition to dietary folate. The USPSTF expert panel advises 400 to 800 µg per day, starting at least one month before conception and continuing through the first two to three months. Higher doses are suggested by specialists for women who have already had a child with a neural tube defect.

Before buying, note the exact form (folic acid or 5-MTHF), the amount per unit, and the stated daily dose. Then look for other sources: a multivitamin, an "energy" or "hair" supplement, a prenatal product, a fortified food. The most common mistake is stacking several labels without adding up the total.

In 2023, EFSA set an upper limit of 1,000 µg per day for all authorized forms of folic acid and 5-MTHF salts combined, for adults, including during pregnancy and breastfeeding. This limit does not concern natural dietary folate. A higher dose can be part of a prescribed treatment: in that case, don't compare it to a supplement and don't change it without the prescriber.

A folate deficiency can cause so-called megaloblastic anemia, with fatigue, headaches, dizziness, shortness of breath, and pallor. But these symptoms have many other causes: they warrant a workup, not self-medication.

EFSA's limit rests on a specific risk: in someone deficient in vitamin B12, high doses of folic acid can correct the anemia while letting neurological damage progress. A B9 supplement should therefore not mask an undiagnosed B12 deficiency: see the article B12: testing, supplementation and risks.

Certain medications interact with folate:

• Methotrexate: this is a folate antagonist. In cancer treatment, a supplement can interfere with therapy; in rheumatoid arthritis or psoriasis, a supplement is sometimes prescribed to reduce digestive side effects. Never decide on your own.
• Anticonvulsants (phenytoin, carbamazepine, valproate): these can lower folate levels, and a supplement can alter their blood concentration.

1. 01Clarify the question: diet, suspected deficiency, pregnancy, general prevention, or an MTHFR genetic test result.
2. 02Take stock of all your supplements and fortified foods.
3. 03Note the form and unit: folate, folic acid, 5-MTHF, µg or mg.
4. 04Compare the total of synthetic forms to the 1,000 µg limit and to any prescription.
5. 05Ask for advice in case of pregnancy, treatment, chronic illness, or a genetic result to interpret.