Folic Acid vs Folate (5-MTHF): Why Your MTHFR Status Changes the Answer

Folic acid is the synthetic, oxidised form of folate used in food fortification and most multivitamins since the US fortification rule took effect in 1998. L-methylfolate (also called 5-MTHF or 5-methyltetrahydrofolate) is the natural, ready-to-use form your body actually puts to work in methylation reactions. Which one you should swallow depends on what you're using it for — and on your MTHFR gene status.

How the body handles folic acid

Folic acid is not directly usable. Your body has to reduce it in two steps with the enzyme dihydrofolate reductase (DHFR) before it becomes 5-MTHF. Human DHFR is fairly slow at this job. At doses above about 400 µg a day, unmetabolised folic acid (UMFA) starts to show up in blood. UMFA itself isn't usable, and there is ongoing debate about whether long-term high UMFA interferes with the natural folate-receptor system. The cautious position has pushed many clinicians toward the natural 5-MTHF form when supplementing above food-fortification levels (Scaglione & Panzavolta 2014; PMID 24494987; DOI 10.3109/00498254.2013.845705).

The MTHFR variants

MTHFR (methylenetetrahydrofolate reductase) is the enzyme that converts 5,10-methylene-THF into 5-MTHF. The common C677T variant cuts enzyme activity by roughly 35% in heterozygous (CT) carriers and roughly 70% in homozygous (TT) carriers. About 30% of people of European ancestry are heterozygous and 10% homozygous; rates vary by population. Homozygous TT carriers tend to have higher fasting homocysteine on standard folic-acid supplementation, and that gap closes faster with 5-MTHF. So for MTHFR TT carriers, methylated folate has a measurable pharmacological advantage.

Preconception and pregnancy

The neural-tube-defect prevention evidence that drove folate recommendations was built on folic acid trials, and folic acid remains the evidence-grade choice for women with normal MTHFR status who are planning pregnancy or in early pregnancy. For women with known MTHFR variants, prior neural tube defects, or recurrent miscarriage, 5-MTHF at 400–800 µg/day is a defensible substitute and is supported by European reviews comparing the two forms (Obeid 2013; PMID 23482308; DOI 10.1515/jpm-2012-0256). The total folate intake from fortified food + supplement matters more than the form for most healthy women.

Depression and methylation

L-methylfolate is approved by the FDA in the US as a "medical food" (Deplin®) for use alongside antidepressants in major depressive disorder that hasn't responded to an SSRI. The mechanism is donation of methyl groups for neurotransmitter synthesis. In two sequential parallel-comparison trials, L-methylfolate at 15 mg/day (but not 7.5 mg/day) significantly improved depression scores versus placebo when added to ongoing SSRI treatment, with a number needed to treat of about six (Papakostas 2012; PMID 23212058; DOI 10.1176/appi.ajp.2012.11071114). A 2014 follow-up suggested the response is greater in patients with MTHFR variants or markers of inflammation. This is a specific clinical use, not a general recommendation.

Practical recommendation

For most adults with normal MTHFR status who just want to cover dietary gaps: 400 µg/day of folic acid is fine. For MTHFR variant carriers, women planning pregnancy after a prior loss, or anyone using folate as part of treatment for SSRI-resistant depression: 5-MTHF at 400–800 µg/day (or 7.5–15 mg/day for depression, under clinician supervision). Avoid chronic high-dose folic acid (>1,000 µg/day) without a specific reason — the long-term safety of persistent UMFA isn't fully settled.