Folic acid

You see “folic acid” on supplement boxes, on bread, on breakfast cereals. It is the synthetic form of vitamin B9, added massively since the 1990s to prevent neural tube defects in infants. A public health victory, and a question that remains open.

Folic acid does not exist in nature. Leafy greens, legumes and liver contain folates, a mix of natural forms that the liver converts into 5-methyltetrahydrofolate (5-MTHF), the active circulating form. Synthetic folic acid must go through several enzymatic steps before it can be used. Once intake exceeds about 200 micrograms per dose, the conversion enzyme saturates and unmetabolised folic acid (UMFA) starts to circulate as is.

The MTHFR bottleneck and UMFA

Conversion of folic acid into 5-MTHF depends on the MTHFR enzyme. Roughly one person in two carries a variant (C677T or A1298C) that reduces its activity by 30 to 70%. In these people, supplementing with high-dose folic acid is like piling up fuel the engine cannot burn. The UMFA that accumulates in plasma is suspected of interfering with cerebral folate receptors, and above all of biologically masking a vitamin B12 deficiency without correcting its neurological consequences. This grey zone is why several research teams favour methylfolate (5-MTHF, already active) or food folates, particularly during pregnancy and in variant carriers.

Choosing the right form at the right moment

In practice: food folates (spinach, lentils, asparagus, liver) remain the primary source, with no known toxicity ceiling. Supplementation becomes useful when planning a pregnancy, with elevated homocysteine, or with persistent fatigue and a mutated MTHFR. In these cases, 5-MTHF (methylfolate) bypasses the enzymatic bottleneck and avoids UMFA build-up. Checking B12 before high-dose folate supplementation is worthwhile, especially after 50, to avoid masking an early neurological deficiency.

Related terms

• MTHFR
• 5-MTHF
• Methylfolate
• Homocysteine
• Vitamin B9