The folate/folic acid distinction explained why the form of B9 matters for MTHFR/DHFR variants
Something that comes up constantly in this sub is the question of folic acid vs folate, and I wanted to write up a clear explanation because I see a lot of confusion, including in many posts that skip a critical enzyme in the pathway.
Folic acid is the synthetic form of vitamin B9. It is found in fortified grains, breakfast cereals, most breads, and many supplements. It is the dominant form in the food supply.
Methylfolate (also labelled as 5-MTHF or L-methylfolate) is the active form your body actually uses. It is found naturally in dark leafy greens, liver, and lentils.
Here is where most explanations get it wrong. Folic acid does not go directly to MTHFR. There are two enzymatic bottlenecks, not one.
Step 1 DHFR (dihydrofolate reductase): Folic acid must first be converted by DHFR into dihydrofolate (DHF) and then into tetrahydrofolate (THF). This enzyme is rate-limited and saturable; it can only process so much folic acid at a time, regardless of your genetics. This is why unmetabolized folic acid accumulates in the bloodstream when intake is high. That problem has nothing to do with your MTHFR status. It happens to everyone.
Step 2 MTHFR: Once folic acid has been converted to THF and passed through further intermediate steps, MTHFR converts 5,10-methylene-THF into 5-MTHF, the active methylfolate. This is where MTHFR variants matter. The C677T variant (rs1801133) reduces this specific conversion step by 30–70% depending on whether you carry one copy (heterozygous) or two (homozygous).
So there are two separate problems stacking on top of each other: DHFR is slow for everyone, and MTHFR is slower still if you carry a reduced-function variant.
Meanwhile, the folate naturally present in food, the 5-MTHF in your dark leafy greens, bypasses both enzymes entirely. It is already in the active form. It absorbs, enters circulation, and goes straight into the methylation cycle without needing DHFR or MTHFR to touch it.
What this means practically:
- Large amounts of unconverted folic acid can accumulate because DHFR cannot keep up, this is a universal bottleneck, not just an MTHFR issue
- Unmetabolized folic acid can compete with methylfolate for the same folate receptors and actually interfere with the methylation cycle
- If you also carry a reduced-function MTHFR variant, you have a second bottleneck downstream making the problem worse
- People in this situation may not only fail to benefit from folic acid supplementation; they may see symptoms worsen
This does not mean everyone with an MTHFR variant should immediately load up on methylfolate supplements. Overmethylation is a real issue (the COMT gene interaction is relevant here). It means the form of B9 in your diet and supplements is worth paying attention to, and that understanding the full pathway, not just MTHFR, matters.
Happy to answer questions. We also wrote a longer piece on this as part of our methylation report blog:
https://noorns.com/blogs/articles/why-so-many-people-start-with-methylation-and-diet-report