Folate matters — within a limit
Folate (vitamin B9) is essential for making DNA and red blood cells, and adequate folic acid before and during early pregnancy lowers the risk of serious birth defects. The NIH Office of Dietary Supplements sets the adult RDA at 400 mcg DFE, rising to 600 mcg DFE in pregnancy [1]. This page is about *intake amounts and the upper limit* — for how the natural and synthetic forms differ, see the folate vs. folic acid comparison.
The upper limit applies only to the synthetic form
The Tolerable Upper Intake Level for adults is 1,000 mcg/day, and it applies specifically to folic acid from supplements and fortified foods — not to folate naturally present in food [1]. NIH explains the ceiling was set 'based on the metabolic interactions between folate and vitamin B12' [1].
The masking problem
Here is the core safety issue. A vitamin B12 deficiency causes two kinds of problems: a blood problem (megaloblastic anemia) and a nerve problem (neurological damage). High folic acid intake can fix the blood part while leaving the nerve part untreated. In NIH's words: 'Large amounts of folate can correct the megaloblastic anemia, but not the neurological damage, that can result from vitamin B12 deficiency' [1].
The danger is that the anemia is often the *signal* that something is wrong. Mask it with folic acid, and a B12 problem can progress silently — sometimes toward lasting nerve damage — before anyone notices. See vitamin B12 deficiency risk.
Who should be especially careful
- People over 50 and those on certain medications absorb B12 less efficiently and are more prone to low B12.
- People following vegan or vegetarian diets get little B12 from food.
- Anyone taking high-dose folic acid supplements on top of fortified foods.
Practical guidance
- Keep folic acid from supplements and fortified foods at or below 1,000 mcg/day unless a clinician advises otherwise.
- If you take folic acid and are in a higher-risk group for low B12, ask about checking B12 status too.
- Folate from whole foods (leafy greens, legumes) is not the source of the upper-limit concern.