Is natural folate from food or supplements safer than synthetic folic acid for kids?

The short version: the evidence still does not answer whether natural folate is safer than synthetic folic acid for children, because no study in this retrieval directly compared them in children.

The retrieved studies compare folic acid to 5-methyltetrahydrofolate (5-MTHF, the naturally predominant form) only in adults and pregnant women. One 2024 review proposes 5-MTHF as a future alternative due to "greater bioavailability and fewer genetic and safety limitations" [1]. A 2020 review notes that unmetabolized folic acid can accumulate at doses above 1 mg/day, while 5-MTHF avoids this issue and does not conceal vitamin B12 deficiency [3]. But both reviews are about pregnancy, not children.

The mechanism difference matters. Synthetic folic acid must be converted by the liver enzyme DHFR into active folate, and this conversion is slow and easily saturated [2, 3]. 5-MTHF is already the active form that makes up about 98% of plasma folate [3]. For a child with an immature or genetically slower DHFR enzyme, unmetabolized folic acid could accumulate at lower doses than in adults. Natural food folate is mostly already in reduced forms, so it bypasses this bottleneck.

What the retrieval does not contain is the critical comparison: a pediatric trial giving children either synthetic folic acid or natural folate from food or 5-MTHF supplements and measuring blood levels, unmetabolized folic acid, and clinical outcomes. That study was not found. The Ontario preschooler study [4] shows that folic acid fortification already pushes some supplement-using children above the tolerable upper limit (7% of users exceeded it), but it does not compare forms.

My call: natural folate from food or 5-MTHF supplements is likely safer than synthetic folic acid for children, based on the mechanism of bypassing a slow liver conversion step and avoiding unmetabolized folic acid accumulation. But this is reasoning from adult data and mechanism, not from pediatric comparative trials. Confidence: low — the retrieval captured no direct pediatric comparison.