Vitamin B9, also known as folate or folic acid (in its synthetic form), has received scientific attention for its potential role in supporting individuals with autism spectrum disorder (ASD). The interest is primarily based on findings that some children with ASD have abnormalities in folate metabolism or transport, particularly those with cerebral folate deficiency (CFD). Research has shown that a subset of children with autism have autoantibodies against the folate receptor alpha (FRAA), which can reduce folate transport into the brain. Supplementation with high-dose folinic acid (a form of folate) has been explored in this subgroup.

Clinical studies, including small randomized controlled trials and open-label studies, suggest that folinic acid supplementation may lead to improvements in language, communication, and core autism symptoms in children with FRAA or CFD. However, the evidence is not robust for all individuals with autism; benefits appear restricted to those with specific metabolic abnormalities. The overall quality and size of studies remain moderate, with calls for larger, well-designed trials.

There is no substantial historical or traditional use of vitamin B9 specifically for autism, as the condition and its biochemical underpinnings have only recently been explored. Most scientific guidelines do not recommend routine folate supplementation for all individuals with autism, but specialist assessment for folate-related issues may be warranted in select cases.