Provided herein are methods of treating neuropsychiatric disorders or improving the symptoms associated therewith administering a medical food, such as L-methylfolate, to a subject having the disorder. The neuropsychiatric disorder may be an autism spectrum disorder or attention deficit disorder with or without hyperactivity (ADD/ADHD). The L-methylfolate may be administered as an adjunct to other therapeutic agents effective to treat the disorder. The subject preferably may be a child and also may have a single nucleotide polymorphism in the MTHFR gene associated with reduced expression of MTHFR enzyme. Also provided is a method of increasing the de novo synthesis of neurotransmitters in vivo by transporting a methyl-donating compound across the blood-brain barrier in a subject.
