Genome sequencing of six children with autism has revealed mutations in a gene that stops several essential amino acids being depleted. Mice lacking this gene developed neurological problems related to autism that were reversed by dietary changes (http://www.sciencemag.org/content/early/2012/09/05/science.1224631">abstract).
According to Joseph Gleeson, a child neurologist at the University of California, San Diego, who led the study, "This might represent the first treatable form of autism."
It is possible that some other forms of autism may also be linked to uncommon metabolic disorders — and so treatable through dietary changes, according to the scientists quoted in the piece."
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