Parent Ratings of Behavioral Effects of Biomedical Interventions
Data on adverse effects and benefits of various diets for autism has been collected by the Autism Research Institute.  More than 25,500 parents filled out reports. The information was gathered from questionnaires designed to collect such information.

The Specific Carbohydrate Diet had 195 subjects and scored a percentage point  higher than the Gluten Free Casein Free Diet which had 2208 subjects reporting. The entire study can be viewed on the Autism Research website.

The Specific Carbohydrate Diet had 1% higher benefit on a much
smaller pool, but also a significantly higher negative effect on behavior, 7% as compared to 3%, which skewed the better: worse ratio
so that it was far lower than that for the gluten-free casein-free diet:  10:1 as opposed to 20:1. 

So it is mixed results. 

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