There is little evidence to support the effectiveness of nutritional supplements or the GFCF diet for improving ASD symptoms.
A dairy-free, gluten-free diet for autistic kids is called GFCF (Gluten-Free, Casein-Free). Parents do often report improvements in behavior after diet change. Currently, there are several reviews that suggest that these improvements are not scientifically measurable, but one metastudy recommends GFCF based solely on parent reporting, and a brand new study (as of 2018) suggests that the GFCF diet can cause significant, measurable improvement for some forms of autism.
"Current nutritional approaches in managing autism spectrum disorder: A review" Nutritional Neuroscience, 8/2017
It is reported
that in individuals with ASD, while the gluten-free
casein-free and KDs, camel milk, curcumin, probiotics,
and fermentable foods can play a role in alleviating
ASD symptoms, the consumption of sugar,
additives, pesticides, genetically modified organisms,
inorganic processed foods, and difficult-to-digest
starches may aggravate symptoms.
"Nutritional and Dietary Interventions for Autism Spectrum Disorder: A Systematic Review" Pediatrics 5/2017
One RCT (high risk of bias) reported significant parent-rated improvements in communication, resistance to communication, social isolation, repetitive or challenging behavior, and overall impairment in children on a GFCF diet compared with those on a usual diet (P values ≤ .007). Children on the GFCF diet also improved significantly on tests of cognitive skills, motor skills, verbal and social communication, anxiety, and reaction to changes in environment and routine compared with control children (P values < .05). Another high risk of bias RCT with 24-month follow-up of participants reported few differences in behavioral measures between children on a GFCF diet and those with no dietary restrictions; ASD symptoms improved significantly in participants in the GFCF diet group versus the no diet group at 12 months, but were not different on any measure in a subset of participants followed for 24 months.
Conclusion: Despite their widespread reported use, little evidence supports the effectiveness of nutritional supplements or the GFCF diet for improving ASD symptoms. Harms reported in studies were generally considered mild, but the long-term effects of these therapies are not well understood. Although the conduct of studies generally improved from those reported in our 2011 review, evidence remains insufficient for most interventions given the small sample sizes, lack of longer term follow-up, and heterogeneous agents and populations.
"Gluten- and casein-free diet and autism spectrum disorders in children: a systematic review" European Journal of Nutrition 3/2018
With few exceptions, there were no statistically significant differences in autism spectrum disorder core symptoms between groups, as measured by standardized scales. One trial found that compared with the control group, in the GFCF diet group there were significant improvements in the scores for the ‘communication’ subdomain of the Autism Diagnostic Observation Schedule and for the ‘social interaction’ subdomain of the Gilliam Autism Rating Scale. Another trial found significant differences between groups in the post-intervention scores for the ‘autistic traits’, ‘communication’, and ‘social contact’ subdomains of a standardized Danish scheme. The remaining differences, if present, referred to parent-based assessment tools or other developmental/ASD-related features. No adverse events associated with a GFCF diet were reported. Conclusion: Overall, there is little evidence that a GFCF diet is beneficial for the symptoms of ASD in children.
"Comprehensive Nutritional and Dietary Intervention for Autism Spectrum Disorder—A Randomized, Controlled 12-Month Trial" Nutrients 3/2018
Modest improvements in CARS-2 and SAS-Pro suggest some reduction in autism symptoms, consistent with parent reports of improvements on the PDD-BI, ATEC, and SRS. Parent reports also suggest improvements in aberrant behaviors (ABC—Irritability, Lethargy/Social Withdrawal, Stereotypy, and Hyperactivity), sensory processing (SSP), and GI symptoms (6-GSI, PGI-2, ATEC), and Overall (PGI-2). ... Three unusual case reports, in which three very different long-term problems were greatly improved, shows the power of comprehensive nutritional interventions in addressing complex, puzzling medical conditions which may involve one or more nutritional deficiencies.