Does it work?
The effectiveness of elimination diets in improving the behavior of children with autism has only recently been scientifically researched (8). This research has almost always examined diets that are both casein- and gluten-free.
One well-controlled study focused on children with autism who had abnormally high protein by-products in their urine, and therefore were more likely to be sensitive to casein and gluten (see What is the theory behind it?). One group of these children was fed a strict casein- and gluten-free diet for 12 months. This group had significantly fewer autistic symptoms than the remaining children, who were not fed this diet (9).
Another well-controlled study of casein- and gluten-free diets focused on children with autism regardless of the level of protein by-products in their urine (10). Overall, the study found no significant differences in behavior between children on the elimination diet and children on regular diets, although individual parents reported behavioral improvements (10). A third well-controlled study reported no significant improvements in speech for 13 children who followed a gluten-free casein-free diet for 6 weeks (11). There were limitations in these studies, including relatively short time periods on the diet and/or small samples sizes.
Regardless, in both cases, the tested diets were casein- and gluten-free, so it is not clear whether it was the elimination of casein, gluten, or both that resulted in any improvements. A third study that did examine the effect of a gluten-free diet on the behavior of children with autism did not compare children on the elimination diet with children not on the diet (12). Therefore, even though there were improvements seen in the behavior of children on the diet, these may also have occurred over the 5 months of the study without the elimination diet
The current thinking is that there is at least some evidence showing that a gluten-free diet, when combined with a casein-free diet, can help improve the behavior of some children with autism. Although the casein-free diet combined with a gluten-free diet is popular, there is little evidence to support or refute this intervention and reviewers have determined that meaningful conclusions cannot be drawn from the existing literature (3, 4).