What is the theory behind it?
Casein is broken down in the intestines into several by-products, including one called casomorphine. These by-products are much more common in the urine of children with autism than in children without autism. Some scientists have concluded that they are leaking from the intestines into the blood of these children (2, 5). Many research studies report that children with autism often have gastrointestinal problems, including intestinal leakage (5). The argument is that, if casomorphine is being absorbed into the general circulation in children with autism, then it could affect behavior (2-5).
In support of this theory, injection of casomorphine in animals activates areas of the brain that have been reported to be involved in autism (6). Moreover, there is evidence that blocking at least some of the action of casomorphine improves the behavior of children with autism (7). Finally, recent evidence of a genetic mutation common among children with autism has been traced to a gene involved in gastrointestinal function (8).