The assumption behind the diets is that people with autism often have gastrointestinal abnormalities that allow unusual amounts of digestive by-products into the body (the so-called leaky gut syndrome). The by-products of gluten and casein, according to one hypothesis, disrupt brain function by altering opioid activity, which is involved in pain regulation and social bonding. Another theory posits that the gut leakage triggers a harmful immune response. These hypotheses are far from rock-solid; in fact, scientists have not even confirmed that people with autism have a higher-than-normal incidence of gastrointestinal problems. But the causes of autism are so poorly understood and the disorder is so variable that some investigators are willing to consider the possibility that gluten and casein may somehow exacerbate symptoms in some children, perhaps just by producing intestinal discomfort.