Gluten exorphine

Gluten exorphins are a group of opioid peptides formed during digestion of the gluten protein. It has been hypothesized that people with autism and schizophrenia have abnormal leakage from the gut of these compounds, which then pass into the brain and disrupt brain function known as the opioid excess theory or a part of leaky gut syndrome. This is partly the basis for the gluten-free, casein-free diet. Two clinical studies of autism patients who followed this diet have found no evidence of benefit. Another found evidence of benefit. Another study suggested the diet may present a greater risk to brain development.

There are four known gluten exorphins with known structure:

...
Wikipedia