Yes: there are opiates that derive from various food proteins that exert peculiar effects on the human brain. The worst? The opiates that come from the gliadin protein of wheat and related grains.
Opiate receptor researchers at the National Institutes of Health originally coined the term “gluteomorphin” nearly 40 years ago when it was determined that the gliadin protein of wheat undergoes partial digestion (since humans lack the digestive enzymes to fully digest proline-rich amino acid sequences in proteins from seeds of grasses) to yield peptides that are 4- to 5-amino acids long. Some of these peptides were found to bind to the opiate receptors of the brain, thereby exerting opiate-like, or opioid, effects, thus the term gluteomorphin (also sometimes called gliadorphin).
This research was performed in response to several observations made in people with paranoid schizophrenia who, upon removal of all gluten sources (that contain gliadin) experienced a reduction of paranoid thinking and auditory hallucinations. Dr. F. Curtis Dohan, while participating in field research in New Guinea, the Solomon Islands, and Micronesia, also made the observation that non-grain consuming natives of these islands developed an explosive level of schizophrenia when allowed to consume Western foods containing grains. Several subsequent studies were performed linking gliadin-derived opioids with schizophrenic behavior but, as often happens in nutritional research, interest waned, as most psychiatric research focuses on drug treatment. More recently the storm of controversy triggered by the Wheat Belly books have rekindled psychiatric research into the link between schizophrenia and gliadin-derived opioid peptides and there does indeed appear to be an association, at least in a subset of people with schizophrenia.
The current opioid crisis, in which people are prescribed potent opioids like Oxycontin and Dilaudid, become addicted and turn to illicit opioids such as heroin, highlights just how powerful the effects of opioids can be on the human brain. The effects of Oxycontin, of course, differ from the opioid effects of bagels and sandwiches, but there is indeed some concerning overlap. No doubt, susceptibility to the opioid effects of gliadin-derived peptides varies from person to person, but it can range from moderate stimulation of appetite to uncontrolled 24-hour-a-day food obsessions as occurs in binge eating disorder and bulimia. Or it can involve behavioral outbursts and deterioration of attention span in children with autistic spectrum disorder or attention deficit hyperactivity disorder (ADHD). Or it can be depression and suicide (at least as suggested by several observational studies such as this one). And, as with stopping prescription and illicit opioids, halting all consumption of gliadin sources yields an opioid withdrawal syndrome, effects readily mimicked by administering an opiate-blocking drug such as naloxone or naltrexone.
Interestingly, recent evidence points towards some species of Bifidobacteria in the gastrointestinal tract as being able to degrade gliadin-derived opioid peptides. This suggests that at least some of the modern phenomenon of wheat intolerance and the mind effects of gliadin-derived opioids may be due to dysbiosis/small intestinal bacterial overgrowth so prevalent among people nowadays. But, as always, don’t misinterpret this to mean that, if healthy bowel flora is restored, wheat must now become something good for you—there’s still too many other unhealthy components to contend with.
Bottom line: That innocent muffin or bag of pretzels yield opioids that have peculiar effects on your brain, emotions, and behavior with results that depend on your individual susceptibility. The result might be depression, it might be appetite stimulation. Unlike having to take an antidepressant, sedative, or antipsychotic, however, just not eating wheat and related grains can represent cure of this particular opioid crisis.