Dysfunctional Metabolism Issues
There are a few variations on the idea that allergies can create or activate autism. One popular form suggests that food allergies are responsible for autism. Traditionally, allergies are thought of as environmental sensitivities. Sensitive people's bodies are unable to handle contact with allergic substances, and a variety of symptoms of irritation may occur. Usually, allergies manifest as irritations of the body, but in the case of autism, it is proposed, that allergens irritate the mind or brain instead.
Some people have difficulty digesting various proteins found in a typical diet, for example, casein proteins (found in dairy products) and gluten proteins (found in wheat products), or, alternatively, refined sugars, additives and preservatives. The allergen model of autism suggests that undigested or partially digested proteins or other compounds from these or other sources set off a chain of events resulting in the production of psychoactive chemicals that affect brain function, possibly by creating opioid-like hallucinogenic substances that act on the brain in a toxic manner. In this model, allergens like various indigestible proteins function as sources of stress that release an underlying diathesis/vulnerability, which is probably the failure of the body to manufacture an enzyme capable of metabolizing the offending proteins in the first place. According to researcher Alan Friedman, the particular enzyme in question is dipeptidyl peptidase IV (DDP-IV), and autistic people either are incapable of manufacturing this enzyme, or manufacture it in inadequate amounts due either to a genetic malfunction, or an autoimmune problem. Despite the attractiveness of food allergy theories, there is no conclusive evidence to date that all autistic people have food allergies. At best, food allergy models may account for only a portion of autistim cases.
A variation on the food allergy theory was introduced by Dr. Andrew Wakefield in a famous 1998 study published in the Lancet, a prestigious British medical journal. Dr. Wakefield described a small number of children with pervasive developmental disorders who presented to his clinic with seriously inflamed bowels which he thought had occurred as a reaction to MMR vaccine exposure (e.g., mercury toxicity/sensitivity). Dr. Wakefield suggested that the intestinal problems he observed were somehow connected to an inflammatory process producing chemical imbalances in his patients' brains, resulting in autism. While other researchers have found limited support for the idea that a subset of patients with autism also have colon inflammation problems, or other gastrointestinal issues (e.g., "leaky gut" problems where undigested proteins leak out of the intestines before the process of digestion is complete), it is not clear at this time that there is any causative link between colon problems and autism itself. Some researchers have suggested that rather than gastrointestinal problems causing a chemical imbalance leading to autism, that the reverse might be true; pre-existing chemical imbalance (specifically a lack of Secretin; a hormone and neurotransmitter that controls digestion) might cause gastrointestinal problems. Still other researchers have suggested that gastrointestinal irritation actually disrupts the brain directly through nervous system hyper-stimulation rather than through chemical means. The jury is still out on this one; more research will have to be conducted before the respective roles of diet, food allergy, gastrointestinal problems and autism are sorted out.
Autoimmune Issues and Myelin Problems
Autoimmune theories suggest that children with autism develop an immune reaction to their brain's own myelin (the fatty insulating layer that covers neurons) early on in their development, perhaps in the wake of a viral infection, or even, it has been suggested, as a result of a reaction to vaccinations (quite independently of their mercury content). Myelin is essential for proper functioning of individual neurons and the brain as a whole. Damage to myelin interferes quite directly with proper brain functioning. Evidence for this autoimmune theory comes from several studies showing that autistic children have measurably more (up to eight times more) immune system antibodies for myelin basic protein and neuron-axon filament protein than typical children and children with mental retardation as well. It is hard to know exactly what to make of this finding, however. Further research will be necessary to illuminate whether autoimmune findings are meaningful with regard to autism.