A disease called celiac disease is under investigation to show a link between autism and behavior.  Celiac disease is caused from mal-absorption of nutrients.  The hair-like projections called villi, in the intestinal tract are flattened from a reaction to gluten.  One who carries the disease is unable to absorb essential nutrients from their food.  Celiac disease affects the carrier both physically and psychologically.  It can cause poor mental functioning as well.  Celiac disease is often overlooked because many doctors are unfamiliar with it.  The results of not treating this disease over an extended period of time can be iron deficiencies that cause other problems.  Since only recently, have researches determined that many children with gluten sensitivity serious enough to damage the gut will show no such symptoms in childhood; it is likely that there are a number of undiagnosed celiac children.  Treatment of celiac disease is a gluten-free diet.  Processed foods such as fast foods and baked goods contain the grain protein gluten and the dairy protein casein.  The proteins from gluten and casein break down into a peptide. Foods that can be eaten on a gluten-free diet include rice,  potato, buckwheat flour, soy, corn, fruit, vegetables, beans tapioca, meat, poultry, fish, shellfish, fish, nuts and eggs.  Meat and poultry are good choices for the diet if the child eats them and does not have adverse reactions. 
A disorder very closely related to celiac disease and requires the same dietary intervention is a skin disease know as dermatitis herpetiformis.  Dermatitis herpetiformis is a skin manifestation of gluten sensitivity.  Dermatitis appears as a bumpy rash usually on the arms, legs or buttocks.  It can be extremely itchy and may also burn.  While there are medications that can help, parents may consider the elimination of gluten in the child's diet to prevent re-occurrence.