Polly: The majority of the autistic children realize benefits from a gluten-free and casein-free diet. (Gluten is a protein found in many grains, and casein is a protein found in many milk products. They are similar proteins.) The parents are advised to start slowly with limiting just the Autism Interventions 17 casein from milk, and then move on to the gluten. There have been a couple of cases of severe reactions from removing them both all at once. Take 3 or 4 weeks to ease into the diet. Give the milk free diet at least a 3 week trial before starting on the gluten free diet. The casein and gluten free diet should be undertaken for at least three months. Younger children will respond quicker. The earlier you can implement this intervention, the better the chance for a significant improvement. If it helps, it is best not to go back to the old diet. You risk regression. Before removing the gluten, doctors suggest getting a test for celiac, because you can’t test for celiac disease after the child is off gluten foods.
Some parents are lucky, and see immediate changes in their children when milk and/or gluten are removed. However, in some cases, several weeks or months are necessary until clear benefits are seen. Many parents have noticed that even small amounts of gluten or casein are sufficient to remove the majority of the benefits. Unfortunately, these substances are hidden in many processed foods and places you would not expect, like in canned fish or the coating on gum.
To implement the gluten free/casein free (GFCF) diet properly, you should read a book by Lisa Lewis, PhD, Special Diets for Special Kids, or log on to the computer and join a list like GFCFKids at http://www.yahoogroups.com. There are websites too, like http://www.gfcfdiet.com and http://www.autismndi.com. The children aren’t necessarily allergic to gluten and casein. The problem is that usually they aren’t digesting these proteins. If the body is having trouble digesting the gluten or casein, then the undigested peptides can form opiate-like neuro-toxins, which may mimic some of the effects of opiate drugs like heroin and morphine. Also these peptides may inhibit the recycling of biotin in the body, thus creating a deficiency.  (Biotin supplements have been particularly helpful to a few. However, note that biotin should be kept balanced with pantethine, alpha lipoic acid, and inositol.)
People are currently debating if these opiatelike substances are actually from the casein and gluten. However, this is the best explanation so far. The casein may also be particularly harmful if a bacteria like Clostridia acts upon it. Clostridia can convert the casein into vasoactive amines, which can induce migraines.
There is a modification to this diet that you might want to consider. The specific carbohydrate diet (SCD) is designed to get rid of intestinal irritation. It is described in the book Breaking the Vicious Cycle by Elaine Gottschall. (There is also a discussion of this diet in Book 3 of this series.) At http://www.amazon.com there is a review of the book by Jennifer Young who had a highly functioning autistic child. She found that just removing wheat and gluten wasn’t helpful. When she put her son on the SCD diet, he came out of autism without implementing any other interventions.