I can understand the sugar free, too much sugar can also cause diarhea even w/o the "leaky gut" . there is something called an Autism panel that allergists can do its a blood test that soley tests for gluten casein allergies, its cheaper and if you don't want to go thru the whole gamut of food testing this one can tell you.
He recommended not only Gluten Free Casein Free but also Sugar (complex carb) Free diet, even without testing for food allergies in our son.
My husband did the food allergies test, and he wasn't allergic to a single one. But because of his digestive problems, the allergist recommended avoiding any processed and pre-packaged foods, because he may be allergic to the additives in them.
He did test for the common environmental allergies, mold, pollen, dust, cat and dog, etc. all of which both my husband and son turn out to be allergic to.