Super brief version:

• The gut cannot properly digest most complex carbs and some sugars such as lactose
• Removing dairy (casein free) and bread (gluten free) removes many of the indigestible complex carbs and lactose
• The individual on the GFCF feels better. (psychological symptoms lessen/disappear, gut begins healing itself)
• To augment the GFCF diet, the individual begins to eat more rice, soy, potatoes--all of which contain complex carbs which aren't properly digested
• Progress on the GFCF stops and/or symptoms return. Trying a "stricter" GFCF diet fails . . the problem is the body's damaged intestine (leaky gut, etc.) and the inability to digest certain carbohydrates.

This pattern/story is often heard when parents switch from the GFCF to the SCD. A strong example comes from one practitioner, an early GFCF adapter who now uses SCD. When first starting the GFCF, the practitioner closely tracked a small group of children (less than a dozen). All of the children showed initial improvement, however, after many months, only two showed sustained improvement/lack of regression. In those cases, the parents had not added 'bread substitutes' such as rice/soy/potatoes/etc.

For more information related to the SCD and ASD, see the video entitled "VIDEO: Gastrointestinal Pathology and the Use of SCD in ASD - Pamela J. Ferro, RN". It appears on this page:
www.gottschallcenter.com/links.php

Also, check the www.pecanbread.com website

(cheers)