I've always instinctively avoided gluten and caseine containing products, because they don't make me feel very good. Simple as that. Gluten is one of the most difficult to digest substances there is, and it's rife in the food chain. You don't have to be coeliac to have issues with it.
If it works for these guys then why should anyone have an issue with it? It's like you taking up issue with me having an egg allergy, because my saying that egg makes me ill is saying all people on the spectrum should avoid eating egg... we're all different, and if it works for a number of autistics then that's brilliant and there shouldn't be any reason why others can't try and see if they have similar success. Why does it automatically have to be a placebo effect, just because it may or may not work for you?
Don't forget, autism is a spectrum and a syndrome - there is no definite cause or reason, it's simply a cluster of symptoms displayed in a large group of people.
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We are a fever, we are a fever, we ain't born typical...