Our autistic son is now 22. When he was 8 we were investigating lots of different approaches, one of which was diet. We went to visit one of the top specialists in the country on allergy/intolerance. After a number of tests, he said that he'd discovered that he'd got mercury in his bloodstream and prescribed a course of selenium to chase it out.
This was a few years before the "mercury" theory appeared.

Obviously the damage had already been done.

Remember the experts said the Titanic couldn't sink.