Mori: Dunno about the celebrity bit, but autism diets are a total thing, and it seems the opiate bit is specifically associated with the gluten-free casein-free diet: https://ndnr.com/autoimmuneallergy-medicine/diet-therapy-in-the-treatment-of-autism/ (It notes that autistic kids apparently tend to really dislike being denied gluten and dairy, as though this proves they're "addicts," but eesh, that's a major diet change, I'd be pissed too! Basically all our dietary sources of carbs are gluten, and the only remaining sources of regular protein we'd get would be soybeans and eggs!)

The diet looks to be nonsense too, at least for autism alone: https://www.urmc.rochester.edu/news/story/gfcf-diets-found-to-be-ineffective-for-children-with-autism