I was fortunate that over time I managed to return myself to full capacity, through reading a ton of research and running dozens of experiments like the above. But it was so damn hard. The symptoms reduced my ability to use my brain to fix myself. And if you're not a careful eater, it's not at all intuitive which foods contain gluten. This was also almost a decade ago while living in a developing country, so it wasn't even apparent that gluten might be a suspect.
I'm currently based in the US - does anyone know how one might get properly tested for chronic giardiasis, as a person who isn't themselves in microbiology? I almost certainly encountered poorly treated water in that period of my life.
Also - I can't help but suspect that a nontrivial percentage of the developing world is living below their full capacity due to something like this. Neglected tropical diseases are a horrendous category.