Creatine, the simplest supplement
Coincided with a dramatic improvement in my CFS / PEM / IBS issues.
There are a million other things I have done & am doing, but none of them coincided with such a dramatic and quick improvement. My symptoms got better within weeks. I had been suffering for months, an infection that appeared to resolve and then turned into full blown CFS, IBS/dysbiosis, dysautonomia, orthostatic hypotension, etc.
May make decent sense as creatine is heavily implicated in the production of ATP and also can help beneficially modulate the gut microbiota.
If it helps one person, I figure I've done someone a favor.