Bertrand why don't you try looking up non-celiac gluten sensitivity. Affects approximately 6 times as many people as celiac disease. Discovered relatively recently, in part as a response to why so many people test negative for celiac disease, but suffer symptoms after eating gluten. But 20 years ago those people would have met with the response "you're imagining it, nothing wrong with you, stop muddying the waters". Not the majority of people, granted, but a significant number of people nonetheless, and it could point to a spectrum of dysfunction caused by the ingestion of gluten, with further reaches of which we are as yet unaware.
Perhaps ponder whether maybe, just maybe, science/medical knowledge doesn't yet fully understand everything there is to know in this area and there are new discoveries/breakthroughs out there to be made.
Perhaps also consider also that lack of evidence is not the same as disproof. The fact that something is not yet conclusively tested does not necessarily make it untrue, it just makes it as yet untested. You may even allow your thoughts to stray to wondering about from whence do the conjectures that form the basis of testable scientific hypotheses stem?
Consider whether it could, at least in part, be from anecdotal evidence and as yet inconclusive observations...perhaps even remember that hypotheses/theories are never proven, they can only be refuted, and absence of refutation does not constitute proof. Maybe realise that even in the world of science, the waters are much, much muddier than we generally realise.
And realise that things other than accurate testing of an hypotheses can lead to it's dismissal. The politics of the day can assert it's influence directly or indirectly- look at the interfaces between Darwinism/Lamarckism/Lysenkoism/Epigenetics and tell me science proceeds only on the basis of evidence not preformed opinion...Science isn't some fully formed omniscient, omnipotent body of objective truth y'know, so don't go dismissing other's opinions with high handed scientism.
You picked big boots to fill Bertrand