Gluten sensitivity comes to mind: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3573730/

The study finds that gluten consumption hasn't increased over the past few decades and that a culprit is more likely "wheat genetics"

Anecdotally, I don't know if you know many genuinely GF people, but the ones I know have all said that they can eat bread while in European countries, but have bad reactions back home.