A family member of mine suffers from severe IBS, including the ever-maligned apparent sensitivity to gluten. Until fairly recently, many doctors considered these complaints to just be hypochrondria. But, if one has severe diarrhea every time one has a trace of garlic or onion, even unknowingly, it's real.

As an aside, IBS sufferers usually aren't sensitive to gluten (unlike with celiac), but fermentable sugars that always occur alongside gluten.