The WHO, back when the had at least the pretense of being a legitimate health organization, published a report on gluten intolerance and in it they laid the blame on vegan diets high in fruit. That too much fructose caused a metabolic bottle neck, leading to more of the bread traveling further down the digestive tract before being broken down.

I'm not a doctor, but the report seemed rather compelling, especially when you consider who typically is gluten intolerant.