There are tons of foods in this world that people are not really accustomed to, but we eat nonetheless. Among these are grains. Humankind only started reeally eating grains around 10000 years when we went from a nomadic to a farming culture. Gluten are the main problem because it is an allergen, this is also one of the reasons it is not allowed on the candida diet. In short, its pretty normal that some people cant cope with certain kinds of foods that we arent really build for to eat.