Raised as a Catholic, I sat through many hours "explaining" the contradiction. The belief is that the bread and wine still look like bread and wine, no matter what physical method is used to observe or test them. So if you look into the stomach, you see bread and wine. It is digested as bread and wine, and it circulates though the blood as bread and wine, i.e. carbohydrates, sugars and vegetable matter, no animal blood or fat. The argument is that it is not really bread and wine, but is the body and blood of Christ, "because He said so". They take the Scripture (in translation) word for word, unlike in other cases where it does not serve their fancy, such as in believing that Jesus' "brothers" are really cousins, so Mary could stay a virgin, for example, or how the priests are called "father" when Jesus says "call no man your father", since "father" used for the priest, is just a title of respect, and has no further meaning.