Mar 8:19 When I broke the five loaves among five thousand, how many baskets full of fragments did you take up?
It is interesting that Christ choose to describe this miracle as breaking bread because what he is truly trying to "break" is our thinking about what bread is and why it is important.
The Greek word used to describe the fragments of bread being collected literally means "to lift" and is used to describe the raising of a child, as Christ is trying to raise our understanding. For Christ, a loaf of bread is a symbol of what the mind and purposeful work can accomplish, and it is also symbolic of what we are given freely by God. While these different aspects of bread may seem contradictory, for Christ, they are one and the same.
We are given by God the mental abilities to make bread. This ability to understand how to create food for ourselves is only given to human beings. this is important because by being productive, we take what is within us and create, revealing ourselves in our actions and creations. What goes into the bread, our work, skill, and especially our spirit, is what Christ sees as important. The fact that the bread goes into us to sustain our lives is less important.
What comes out of us defines our character. What comes out of us, not what goes into us, defines what we are (see Mar 7:15, discussed here). It is what goes into the bread that is important because it comes out of our effort and heart (emotions).
The role of bread as nutrition is less important. While it seems that bread is important to our survival, that is an illusion. God determines our survival. Christ points out in this verse that God can create bread without our having to work for it. This point is made directly elsewhere when Christ tell us not to worry about what we eat, saying that the the birds of the air do not sow or reap (Matthew 6:26, discussed here), but they still eat. He cover this idea even more directly in the first temptation (Matthew 4:4, discussed here.)
Christ simply asks his apostles to remember that miraculous bread. As nutrition, bread can miraculously appear out of nothing (or fall from the sky like mana) if that is God's will. Christ's concern is about what we control. He worries about what goes into the bread and comes out of us, the "leaven" of the religious leaders. It was their contaminating spirit that Christ wants his followers to understand and see. The apostles think that Christ is worried about leaven and bread as nutrition, but Christ is concerned not about what goes into a person, but what comes out of them.