And they began discussing with one another the fact that they had no bread.
Leaven and bread are two different things. Leaven is put in bread to make it rise. It is an agent of change. Bread is what we eat. The disciples were so focused on their bellies, on their immediate needs, that they were unable to comprehend the spiritual needs -- the eternal needs -- Jesus was addressing when he said, "beware of the leaven of the Pharisees and the leaven of Herod."
How often are we so focused on the temporal, the literal, that we miss the metaphor, the greater teaching? So like Martha running after the work of the kitchen instead of like Mary, setting ourselves to the work of the soul.
We see the immediate need to fix our hunger, and so neglect that our souls are in immediate need as well. It is when we are hungry that the leaven of the Pharisees can take root, and spoil the loaves that are our lives.
Lord, help us to remember you always meet our bodily needs, so that we may focus our minds and hearts on You. Let Your leaven work in us that we may always work in You. Amen.