Now the disciples had forgotten to take bread, except for one loaf they had with them in the boat. “Watch out!” He cautioned them. “Beware of the leaven of the Pharisees and of Herod.” So they began to discuss with one another the fact that they had no bread. Aware of their conversation, Jesus asked them, “How can you fail to see that I was not speaking about bread? Do you still not understand? Are you that dense? ‘Having eyes, do you not see? And having ears, do you not hear?’ Mark 8:14-18
When they crossed to the other side, the disciples forgot to take bread. “Watch out!” Jesus told them. “Beware of the leaven of the Pharisees and Sadducees.” They discussed this among themselves and concluded, “It is because we did not bring any bread.” Aware of their conversation, Jesus said, “You of little faith, why are you debating among yourselves about having no bread? Do you still not understand? Do you not remember the five loaves for the five thousand, and how many basketfuls you gathered? Or the seven loaves for the four thousand, and how many basketfuls you gathered? How do you not understand that I was not telling you about bread? But beware of the leaven of the Pharisees and Sadducees.”
Then they understood (saw) that He was not telling them to beware of the leaven used in bread, but of the teaching of the Pharisees and Sadducees. Matthew 16:7-12
(BTW this shows that the feeding of the multitude was not about eating magical fish sandwiches)
"Till now I have been using figures of speech" John 16:25
Bread is a metaphor for teaching. Bread from heaven, teaching from God, became the flesh of Jesus , a new metaphor for Divine instruction. Unless you eat that flesh you can have no life in you
"Man does not live on bread alone, but on every word that God utters." (Jesus H. Christ!)
Is there something that you still don't understand, or see?