"'I am the living bread which came down from heaven; if any one eats of this bread, he will live for ever; and the bread which I shall give for the life of the world is my flesh.'
"The Jews then disputed among themselves, saying, 'How can this man give us his flesh to eat?'"
It's not uncommon, in the Gospels, for Jesus to be misunderstood by those who hear Him. Especially when Jesus starts talking about food, His hearers repeatedly make the mistake of taking Him literally when He is really speaking metaphorically.
Take, for example, this exchange recorded in the Synoptic Gospels between Jesus and His disciples: "Jesus said to them, 'Take heed and beware the leaven of the Pharisees and Sadducees.' And they discussed it among themselves, saying, 'We brought no bread'" (Matthew 16:6-7). Jesus responds, "O men of little faith, why do you discuss among yourselves the fact that you have no bread?" (16:8) and continues, "How is it that you fail to perceive that I did not speak about bread?" (16:11). Matthew narrates the disciples' delayed epiphany: "Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees" (16:12).
A similar episode occurs in the Gospel of John just after Jesus has spoken with the Samaritan woman: "Meanwhile the disciples besought him, saying, 'Rabbi, eat.' But he said to them, 'I have food to eat of which you do not know.' So the disciples said to one another, 'Has any one brought him food?' Jesus said to them, 'My food is to do the will of him who sent me, and to accomplish his work'" (John 4:31-34).
The pattern in both these episodes is the same: 1) Jesus speaks metaphorically about food; 2) His hearers mistakenly interpret His words literally; 3) Jesus explains that He was not speaking literally, but rather metaphorically.
One would expect this pattern to hold true in the above passage from John 6, in which Jesus speaks of eating the living bread which is His flesh, and His hearers clearly understand Him to be speaking literally ("How can this man give us his flesh to eat?"). So how does Jesus explain to them that He was not actually speaking literally? Let's see:
"So Jesus said to them, 'Truly, truly I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; he who eats my flesh and drinks my blood has eternal life, and I will raise him up on the last day. For my flesh is food indeed, and my blood is drink indeed. He who eats my flesh and drinks my blood abides in me, and I in him.'" (John 6:53-56)
Why does Jesus neglect to clear up His disciples' misunderstanding, even when many begin to leave Him because of this teaching? Why does He not explain that He was only speaking metaphorically?
Maybe because He was speaking literally.