In the I am statements in John, when Jesus says, “I am the Bread of Life” he is speaking metaphorically as he did in other places where he speaks of himself as “shepherd, door, etc”.
However, the movement in John 6 is from the figurative to the literal.
In other “I AM” discourses the movement is the same.
John 10:7-10 Jesus speaks in figure of himself as a gate, then of providing pasture, then literally of eternal life. Eternal life is not symbolic. It is real. So we see the movement from metaphor to literal.
John 10:11-18 the movement is the same, he speaks figuratively of the good shepherd laying down his life for his sheep, yet in 10:17 of his literal laying down of his life.
No one assumes that laying down his life is metaphorical. Again, he uses metaphor then proceeds to the literal application of the metaphor in Christ.
John 6: 35-40 Jesus is speaking in metaphor
John 6: 43-51 the language is symbolic but not merely symbolic. That he is speaking of a real and literal “bread that I will give” is clear from the context — this is a discourse after the feeding of the 5000 and he connects the bread he will give with the manna sent from heaven which was literally bread
John 6: 52-59 the language is very realistic and literal his words are not “body” but “flesh” and the word “to eat” is “to gnaw” as one does to flesh
The whole discourse put in context reveals that there is to be a literal bread that is to be his real flesh.
If in John 6:61-64 Jesus is clarifying that he is speaking merely symbolically why is it that the result of his clarification many of his disciples leave and return to their former way of life.
Jesus says “the bread that I will give is my flesh”.
If he gives us mere bread in the Eucharist then he lies because “the bread that I will give is my FLESH”
If he gives us his flesh as flesh then he lies because “the BREAD that I will give is my flesh”
If he gives us his flesh as bread then he fullfills what he promised.
Therefore, his flesh is bread. This is the Eucharist.
So again, the I am statements follow this progression.
The metaphor (I am, the gate, bread of life, vine, etc)
Then he explains how the metaphor applies.
Then Jesus proclaims that He is the literal application of the metaphor.
I am the bread of life (metaphor)
If anyone eats of this bread, he will live forever. This bread is my flesh, which I will give for the life of the world." Whoever eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day.
(application of how the metaphor applies)
For my flesh is real food and my blood is real drink. (literal fulfillment of the metaphor)
Hope that helps.