In chapter 6 of the Gospel of John , Jesus is preaching a parable to a crowd that followed him after the miracle of the loaves and fishes, and says "Verily, verily, I say unto you, Ye seek me, not because ye saw the miracles, but because ye did eat of the loaves, and were filled. Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you....
He goes on to say "I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst."
At which point the crowd murmured against him, "because he said, I am the bread which came down from heaven. And they said, Is not this Jesus, the son of Joseph, whose father and mother we know? how is it then that he saith, I came down from heaven?"
To which Jesus tries to be more clear: "I am that bread of life. Your fathers did eat manna in the wilderness, and are dead. This is the bread which cometh down from heaven, that a man may eat thereof, and not die. I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world."
At this point the crowd is still not taking Him seriously, saying "How can this man give us his flesh to eat?"
At which point Jesus speaks in total bluntness:
" Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you. Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day. For my flesh is meat indeed, and my blood is drink indeed. He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him. As the living Father hath sent me, and I live by the Father: so he that eateth me, even he shall live by me. This is that bread which came down from heaven: not as your fathers did eat manna, and are dead: he that eateth of this bread shall live for ever."
At which point the crowd was understandably gobsmacked, and even his disciples were saying "This is an hard saying; who can hear it?"
"When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you?...{and f}rom that time many of his disciples went back, and walked no more with him."
Which, of course, was Jesus's cue to say "Hey, come back! I was just being metaphorical"
But what was His response to this exodus? He turned to the twelve, His chosen apostles, and said, "Will ye also go away?" In other words, this was something that was non-negotiable.
And this is why Catholics believe that the Eucharist we receive is truly, essentially, in every way, the flesh and blood of Jesus Christ.