Except that what he was saying, and the way he was saying it, was profoundly offensive. "Eat my flesh and drink my blood" ... he then changes Greek verbs to something that translates more like "Munch my flesh like a pig and chug my blood" (just to increase the offense).
(Eating human flesh and drinking human blood is only used in Scripture as a metaphor for overwhelming military defeat -- the kind of thing that causes people to resort to cannibalism. It's also a well-known Middle Eastern expression for reviling someone viciously.)
So Scripture records that many of his followers murmured "this is a hard teaching, who can accept it," then left him. He doesn't try to explain his "metaphor". He goes to the Apostles and says plaintively, "Do you also want to leave?"
Why the offense? Why the ugly imagery? And why did he happen to pick this particular theme for his preaching on Passover, exactly a year before the Last Supper?
Jesus clearly explains what He is talking about in verse 29 at the start of the second dialogue. Then neatly wrapped up in verse 63.