As I've shared recently in my answer to a different question with similar content, in the Protestant tradition, not the Roman Catholic, Jesus debunked the word "literally" vis a vis His words regarding the eating of His flesh and the drinking of His blood (see John 6:26-58) in John 6:63, where we read,
"'It is the Spirit who gives life; the flesh profits nothing; the words that I have spoken to you are spirit and life.'"
Instead of saying
". . . the words that I have spoken to you are literal and life,"
". . . the words that I have spoken to you are spirit and life" (my emphasis).
Parenthetically, Christians should be cautious when "spiritualizing" what is meant to be literal, and "literalizing" what is meant to be spiritual. (Hey, I like that sentence; it has a certain ring to it!) That is, in essence, was what Jesus was saying to His grumbling disciples who took offense at Jesus' "difficult statement" (v.60).
"Guys, don't be so literal. I'm speaking spiritually. I can understand why my opponents are offended by my words, but I expected more from you!"
In other words, Jesus' hyperbole, if you will, in speaking of His flesh and blood was designed, purposely, to offend His critics, to get them to see how spiritually blind and deaf they were. Jesus used this "tactic," if you will, quite frequently. See, for example, Matthew 13:13 NIV,
"'This is why I speak to them in parables: 'Though seeing, they do not see; though hearing, they do not hear or understand'".
Also, Matthew 13:14 NIV,
"'In them is fulfilled the prophecy of Isaiah: 'You will be ever hearing but never understanding; you will be ever seeing but never perceiving.'"
Also Mark 4:12 NIV,
"so that, 'they may be ever seeing but never perceiving, and ever hearing but never understanding; otherwise they might turn and be forgiven!'"
Also Luke 8:10 NIV,
"He said, 'The knowledge of the secrets of the kingdom of God has been given to you, but to others I speak in parables, so that,' 'though seeing, they may not see; though hearing, they may not understand.'"
While I respect many of Roman Catholicism's traditions and teachings, their teaching on transubstantiation, in my perspective and in the perspective of most if not all Protestant denominations, is that Jesus' body and blood, which are represented symbolically in the fruit of the vine and in bread, are just that: symbols of His earthly body and blood which He offered up at Calvary for the forgiveness of sins.
Furthermore, we can no more partake of the life of our Savior and His life-giving words by "taking communion" than we can by simply mouthing the words, "I believe Jesus is the Savior of the world." In order for us truly to benefit from Jesus' life-giving words, we must receive them into our hearts in childlike faith, and not simply give mental assent to them just because they have been taught to us, or because other people are pressuring us to say them.
In conclusion, Jesus' "difficult saying" caused some of His disciples to separate themselves from Jesus. Perhaps they were "easy believers" who were more impressed with Jesus' miraculous provision of bread (see John 6:1-14, and 6:26 and 27) than they were with the heart of His message, which was to believe in Him, even when their belief was challenged.