I struggle with this same issue.
In John 6, immediately after Jesus tells the disciples that they must eat His flesh and drink His blood, this was the response:
John 6 (NASB95)
60 Therefore many of His disciples, when they heard *this *said, “This is a difficult statement; who can listen to it?” 61 But Jesus, conscious that His disciples grumbled at this, said to them, “Does thiscause you to stumble? 62 “*What *thenif you seethe Son of Manascending to where He was before? 63 “It is the Spirit who giveslife; the fleshprofitsnothing; the words that I have spoken to you are spirit and are life. 64 “But there are some of you who do not believe.” For Jesus knew from the beginning who they were who did not believe, and who it was that would betray Him.
New American Standard Bible : 1995 update. 1995. The Lockman Foundation: LaHabra, CA
We know that Jesus often made statements which His disciples misinterpreted. An example of this is when He tells them to beware of the leaven of the Pharisees and of Herod, and they think that He is chastising them for not bringing bread. Could it be possible that they have taken His words too literally here also, resulting in Him having to tell them (regarding the issue of drinking His blood and eating His flesh): “It is the Spirit who gives life; the flesh profits nothing; the words that I have spoken to you are spirit and are life”?
Thanks for your help,
bajolyn