He said this while teaching in the synagogue in Capernaum. On hearing it, many of his disciples said, “This is a hard teaching. Who can accept it?”
FF Bruce wrote a book entitled "The Hard Sayings of Jesus". It was not that what he said was hard to understand, but they were hard to accept and go along with. These are among the hardest. They are of course metaphorical, but even so they demand a level of committment that most people are inwilling to accept. Shortly we will see 'disciples' beginning to desert Jesus. It is easy to follow a miracle worker of a good speaker, but someone who demands that we change our lives is another matter.
You can't have Jesus and your old ways; he is much more demanding than that.