John 6:63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.
He wasn’t talking about the carnal flesh in verse 53 that you quoted. If Catholics would study scripture instead of the RCC they would know that.
And many of his followers left him because that was too hard of a saying.
And many non-Catholics have forsaken the Body and Blood of Jesus Christ in the Sacrament of the Eucharist too.
A loss to those people who left Jesus, and a loss to those who refuse to believe in transubstantiation.