The life born in Baptism of water and Spirit is fed not with perishable food, but with another kind of food, eternal heavenly food. Jesus alone gives this food. He alone bears the Father's seal of approval, the testimony that He is the beloved Servant-Son sent to feed us, and only He gives food that endures to eternal life. That Jesus gives this food means that we do not work for it. And that is the second barrier through which Jesus must break.
Bread usually means work.
The question was a natural one. Jesus could have replied with the Ten commandments or the two commandments to love God and love the neighbor. Those are the works of God. But Jesus stands the question on its head. It is not our works but God's work that counts.