But I said to you that you have seen me and yet do not believe. The Jews understood that God promised them mana from heaven to sustain them on their journey to the promised land.
Bread is the very staple of life.
We could not live without food for very long. Bread sustains us. Jesus clearly meant something more than mere physical existence. The life Jesus refers to is connected with God, the author of life. Real life is a relationship with the living God, a relationship of trust, love, obedience.