But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God.
Matthew 4:4 (KJV)
We do not live by bread alone because there is more to us than our physical bodies. Just as our body requires physical food, so too our recreated spirit requires spiritual food. We are being fed spiritually anytime we receive the Word of God, when we meditate on it and take it into our hearts. The more that our spirit is fed, the stronger our spirit becomes. And when our spirit is strong, our ability to be victorious over the flesh increases.