God's Word is just as essential to spiritual life as food is to physical life. Just as one must discipline himself to provide and eat physical food, so must one exercise discipline to seek and ingest spiritual food. If one will not do this, then, just as physical health will decline without adequate food, a person's inadequate spiritual diet will lead to spiritual disease. At the very least, one's quality of life will be severely compromised.
In Deuteronomy 8, God merely states that life has psychological aspects and does not indicate whether bread or God's Word is more important. It only states that God's Word is needed for life. By drawing attention to everlasting life in John 6:26-27, Jesus clarifies that what goes into the mind for processing and assimilation is far more important than what goes into the stomach.
The quality of what enters the mind will be the major factor that determines the quality of life. Jesus first emphasizes, using the imperative tense, that we should strive for the food that endures, that is, satisfies forever. He wants us to recognize its potential. Merely reading God's Word and putting it into the mind is just the beginning of its usefulness. We must combine further actions to our reading because the Word's ultimate effect does not magically happen. Principally, we must also believe it and put it into action.