From A Hunger for God, John Piper, pp. 21-23:

"Bread magnifies Christ in two ways: by being eaten with gratitude for his goodness, and by being forfeited out of hunger for God himself. ... In the heart of the saint both eating and fasting are worship. Both magnify Christ. Both send the heart--grateful and yearning--to the Giver. Each has its appointed place, and each has its danger. The danger of eating is that we fall in love with the gift; the danger of fasting is that we belittle the gift and glory in our willpower."

"Fasting proves the presence, and fans the flame, of that hunger [for God]. It is an intensifier of spiritual desire. It is a faithful enemy of fatal bondage to innocent things. It is the physical exclamation point at the end of the sentence: 'This much, O God, I long for you and for the manifestation of your glory in the world!'"

"If you don't feel strong desires for the manifestation of the glory of God, it is not because you have drunk deeply and are satisfied. It is because you have nibbled so long at the table of the world. Your soul is stuffed with small things, and there is no room for the great. God did not create you for this. There is an appetite for God. And it can be awakened. I invite you to turn from the dulling effects of food and the dangers of idolatry, and to say with some simple fast: 'This much, O God, I want you."