"Christ...is the new Adam. At the beginning of his ministry in the gospel of Matthew, we read, 'When He had fasted 40 days and 40 nights, He became hungry.' Hunger is that state in which we realize our dependence on something else - when we face the ultimate question: on what does my life depend? Satan tempted both Adam and Christ, saying, 'Eat, for your hunger is proof that you depend entirely on food, that your life is in food.' Adam believed and ate. Christ said, 'Man does NOT live by bread alone.' This liberates us from total dependence on food, on matter, on the world. Thus, for the Christian, fasting is the only means by which man recovers his true spiritual nature. In order for fasting to be effective, then, the spirit must be a part of it. Christian fasting is not concerned with losing weight. It is a matter of prayer and of the spirit. And because of that, because it is truly a place of the spirit, true fasting may well lead to temptation, and weakness and doubt and irritation. In other words, it will be a real fight between good and evil, and very likely we shall fail many times in these battles. But the very discovery of the Christian life as 'fight' and 'effort' is an essential aspect of fasting."
- from the USCCB website