On Fasting...

Fasting is the earliest commandment known to mankind, for God commanded our ancestor Adam to refrain from eating a certain fruit from a certain tree (Gen 2:16,17) but allowed him to eat from the rest.

In this way, God set for the body certain limits beyond which it should not go. Thus, man did not have absolute freedom to take whatever he laid eyes on and whatever he desired. He had to abstain from certain things and control his inclination towards them. Thus since the very beginning, man has had to control his body. A tree may be “good for food and ... pleasant to the eyes” (Gen3:6) and yet one must turn away from it.

By abstaining from food, man rises above the level of the body and above matter, and this is the wisdom behind fasting. Had the first man succeeded in triumphing over his bodily desire for food, and controlled his bodily senses that saw the tree as an appetising sight, it would have proven that his soul had overcome his bodily desires and he would have been worthy to eat from the Tree of Life.

Nevertheless, his own body, which dominated over him, defeated him. Man went on committing several other bodily sins, one after the other, until he was condemned to walk after the flesh and not the spirit. (Rom 8:1).

Then the Lord Jesus Christ came to restore man to his initial status. Since man had erred into the sin of eating the forbidden fruit by obeying his body, Christ's first triumph over temptation addressed this particular point, to overcome the desire for food in general and over that which was legitimate.

For more on fasting follow the link:
http://www.copticchurch.org/Texts/Spirituals/fasting.pdf