Fasting is done both as an aid to prayer as noted above, and as a method of Spiritual training. As we learn to control our food and sensual passions, we learn to control those other passions that do defile us before God. St. John Chrysostom (4th c AD) spaks of this in a short sermon on fasting:
Do you fast? Give me proof of it by your works.
If you see a poor man, take pity on him.
If you see a friend being honored, do not envy him.
Do not let only your mouth fast, but also the eye and the ear and the feet and the hands and all the members of our bodies.
Let the hands fast, by being free of avarice.
Let the feet fast, by ceasing to run after sin.
Let the eyes fast, by disciplining them not to glare at that which is sinful.
Let the ear fast, by not listening to evil talk and gossip.
Let the mouth fast from foul words and unjust criticism.
For what good is it if we abstain from birds and fishes, but bite and devour our brothers?
May HE who came to the world to save sinners strengthen us to complete the fast with humility, have mercy on us and save us
The Orthodox Church has been practicing fasting from the day that Christ commanded us to fast until this present day. Details on the practice, what and when can be found on this article concerning
Fasting & Fast-Free Seasons of the Church