In the Scripture, we see the necessity to observe fasts, firstly from the example set by Christ Himself, who fasted 40 days in the wilderness, even though He was God and had no need of this. Secondly, to the Disciples’ question as to why they could not drive out the evil spirit in a person, He replied: "Through your unbelief;" and then added: "This kind can come out by nothing but prayer and fasting." (Mark 9:29) Apart from this, there are directions in the Scripture that we must observe Wednesdays and Fridays as days of fasting. On Wednesday, Christ was given up for crucifixion and on Friday, He was crucified.
Plain food is not nasty food. It does not corrupt the body but fattens it. And St.Apostle Paul says: "Even though our outward man is perishing, yet the inward man is being renewed day by day" (2 Cor 4:16) He calls the outward person as body and the inward, as the soul.
Elder Ambrose of Optina