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.
The Bible states that every deprivation and every constraint is precious before God: "...the Kingdom of Heaven suffers violence, and the violent take it by force" (Matt. 11:12) Those who audaciously and wilfully violate the established fasting rules, are called enemies of the Cross. To them, their God is their stomach and their glory lies in their shame. It is stated in the Psalms: "lost through the stomach." Understandably, its another matter if a person violates the fast due to an illness and weakness of the body. But the healthy become healthier and on top of that, they sometimes live longer even though they may look gaunt. With fasting and self-restraint, the body does not rebel as much, we don’t succumb to sleep as much, less empty thoughts invade our mind, and spiritual books are read more eagerly and with greater understanding.
And so, if by the grace of God you developed a desire to cleanse yourself of your internal iniquities – let it be known to you that their origins cannot be extracted except through sincere prayer and fasting, and then sagaciously. Otherwise, you may have a foolhardy example that occurred here. One landowner, living an indulgent life suddenly wanted to observe a severe fast: he resolved that all he would eat during Great Lent is ground hemp-seed washed down with kvass (a Russian soft drink). From this severe change of indulgence to fasting, his stomach was so damaged that it could not be remedied after one whole year. Incidentally, there are words of Holy Fathers that tell us we should not be body killers but vice killers.
Elder Ambrose of Optina