“Fasting in the body, O brethren, let us also fast from sin.” This is the Church’s song in the lenten season of fasting. It is also the teaching of the saints.
…in fasting one must not only obey the rule against gluttony in regard to food, but refrain from every sin so that, while fasting, the tongue may also fast, refraining from slander, lies evil talking, degrading one’s brother, anger and every sin committed by the tongue. One should also fast with the eyes, that is, not look at vain things…not look shamefully or fearlessly at anyone. The hands and feet should also be kept from every evil action.
When one fasts through vanity or thinking that he is achieving something especially virtuous, he fasts foolishly and soon begins to criticize others and to consider himself something great.
A man who fasts wisely … wins purity and comes to humility and proves himself a skillful builder’ (St. Abba Dorotheus, 7th c., Directions on Spiritual teaching).”
(Fr. Thomas Hopko, Spirituality: Vol. 4, p. 135)