The leader of feats and our Saviour, the Lord Jesus Christ, before setting out on the feat of redeeming the human race, fortified Himself with a lengthy fast. And all ascetics, proceeding to work for the Lord, armed themselves by fasting and did not set out on the path of the Cross without the feat of fasting. They measured the very success of their ascetism by their success in fasting.
Despite their fasting, and to the surprise of others, the holy fathers did not know weakness but always remained hearty, strong and ready for the task at hand. Illnesses were rare among them and their lives were extraordinarily prolonged.
During the time that the body of one fasting becomes thin and light, the spiritual life attains to perfection and reveals itself through miraculous manifestations. The spirit then performs its actions as if in a bodiless body. External feelings are as shut out, and the mind, renouncing the worldly, ascends to the heavenly and becomes completely immersed in the contemplation of the spiritual world. Yet not everyone can take upon himself strict rules of abstinence from everything, nor deprive himself completely of all that serves to relieve infirmities: "He that is able to receive it, let him receive it" (Mt. 19:12).
One should take enough food everyday to strengthen the body, so that it can be a friend and helper to the soul in accomplishing virtues: otherwise it can happen that through the exhaustion of the body the spirit can weaken. On Wednesdays and Fridays, particularly during the four Lenten periods, follow the example of the Fathers and take food once a day — and the Angel of the Lord will affix himself to you
Saint Seraphim of Sarov
http://agapienxristou.blogspot.ca/2014/01/why-do-we-fast-saint-seraphim-of-sarov.html