From the Greek Orthodox site:http://www.goarch.org/en/ourfaith/articles/article7057.asp
From The Russian Orthodox Cathedral of St. John the Baptist, Washington, D.C. site:
The meaning of fasting consists, In the first place, in abstinence (in theology it is called self-denial,) in forcing oneself to do not only what is pleasing to "my little soul," as the Russians would say. The meaning of fasting lies in controlling one's egoistic desires, In submission to something other than the continuous dictates of the self-loving ego. Why Is this necessary? Why is it good for us periodically to tame and humble ourselves? Because our nature Is a part of the world which is wallowing in sin In the words of John the Evangelist; man carries In himself the fruits of the original sin, and his voluntary renunciation of the ties to this natural world elevates him -above that world and frees him from It thus beginning the process of freeing him from the shackles which keep him in bondage to sin.
It would be wrong to think that fasting is in some way similar to suicide. Atheists and materialists try to force this Idea upon us arguing that religion Is a renunciation of life, a disregard for life In the name of a non-scientific and abstract Idea of the Spirit-GOD. Any religion, they aver, is the opium of the masses, and fasting blunts man's physical activity. This assertion is entirely unsubstantiated. The practice of fasting was Instituted In order to revitalize, not destroy, our nature. Fasting Is the hygiene of the soul, the means of Its cleansing and healing. In the physical sense alone fasting Is good for one, and one can speak about the necessity to fast from the medical point of view.
Fasting gives us back our lost equilibrium and brings into harmony our mind, our soul and our body. Abstaining from excessive food results in the lightening of the capabilities of the mind and the soul. Food is conducive to sleep rather than activity, be It physical -or mental. incidentally, St. Seraphim of Sarov used to say that It is not meet to speak of things Divine having "stuffed one's gut."
Man must eat In orderto survive. Eating is certainly not sinful from the spiritual point of view. Heaviness and obesity are harmful for spiritual life: the Bible tells us that God warned Israel on Its way to the Land of Canaan: "Israel, Israel! Thou goest to the land that floweth with milk and honey. Beware, Israel, lest thou become heavy and forget thy God!"
It pains us to see hunger In this world. Yet It not less painful to see what the poet Marina Tsvetaeva called the "satiety of the sated."
Fasting leads to the total equilibrium and harmony of our mental, spiritual and physical selves. if our physical self becomes dominant, it is good to reduce physical activity so as to liberate the spirit and establish the equilibrium of spiritual purity. Even the stoics taught that spiritual purity leads to the harmony of man's entire being.
Why does the Church prescribe the choice of foods for fasting periods?
Our physiological activity Is a function of the food we eat. The more we eat foods which are located higher up on the ladder of organic and animal world, the more active we become In the physical sense. Vladimir Soloviov.gives the following gradation of physical activity. Physical activity Is the least pronounced in the plant world. For this reason, the most "fasting" food consists of vegetables, fruits and cereals. Fish are more highly developed than plants, and birds are more highly developed than fish. That Is why the physical energy derived from birds Is proportionately stronger than that derived from either fish or plants. The most highly developed animals are mammals which Is why they are Justly considered the most "un-lenten" food.
Apart from its physiological aspect, fasting Is Important for spiritual development and the training of will.
The Lord gave man the freedom of will so that he may do good deeds, seek the Truth and create beautiful things. But the freedom of will does not mean the absence of will, lack of volition, weakness of will or apathy. If man is given his freedom of will, than he must know what he wants, towards what end his will should be directed. The freedom of will must never lead man to immorality, lack of concern, indecency, licentiousness. Having decided what is good and what evil, man Is under an obligation to use his will to do good which would lead him to God. Man Is called to the doing of good deeds, to spiritual activity, the building of the City of God. For this reason, fasting, as every form of self-denial, Is a marvelous instrument in the education of man's will and the taming of his licentiousness.
The discipline of the Church fast, as every discipline makes man more compatible with others, less solipsistic, less self-willed. Man must make his will conform with the will of the Church. It Is good for each of us to know what the will of the entire Church is. it is good for us to fast at times and in the manner prescribed by the Church. It is good for us, for our salvation, to be subject to the discipline of the Church, to be together with all others, to have the personal ego conform with the general desire of the Church. It Is Imperative for a man to be active not only in his own cause but also in the 'Common Cause" and to submit to the discipline of the Church. Incidentally, the word 'liturgy" means "common cause." According to the Church's definition, Christians are "Christ's soldiers". Together, they make up "Christ's Army." The earthly church is the "Church militant", the church fighting for the salvation of Its children. Discipline in the army of the Church, In the army of Christ, is just as necessary as It Is In the ordinary army. This is In part the essence of Christian catholicity.
Fasting, of course, is not merely an exercise In abstinence. It also consists In performing good deeds. The Church insists on this aspect of fasting in Lenten hymns. it is evident, for instance, in the following liturgical text: "While fasting, brethren, In our bodies let us also fast in our spirits, let us destroy every alliance of deceit, let us give bread to the hungry and let us bring the poor and the homeless into our homes."
On the eve of the Lenten Fast, during the liturgy on the Sunday of Repentance the gospel reading speaks of conditions on which is predicated our liberation from the slavery to sin , to 'this world." The first condition for our liberation from the dictatorial will of the flesh, the matter, is fasting. But for the fast to be real, authentic, the Gospel indicates, we must not fast hypocritically, ostentatiously. In the words of Christ, when we fast must not appear to fast unto men but unto our Father which Is In secret. The second condition Is forgiveness. "If ye forgive men their trespasses, your Heavenly Father will also forgive you." The triumph of sin and the principal sign of Its dominion over the world are quarrels, discord, dissension, hatred. The first breach in the fortress of sin is forgiveness, that Is the return to unity, concord, love.
At the-end of the vespers on the Sunday of Forgiveness, the worshippers come up to the celebrant and ask for forgiveness after which they turn to each other asking each other for forgiveness. It Is precisely this act of love, this act of becoming one, that marks the beginning of the Fast.
To-day, there are not many who stop to think about the meaning of fasting. This is a result of the overall spiritual weakening, of the lack of spiritual vigor and responsibility.
O, If only we remembered what we were called for, what feats of spirit the Lord expects of us! If only we remembered more often that we were appointed laborers in the vineyard of the Lord and that we shall be called by the Master to account for our labors and for the fruits brought forth by our labors In His vineyard! Perhaps It is to us that the terrible words of the Master in the parable refer: "The kingdom of God shall be taken from you and given to a nation bringing forth the fruits thereof!" (Matt. 21, 33-43.)
Archpriest Victor Potapov
The Lenten Fast, 1988