Several Times in the Year the Byzantine Church prescribes periods of fasting. The least important (but most visible) aspect of this fasting is a change in the quantity and quality of food: we eat one meal a day, with no animal products. The practice of fasting comes highly recommended by the Holy Fathers and Mothers and, indeed, by the word and example of our Blessed Lord Himself, it is always seen as a means of purification, usually before some significant work to be done or decision to be made.
The object of fasting is not simply self-discipline, it is that turning of the soul to God, the re-shaping of the will, that the Greeks call Metanoia (usually translated into English as “repentance”). If one’s health allows one to observe the fast with regard to food, one should feel the need to do the best that one can, but all should fast of the spirit. In the words of St. John Chrysostom the fast is of no advantage to us unless it brings about our spiritual renewal.