In the spiritual fast the measure is the same and important for all – complete absence for everything that leads to sin. Regarding our abstaining from food, there the measures vary and the manners in which it is used are different: it depends on one’s age, occupation and physical shape. Thus, it is impossible to place everyone in the school of piety under the same rule. Yet, determining a measure of abstinence for healthy ascetics, we indulge prudence to the superior that he make wise changes. The diet of those who are sick, or weary from hard work, or one who go to heavy labor, on the road or whatever difficult job, the superior must manipulate optionally. For it is not wise to take food which is for the nourishment of the body and hinder it from carrying out the commandments. In every food group we must assume the one that is easiest to obtain, so that we not concern ourselves with the preparation of favorite and expensive foods, all with the excuse of the Fast.
St. Basil the Great