On Fasting:

In regard to the abstinence from food, the same rule cannot be applied to everyone because not everyone has the same physical strength. The virtue of fasting is not observed with just the strength of the soul, but must also be proportionate to the toughness of the body. The holy Fathers determined the measure of our abstinence from food as follows: we should stop eating while we still want to eat. Guided by this rule, even a person weak in body can exhibit the virtue of abstinence on the same level as the robust and healthy, if he applies his willpower in restraining the craving for food when it is not demanded by his physical infirmity. For even the Apostle states: "and make no provision for the flesh, to fulfill its lusts" (Romans 13:14).


Saint John Cassian