Hippocrates admits the possibility of fasting more than six days without a fatal issue; but Pliny and others allow a much longer time, and both the ancient and modern literature of medicine are replete with examples of abstinence to almost incredible lengths of time.

Formerly, and particularly in the Middle Ages when religious frenzy was at its highest pitch, prolonged abstinence was prompted by a desire to do penance and to gain the approbation of Heaven.
In many religions fasting has become a part of worship or religions ceremony, and from the earliest times certain sects have carried this custom to extremes.

It is well known that some of the priests and anchorites of the East now subsist on the minimum amount of food, and from the earliest times before the advent of Christianity we find instances of prolonged fasting associated with religious worship.

The Assyrians, the Hebrews, the Egyptians, and other Eastern nations, and also the Greeks and Romans, as well as feasting days, had their times of fasting, and some of these were quite prolonged.
At the present day religious fervor accounts for but few of our remarkable instances of abstinence, most of them being due to some form of nervous disorder, varying from hysteria and melancholia to absolute insanity.

The ability seen in the Middle Ages to live on the Holy Sacrament and to resist starvation may possibly have its analogy in some of the fasting girls of the present day.