This section is from the book "The Hygienic System: Fasting And Sun Bathing", by Herbert M. Shelton. Also available from Amazon: The Hygienic System Vol III Fasting and Sun Bathing.
It is estimated that fasting for the alleviation of human suffering has been practiced uninterruptedly for 10,000 years. No doubt it has been employed from the time man first began to get sick. Fasting was part of the methods of healing practiced in the Ancient Asculapian Temples of Toscurd Guido, 1300 years before the time of Jesus. Hippocrates, the mythical Greek "Father of Physic," seems to have prescribed total abstinence from food while a "disease" was on the increase, and especially at the critical period, and a spare diet on other occasions. Tertullian has left us a treatise on fasting written about 200 A.D. Plutarch said: "Instead of using medicine rather fast a day." Avicenna, the great Arab physician often prescribed fasting for three weeks or more.
I think that there is no room to doubt that man, like the lower animals, has always fasted when acutely ill. In more modern times the medical profession has taught the sick that they must eat to keep up their strength and that if they do not eat their resistance will be lowered and they will lose strength. The thought behind all of this is that unless the sick eat they are likely to die. The reverse of this is the truth--the more they eat, the more likely are they to die. In his Eating for Strength, M. L. Holbrook, an outstanding Hygienist of the last century, says: "Fasting is no cunning trick of priestcraft, but the most powerful and safest of all medicines."
When animals are sick they refuse food. Only when they are well, and not before, will they resume eating. It is as natural or normal for man to refuse food when sick as for animals to do so. His natural repulsion to food is a safe guide to not eating. The aversions and dislikes of the sick, especially to food, noise, motion, light, close air, etc., are not to be lightly dismissed. They express protective measures of the sick body.