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.
Due to many causes waste accumulates during the entire life period of the body. The older the body and the more gluttony and sensuality have been indulged in, the greater the toxin saturation. These toxins are lost to the body during a fast. It is largely for this reason that fasting proves to be so valuable in chronic "disease." The purification of the organism and the regeneration of its tissues restore the youthfulness of the body.
Dr. Lindlahr likened the digestive tract to a sponge. In health it absorbs the elements of nutrition; but in acute disease the process is reversed; the sponge is being squeezed and it throws off toxins. When the sponge is being squeezed the processes of digestion and absorption are at a standstill. In fact, as he said, the entire organism, in acute disease, is in a state of weakness, prostration and inactivity. The vital energies are concentrated upon the work of cleansing the body of its accumulated toxins. I need only to add that in chronic disease, when the patient fasts, the whole digestive tract enters upon the work of elimination and assists in freeing the body of its accumulated toxins.