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.
A few words about the effects of fasting upon the so-called spiritual powers may be appropriately introduced here. In detailing his experiences during his forty days' fast, taken some years since, Dr. Tanner said: "My mental powers were greatly augmented, to the very great surprise of my medical attendants, who were constantly on watch for mental collapse, which was freely predicted, if I persisted in the experiment until the tenth day. About the middle of my first experiment, I, too, had visions; like Paul of old, I seemed to be intromitted to the third heaven and there saw things which not even the pen of Milton or Shakespeare could portray in all their vivid reality. As a result of my experiment, I came to comprehend why the old prophets and seers so often resorted to fasting as a means of spiritual illumination."
That the mental powers of the faster are elevated instead of decreased by the fast, I have shown, but I pause here to express my opinion about these visions that fasters see. They are, I believe, due to hysteria, or auto-hypnosis. They are seen by people who are called psychic which means they are easily swayed by suggestion, particularly auto-suggestion. Fasting tends to increase temporarily this suggestibility and for this reason was and is employed by all mystic religions for purposes of "illumination." Auto-suggestion, during these religious fasts takes the form of frequent and repeated prayers. To add to the religious power of the fast, sexual desires disappear and thoughts of sex cease to obtrude upon the mind. In India the priests connected with the sacred temples are pledged to the strictest chastity. The Hindu high priest is forced to undergo a long period of training and purification, and to pass through many severe trials to prove that he has thoroughly conquered his sexual appetites and passions and has them well under control of the higher powers of mind before he is admitted to the priesthood. In these days when the fallacies of psychology and psycho-analysis are on the lips of everyone and when feminine leaders declare chastity and continence to be neither desirable nor practicable and insist that they would be harmful if put into effect, methods of attaining self-control in matters of sex are frowned upon. This feature of fasting may not, therefore, appeal to many who read these lines. Fasting does increase one's control over all his appetites and passions and this will account in some measure for its use by high priests and others in the religions of old.