Fasting is part of the spiritual tradition of all religions. It also has physiological, psychological, and metabolic implications that affect health. A fast means different things in various religions–from the Jain fast where there is no food for 36 hours to partial fasts that allow one main meal and beverages and fruit. There are indications that some form of fasting may have physiological benefits but it must clearly be done in consultation with a doctor. Many medical tests require some type of fasting that we know cause metabolic changes.
Looking and sifting through various sources for information on fasting, the following are some of the most informative ones.
Hindu Science of Fasting by Swami Muktibodhananda Saraswati is about the spiritual significance of fasting as a form of purification. The lunar cycle plays a major role.
Fasting for Health and Spiritual Development by Swami Satyananda Saraswati briefly touches on the psychosomatic and hormonal effects; effect on meditation; and the important days for fasting. However, some spiritual beliefs and discipline regarding fasting are not suitable for most of us.
Psychophysiology of Fasting by Dr. Swami Karmananda Saraswati is a brief summary of fasting and the brain, nervous system, and endocrine system.
Some fasting may reduce cancer risk as indicated by the study on WorldHealth.net.
The following abstract from a study from the National Academy of Sciences summarizes the health benefits for alternate-day fasting which is impractical and unrealistic on a long-term basis.
Abstract: Dietary restriction has been shown to have several health benefits including increased insulin sensitivity, stress resistance, reduced morbidity, and increased life span. The mechanism remains unknown, but the need for a long-term reduction in caloric intake to achieve these benefits has been assumed. We report that when C57BL/6 mice are maintained on an intermittent fasting (alternate-day fasting) dietary-restriction regimen their overall food intake is not decreased and their body weight is maintained. Nevertheless, intermittent fasting resulted in beneficial effects that met or exceeded those of caloric restriction including reduced serum glucose and insulin levels and increased resistance of neurons in the brain to excitotoxic stress. Intermittent fasting therefore has beneficial effects on glucose regulation and neuronal resistance to injury in these mice that are independent of caloric intake.