The best way to comprehend what fasting actually is in principle and also what fasting is not is through analogy. An analogy will clarify what voluminous writing might not. Every schooling tradition has used analogies to teach concepts and even the Bible uses analogy in the form of parables and metaphor towards the same purpose describing sometimes complex spiritual concepts. This is pertinent because fasting fundamentally, is more a spiritual practice that also happens to help the physical body.
Everyone knows what water is and recognizes the difference and infinite usefulness of clean, clear water and the near uselessness of dirty water. A natural water source flowing in the open air and sunshine will naturally and perpetually keep itself clean and clear for as long as nothing detrimental is added to it. If something is added to it that was not there before the water will lose its clarity and often also ability to sustain life. Compare this to the life fluids within any living thing and what would happen to them if their fluids would thicken from being forced to carry within them what they were not created to be carrying.
When water is clear and and clean, like any other fluid, it will flow easily. When it is filled with pollutants it becomes "thicker" and because it is carrying more than just itself will have a more burdened flow. Sometimes a river bed may need to be dredged because of the deposited pollutants that the water had been carrying. But, in general, if the pollutants are no longer being dumped into the water, it will, in time, clean itself.
This is the best example to describe the effects of fasting within the organism of the human body.
There are two main reasons for fasting. One is to keep the fluids of the body clear, clean and free flowing within their vessels preventing them from depositing their wastes onto the vessels and within the bodily tissues, ultimately clogging them to proper circulation; the other is to constitutionally strengthen psychologically and spiritually. Spiritually and psychologically fasting results in the triumph of the spiritual will power over psychological tendencies, inclinations and temptations. Even the most uneducated will understand that to abstain from habitually consuming and over consuming certain types of foods and chemicals that do not digest well will end up thickening the blood and lymphatic fluids resulting in disease.
Every religious tradition has fasting as part of its teaching. Catholics would fast during the time of lent; starting from Mardi Gras - Ash Wednesday until Easter. Muslims and Jews also have traditional fasting as part of religious rite. To condemn fasting would be as much as to condemn and diminish all religions. People who fast all claim a sort of mental clarity and a great spiritual acuity. In some spiritual retreats people abstain from any manner of talk for an entire weekend or even weeks on end. Again, the purpose is to make the individual focus inwardly.
Before discussing the health benefits of fasting it is best to first clarify a few basics points. First of all, fasting has nothing to do with starvation or the modern, historically unprecedented, insanity that is anorexia.
To someone who is a heavy meat eater, a fast might simply consist of refraining from red meat and replacing it with fish or chicken and more fresh fruit and vegetables than they might be accustomed to. For a fast food junkie, it could mean simply 
eating better food. For someone obsessed with television, a fast might consist of limiting the time they view television down from 4 hours a day down to 1 or 2 hours or even no television at all. A person who is perpetually subject to their own anger might fast emotionally by not allowing themselves to indulge in their anger. A lazy person might fast by deliberately making themselves more productive. A person who is perpetually late could fast by making it a point of being on time. An addict to alcohol, drugs and the like might muster their strength to leave their addiction behind. As is evident the purpose of fasting is to make the physical body subject to the will of the mind (spirit). It is an exercise in inner discipline more than anything else. Best of all, doing any of these things is completely free of charge attesting again that the best things in life are often free.
The whole purpose of making these very deliberate and conscious choices, contrary to one's inclinations is intended to make a person objectively take stock of themselves and forcing themselves by their own volition and better judgment, to bring themselves under their own self control by practicing self restraint without becoming fanatically ascetic. By, ultimately triumphing over their own detrimental and often unconscious habits they lift themselves of a burden that had made them and the people around them subject to. In essence, this is all that fasting really is! Used in this manner and with this awareness, fasting will put a person back in control of their own life. Disease, be it physical, emotional or mental is the result of being out of control. Complete health is the result of being in control of one's own being from the top down - spiritually, astrally (in the soul) and physically.
This whole practice of deliberate abstention is to give the entire body and its organs a much needed rest from the work of keeping itself alive. Who would argue that a rest is detrimental.
The average person from an industrialized society should not fast without some considerations. For the most part citizens of these societies are too filled with a lifetime of toxic poison and chemical residue from their very poor and over abundant food choices and food preparation methods. Fasting is also contrary to their cultural make up. They have been socialized for generations to simply consume irrespective of quality. Whether it is food, completely useless or unnecessary products or even the cheapest of often disposable utilitarian products, it does not matter; just consume and consume without restraint or question! This compulsive mantra of binging extends to much more than just food. This mindlessness becomes part of the identity of the person. The result of these completely unnatural excesses is self evident everywhere and in everything resulting in an empty vacuous existence based around "stuff," especially in, North American and requires no further explanation. Should a person that has lived this way want to fast, they should be moderate or gradually prepare their body for it or seek out professional council on this manner of healing otherwise they will make themselves severely ill or perhaps even worse. A healthy person that had always been somewhat reasonable and conscientious of their health should generally have no major difficulty or problems with fasting, only benefits.
As previously stated the benefits of fasting are spiritual first and physical second. In a world that only pays lip service to matters of the spirit and officially sanctions matters physical because that is all it can bring itself to believe, it makes sense that fasting is considered dangerous by the orthodox healing professions. (Enough said about doctors, nasty corporate interests and conspiracy theories. Anyone with any amount of common sense and and elementary power of deduction should be able to put this together themselves by now.)
What Fasting Does
When the body is presented with a change in food, provided that change in food is an improvement over what it had become accustomed to, the body starts disposing of its accumulations. This happens because the body and all of its organs, generally. only have two basic functions. They are either digesting food or eliminating the metabolic wastes from the digestive processes. The body and its organs cannot do both at the same time. The body of a person who constantly eats is preoccupied with digestion and less with elimination. Naturally, the result is accumulation until such time as the body is given the time to eliminate.
When a person fasts, the body has the time to eliminate and does. Therein lies its modus operandi. It is no more complicated than that. A person accustomed to fasting being without food produces almost no symptoms. Someone used to over eating or eating poorly will have a great deal of problems when they are without food for even a single meal.
What To Expect When Fasting
When the body starts to eliminate it begins to put its wastes from the organs and bodily tissues into the blood stream and, if there are too many toxins, they will also back up into the lymphatic system. This can produce some noticeable effects. Rashes and fatigue are not uncommon. The elimination of the accumulated toxins will displace some of the oxygen in the blood stream producing tiredness. The body, finding itself full of the toxins it had previously compartmentalized will decide which way they are going to be disposed of. They might come out through the skin in the form of rashes. The main organs of blood filtration, the liver and kidneys, will find themselves working a little harder to remove these toxins from the blood it is their job to clean. These organs may produce their characteristic symptoms but, generally, the symptoms that previously existed tend to subside.
Different types of Fasting
Raw Fruit and Vegetables
Pulse fast out of the Book of Daniel - the same as Samuel Hahnemann's and Jakob Lorber's homeopathic diet.
Juice Fasting - Rudolf Breuss Cancer Cure
Water Fasting
The Master Cleanse Fast
