Fasting is a common "ritual" when it comes to both, spiritual quest and physical/mental health but what must be understood is that there are many kinds of fasts and certain reasons why one would be challenged by one form vs. another. The full fast that most of us think of -- abstaining from food stuffs for a given period of time -- is observed monthly by certain religious groups, the purpose being to remind us to be both, humble and grateful in life but likewise, these short 24 hour fasts can be used as a way to aid one with health related issues. If you have digestion issues or constipation you can rely on the consumption of water -- lots of water -- throughout the 24 hour period so as to "cleanse" your system. If the issue centers of actual digestion you may be encouraged to do what some call the Cayce Fast -- living on a concoction composed of lemonade, cayenne pepper, and maple syrup (real syrup). This is to be the only thing you consume for as short of a time as a week to well over a year. It really does some rather remarkable things for the body when done in moderation, I cannot encourage anyone to take this to any kind of long-term extreme but then I feel that way about most of them.
Certain of the Trend Diets involve Fasts that focus on specific food groups such as only protean or only carbs or only certain types of fruits/vegetables. Frankly, I don't see how a Pasta Only "Fast" does anything but make the Pasta Makers happy, same goes with the protean only diets; while it will benefit your butcher I can't say that it will do much for you body, mind or soul.
Fast Drinks have existed for centuries, frequently being composed of mild hallucinogenic additives, be it wine/mead or beer/ail or something far more potent like peyote. In more recent times the "New Age" elements have introduced certain high-sugar or high-caffeine elixirs and even Fasts in which it was ok to grab a Slim-Fast shake or similar product once or twice a day during the ritual. My personal theory when it comes to this sort of Fasting exercise, is that it's a safety factor for persons that juggle sugar imbalances.
Long story made a hint shorter. . . Fasting can help if you are focused on it in a positive manner for the sake of reaching a positive goal. Religious observations might seem a bit primitive and even silly to many these days, but when observed in the manner prescribed there is a very real psychological result in the practitioner's personality & mannerisms; a greater sense of grace and graciousness as well as an appreciation of the simpler things in life and life itself. So it's what you put into it that decides what will come from it.
OH! There is one fast everyone on this forum has participated and I'll bet you aren't aware of it.
I'll give you a hint . . . why is the first meal of your day called BREAK-FAST?
