I'm going to jut in at this stage to dispel a few myths.
Our human bodies are categorized into 3 metabolic types: protein, mixed and carbo types. Other names used are meat-eaters, hunter gatherers, agriculturists etc.
Imagine a line with protein and fats at one end of the spectrum and carbs on the other end. Your optimum need for proteins/carbs lies somewhere on this line. So far so good... Where's the catch?
Because each of us are different, our optimum food combination lies somewhere along that line. Fasting works very very well for agriculturists(or carbo types... me for example
). Agriculturists are found in countries adjacent to the equator. These geographical locations have plenty of sunlight and trees and people in these regions derive their food from plants, vegetables, fruits and grains. On the other end of the spectrum are people (or their ancestors) living in cold frigid conditions with very little sunlight and negligible vegetation (think of Eskimos). These people have survived eating animal protein and fat for centuries.
So, if you are a protein type(hint: you do well on the Atkins' diet), avoid fasting at all costs. Protein types need to eat every 3-4 hours. Otherwise they feel hungry and dizzy/weak if they haven't eaten for a while.
Agriculturists like me .... the less we eat, the better we feel. Yeah, it's a conundrum but true. You'd see fasting in religions whose diet is predominantly agrarian(people from India). Fasting is unheard of in cultures whose diet is animal protein and fat.
An Eskimo has meat and fat for breakfast, lunch, dinner and snacks. The amount of fat they eat everyday is astounding. However, if they start fasting and start going vegetarian/vegan, they WILL die of coronary heart disease. Same goes for a healthy agriculturist who switches to Atkins diet.
Why does this happen? The answer is Evolution. Gradual adaptation to the environment over thousands of years. Supposing you are from Ireland/Iceland, your grandfathers and his grandfathers thrived on meat. They had very little access to vegetation. The exceptions were root vegetables they cultivated.
Not all people are cut for water or juice fasting. A majority would do well but a minority would suffer greatly. I'd guess that about 20% of the world's population has a protein metabolism and should never fast.