I recently did a water fast (no food or juice, just water) for a week. First few days you feel hungry but towards the end you adapt. Fasting also causes the body to detox, so if it is the first time fasting you will probably feel like crap. I'm going to see about doing it for longer than a week come New Years. I'm thinking it should be much easier the 2nd time around if I don't have to detox as much.
I lost about 10 lbs in 7 days but most of it was water weight and I regained most of that back in about 2 weeks. I have seen statistics on 30 day fasts and the body burns about half a pound of fat a day consistently starting from about the 3rd day. Loss from water weight slows down after the first week.
A lot of the above information about fasting is wrong or misleading. Studies have shown increased mental and physical capacity when the body is operating in a ketogenic state. Extended duration fasting is the simplest and easiest way to enter a ketogenic state. That is to say, the body is burning fat (which gets converted to ketones) as its primary fuel source. Without going into too much detail, the reason is that fat gets burned at an even rate as the body needs energy. Most diets are high carb and result is spikes of blood sugar and insulin. Insulin is necessary but the excessive amounts which are common in modern society have devastating effects.
Fasting causes the body to go into a ketogenic state, but until it goes through ketoadaptation, the body will experience the effects described in some of the above links (headaches, nausea, etc). These are normal and go away. An easier way to ease into fasting is to ketoadapt before fasting. This is accomplished by eating a diet low in carbs, low in protein, and high in fat. The reason for keeping protein low as well is because proteins can get converted to glucose through a process known as gluconeogenesis. As long as glucose is present the brain and other organs will not adapt to using ketones and this causes the negative symptoms described above. Once ketoadaption takes place the need for gluconeogenesis (which results in consuming muscle and body tissue for energy) drops off.
Caloric restriction has been scientifically proven to reduce aging and extend life expectancy. As far as how far you can go is not certain but one study showed at 30% restriction resulted in a 30% longer life span in rats. The reason is because lower calories means less food is converted into energy, and energy production is one of the primary ways free radicals get created. Reduce free radicals and you age less.
I haven't found much information on fasting yet, I am currently experimenting with it to learn more about it. However, a lot of the knowledge of what fasting does to the body can be understood by reading about ketogenic diets. I recommend the book "The Art and Science of Low Carbohydrate Living" if you are interested in learning about how the body works under different nutritional inputs. If you want to lose weight and maintain health you want to see about weight training. I highly recommend the book "Body by Science" for that. It is one of the most technical but still readable without a biochemistry background.
Some links on fasting you might find interesting:
http://healthcorrelator.blogspot.com...s-fat-and.html
http://www.falconblanco.com/health/fasting.htm
For weight loss and ketogenic diets I recommend the 2 books mentioned above. This link will provide the basics:
http://www.ketogenic-diet-resource.c...s-program.html
I'll keep you posted if I manage to keep with it for longer than a week and what the results are.