Stupid. Don't do that. Any doctor, physiologist, or biochemist would say that a dumb F'n idea.

If you want to fast, just do a 24 hour fast every other day.

I tried a five-day fast and wound up with suspected refeeding syndrome and hypophosphatemia, and I was sucking down low-carb Powerade and diet sodas in an attempt to replenish phosphates because I couldn't find supplements anywhere. It was the stupidest thing I've ever done. I started reintroducing food very slowly, no carbs, but it didn't seem to make a difference -- I became very weak and exhausted and scared myself half to death. Never again.

If you have to do this, find a doctor to oversee it. Otherwise, alternate day fasting is great. Refeeding syndrome is a very real danger, and you don't want to mess with it.

Maybe I'm the only one, but I had no problems water fasting (although I was eating SAD at the time). I fasted for 2 weeks; I was hungry at first but that quickly faded and I felt fine. I would go to lunch and sit around smelling everyone's food without wanting it. I was in high school at the time, and I kept up my normal levels of activity without any problems (although I wasn't really exercising - an hour a day of half-assed participation in mandatory C-squad softball doesn't count for much, since I spent most of it standing in the outfield doing homework in my head). I had no skin issues before, during, or after, so I can't comment on that, though.

I really don't understand why people are so hostile to longer fasts - obviously if you're super athletic or have a physically demanding job, it's a terrible idea, and if you're not generally healthy I wouldn't try it, but I don't see how it's so obviously harmful otherwise.

Water fasts are really intense, especially a 7 day water fast. I've done juice fasts for that long but never a water fast. My biggest concern with doing one would be losing a bunch of muscle during the fast. Plus, if you don't know what you're doing and haven't built up to a week long fast you're asking for trouble. I'd never do a 7 day water fast just for "spring cleaning." If you're drawn to fasting you might experiment with intermittent fasting (IF) or with juice fasting (for shorter periods of time.)

Yes, people have been fasting for up to a week or more forever. A lot of animals fast out of instinct when injured. Absolutely do not do this without supervision the first time, and quit if any problems arise. Wrap it up slowly by starting with a day of just juice, then a day of soups and soft foods, followed by returning to paleo. It is much easier to juice fast or master cleanse (lemonade diet), with similar results. I wouldn't attempt any rigorous routines while doing it.

I was on a diluted juice fast for 3 days (72 hours total). It was juice I made myself and the diluted in half with water. I had a lot of really tripped out dreams, and felt very very light. I was a vegetarian at the time, and a long distance runner. I did not run for those 3 days, but I did light yoga. I used to fast for 24 hours once a week back then.

I did a 5 day fast,was tough but strangely enjoyable. Now I just IF 18hr every day.

a well-known detox solution is birch sap for a few weeks, straight from the tree; I've done it for a month and did not notice anything [good or bad], but perhaps I am already healthy :D ?

In fitness centers, lots of detox drinks are offered; perhaps you can check it out.