http://www.burnthefat.com/colon_cleansing_scam.html
Consistent, long term body weight/ body fat loss can rarely exceed 2 lbs per week or 1% of total body weight per week (ie, 3 lbs if you weigh 300 lbs) and weight lost beyond that is usually
not body fat.
The contents of the digestive system add to your body weight. If you have fasted or done a light food or juice "cleanse" for any significant period of time, OF COURSE YOU WILL WEIGH LESS! And by the way, that's the
normal contents of your digestive tract clearing out - not "icky stuff" that has been clinging to your insides for months or years.
Also, when you fast or go on a very low calorie "cleansing" protocol - you often generate a huge caloric deficit. A large caloric deficit will create a large weight loss. With a large enough caloric deficit, you can also get FAT loss that is above average (usually 1-2 lbs a week, but could easily be 3-3.5 lbs in a week with a big deficit). Combined with water weight, glycogen and lean tissue loss, (plus the clearing out of the contents of the digestive tract), the total body weight loss can be very large.