According to The Sun, a Malaysia newspaper, dated on June 14 2007, page 30 in the lifestyle section, there was an article with the title "The 'hidden' secrect to slimming."
In the article, it mentioned on an average, most people carry between 2kg to 5kg of old, dried-up faecal matter in their intestine.
In my own thoughts, I think some people definitely has more than that. Especially, those who are obese or seriously over-weight.
Thus, when a person is going through a detox programme, it will help to flush out these accumulated waste and makes us feel 'lighter'.