I have asked that question many times of 'detox' proponents. I never get an answer.
Most medical scientists agree that the alt med concept of the 'toxins' in question do not exist, and there is no evidence that they accumulate in your body. 'Detox' has no meaning outside of the clinical treatment for drug addiction or poisoning.
If you really had toxic levels of poisons in your body, then you need a hospital. Do not fall for the alt med concept of detox, it's mostly a scam to sell products that has turned into a fad. The human body has evolved to get rid of unnecessary substances through your liver, kidneys, and colon. It isn’t possible to improve their function without medical assistance.
From Wiki:
"Body cleansing and detoxification have been referred to as an elaborate hoax used by con artists to cure nonexistent illnesses." http://en.wikipedia.org/wiki/Body_cleansing
Good question. I doubt you'll get a good answer.
More reading:
http://www.theness.com/neurologicablog/?p=452
http://www.senseaboutscience.org.uk/index.php/site/about/282/
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EDIT: @Michelle, interesting theory.. what is the evidence that supports this?