Random fat sampling by the EPA shows that all Americans store fat-soluble toxins from common chemical exposures in their fat. This is a testament to the amount of pollution we face. Some of these, like PCBs, which are proven to elevate in your blood as they are released from fat when you lose weight, can be quite toxic.
Note that the detoxification process works by shifting the focus from metabolizing unhealthy foods towards cellular regeneration and internal cleansing of the body.
Before you go on a diet, you must be able to get toxins out of your system completely as you lose weight, otherwise your fat will simply not release them easily and you will struggle to lose weight past a certain point. However, medical science offers no support for the value of a detoxifying cleanse. Nevertheless, a short pre-diet cleanse might help you get in the proper mindset for success.