It came about as a marketing gimmick due to the scientific reality of widespread contamination of food, water and air with persistent chemicals that accumulate

It came about as a marketing gimmick due to the scientific reality of widespread contamination of food, water and air with persistent chemicals that accumulate in human bodies, e.g. PCBs in breast milk and so on. It's also a reality that many 'natural' substance accumulate and persist in the body and are only slowly excreted particularly if they have a nonpolar oily affinity (this is why THC testing, i.e. marijuana urine tests, work). The reason it's batty is that nobody who markets these products ever does a scientific study to see if their 'cleanse' program has any measureable effect on the rate your body excretes various pollutants and contaminants, relative to the normal background turnover. Generally, removing the source of the contamination is more important than anything else, e.g. lead poisoning from contaminated water supplies and so on.