The premise of body cleansing is based on the Ancient Egyptian and Greek idea of autointoxication, in which foods consumed or in the humoral theory of health that the four humours themselves can putrefy and produce toxins that harm the body. Biochemistry and microbiology appeared to support the theory in the 19th century, but by the early twentieth century, detoxification based approaches quickly fell out of favour. Despite abandonment by mainstream medicine, the idea has persisted in the popular imagination and amongst alternative medicine practitioners. In recent years, notions of body cleansing have undergone something of a resurgence, along with many other alternative medical approaches. Nonetheless, mainstream medicine continues to view the field as unscientific and anachronistic.
You’ve probably come across all kinds of cleansing and detox programs online. There is a whole industry based around selling these plans and supplies to people trying to live healthier. Unfortunately, doctors largely agree that most of these plans have no real health benefits. They might even be harmful. It's best to skip them and follow a healthier lifestyle instead.