Some colon cleansing fasts are designed to help you rid your body of built-up wastes including pesticides, preservatives, and dyes that may be in the food you eat. If you take any medication or drugs, residues from them may remain in your body as well. Many people believe that these contaminants lie in the mucoid plaque that is on your colon wall and they use regular colon cleansing systems to remove the plaque and the contaminants along with it.