A healthy bowel receives food residue from the stomach and small intestine. As the bowel fills, fecal matter stimulates nerve endings, causing muscle contractions that expel the residue from the body in a bowel movement. Laxatives and enemas artificially stimulate nerve endings in the large bowel, which is also called the colon. The colon is one of the last structures in the digestive tract. By the time food arrives there, nothing is left but indigestible fiber and other non-nutritive material.
Laxatives remove lots of water from the colon as well as food residue.
The scales indicate weight loss after a laxative-induced bowel movement, but it is false weight loss. The ounces or pounds return as the body rehydrates after liquid intake consumption. Laxatives and enemas cannot stimulate the small intestine, the part of the GI tract where food is digested and where nutrients and calories are absorbed. The small intestine does not even have the kinds of nerves that occur in the colon and respond to artificial stimulation. In one experiment, a group of laxative abusers ate a high calorie meal.