Pooping to lose weight is actually a really bad idea
Of course, there are those out there who see "poop can weigh a pound" and will try to up their poop game by taking laxatives. Bad idea. Robert Herbst, an 18-time world-champion powerlifter and one of the drug-testing supervisors at the Rio Olympics, says laxative-driven weight loss happens even at the highest levels of sport, and it isn't pretty.
Herbst confirms that dropping a deuce will in fact budge the number on the scale, though it won't alter your body composition or muscle percentage, saying, "One pound in does not guarantee one [pound] out," because food is metabolized differently. Certain foods are absorbed more efficiently, while others pass right through (looking at you, corn).
So while a pound of lettuce may work its way out to the porcelain water slide, a pound of pie will most likely stick to your thighs. Pooping isn't a total elimination of all the calories you eat, since that wouldn't make any sense. Your body needs energy, so it's not going to shit it all out.