During a bowel movement, you lose an insignificant amount of weight, which is mostly water. The rest of the poop is undigested food, gut bacteria, and mineral content from your diet. Your poop is also a source of stercobilin, which gives it its color from hemoglobin derived from broken down red blood cells.
The number of calories lost in a bowel movement varies from person to person and depends on the frequency, size, and weight of stools, as well as the food consumed and the type of stool produced. For example, if you eat foods that are high in fiber, you will have a higher-volume stool with a lower calorie content. Similarly, foods that are high in fat can result in a fatter poop.
While pooping does burn a small amount of calories, it is not enough to help you lose or gain weight. If you want to lose body fat, it is important to focus on burning more calories than you consume through diet and exercise.
However, while pooping does not burn any significant amount of calories, it does tense your muscles. This can help you burn a few more calories than if you were sitting at rest. This is especially true if you are having a bowel movement that requires straining. However, it is important to not strain excessively because this can cause painful and dangerous complications such as hemorrhoids. A study by Dr. Bridget Cassady found that people who chewed their food more often (up to 40 times) had fewer calories in their stool.