I thought about what gaining weight means. Isn't the following (obviously) correct? When you eat X grammes of something you momentarily gain X grammes of weight. The only way to lose weight is to go to the toilet or sweat. However, when you sweat most of the mass is water so this is easily calculable. Now it's probably quite doable to calculate which part of the food mass stay in the body after the first digestion cycle. The rest is weight gain. Unless it is decomposed again and "flushed" away. Does that makes sense? Have I missed something? It seems more direct than calorie thinking. Shouldn't it be easy to directly calculate the mass you lose when you exercise? Like this biochemical process will transform mass into waste mass.