Where does all the food go? Food goes in, excrement come out. This seems obvious. But it seems like the mass of the excrement is less than the one of the food. Most people I asked answered: "There's a difference because it's transformed into energy, like heat...". I think that's _bull-excrement_. There's still the law of conservation of mass: our body does no nuclear reactions, **or does it?** Example: Someone's on a "high protein / low carb" diet, he may ingest like 1kg of food, he isn't really productive on the toilet (it's difficult to find any fibers in those diets), he loses 500g in a day... My question is: where does the 1.5kg of matter go?

Whatever the diet, the food intake contains macronutrients: carbohydrate, fat and protein. When they are metabolised all of these molecules will end up, for the most part, as carbon dioxide, water, and ammonia, unless they are incorporated into components of the body.

These waste products are, of course, lost via breathing, and via the urine. That's where the "missing mass" goes.

A related question is here.