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?