Calories are not calories, because they're a unit of energy, and humans aren't batteries. We "use" energy in the form of countless chemicals and reactions, so

Calories are not calories, because they're a unit of energy, and humans aren't batteries. We "use" energy in the form of countless chemicals and reactions, so things get complicated fast. It's far easier to instead talk about mass, because one gram is indeed one gram, until humans evolve anti-matter producing organs. If you eat X and excrete Y, your body mass will change by X-Y. If you want to lose 1 kg, you will have to exhale 0.8 kg [1] of carbon atoms (you breath in O2 and exhale CO2). [1]: There are a lot of different "calorie" molecules in your body, but generally all of them are some combination of C, H, O, which turns into CO2 and H2O at roughly 4:1 ratio by mass. So 80% is lost via exhaling C, and the H2O gets processed by your body normally, and is generally lost as urine or sweat, unless you're dehydrated.