Where does your body fat go when you lose it? "Into thin air," quite literally, says Ruben Meerman. A new study out of Australia looks at how we shed pounds and the "surprising ignorance" around the process. Contrary to commonly assumed mechanisms—that excess fat is converted to heat or energy, which "violates the law of conservation of mass," per the study—most of our lost mass is breathed out as carbon dioxide, says Meerman; he's the lead author of the study, published yesterday in the British Medical Journal. How the researchers' "novel calculations" take shape: They write that losing 10 kilograms of fat (that's 22 pounds) requires 29 kilograms of oxygen to be inhaled; that in turn produces 28 kilograms of carbon dioxide and 11 kilograms of water.
"This tells us the metabolic fate of fat but remains silent about the proportions of the mass stored in those 10 kg of fat that depart as carbon dioxide or water during weight loss," they write. So they set to work tracing how every atom exits the body, and determined that the majority of the fat—8.4 kilograms—departs via exhalation. The remaining 1.6 kilograms become water, and leave the body via a variety ways: urine, feces, sweat, tears, etc. The researchers note that the biochemistry behind their findings isn't new (their study cites research from 1949), but "it seems nobody has thought of performing these calculations before," they say per a press release. The million-dollar question: Can we lose weight by just breathing, well, more? Nope. That's a ticket to hyperventilation, not weight loss, they say. (More on weight loss: Why one woman lived on dog food for six days.)