Where does all that fat go when you lose weight? According to a new UNSW Australia study, published today in the British Medical Journal, a startling number of health professionals don't know the answer to that question. The most common misconception, expressed by more than half of the 150 doctors, dieticians and personal trainers surveyed, is that your body burns it all up into energy. But the truth is that most of the fat mass is breathed out as carbon dioxide. That's right, you exhale it into thin air. The team of researchers traced every atom's exit pathway from the body and discovered that when 10 kg (22 pounds) of fat are fully oxidised, 8.4 kg (18.5 pounds) of fat departs through your breath as CO2, while the remaining 1.6 kg (3.5 pounds) drains out as good old H2O. So remember that the next time you shed some weight. You're not sweating, peeing or pooping it out -- you're breathing it out into the world around you. Now let's watch some CO2 production ​