 Ruben Meerman , researcher , Andrew J Brown , professor 1School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney, 2052, Australia Correspondence to: R Meerman rubenmeerman{at}me.com Accepted 14 November 2014 
 
 Abstract Ruben Meerman and Andrew Brown explain why the answer might not be what you expect 
 
 Considering the soaring overweight and obesity rates and strong interest in this topic, there is surprising ignorance and confusion about the metabolic process of weight loss among the general public and health professionals alike. The most common misconception among doctors, dieticians and personal trainers is that the missing mass has been converted into energy or heat. We suspect this misconception is caused by the “energy in/energy out” mantra and the focus on energy production in university biochemistry courses. Other misconceptions were that the metabolites of fat are excreted in the faeces or converted to muscle. The complete oxidation of a single triglyceride molecule involves many enzymes and biochemical steps, but the entire process can be summarised as: C 55 H 104 O 6 +78O 2 →55CO 2 +52H 2 O+energy Stoichiometry shows that complete oxidation of 10 kg of human fat requires 29 kg of inhaled oxygen producing 28 kg of CO 2 and 11 kg of H 2 O. Human fat cells store triglyceride, which consists of just three kinds of atoms; carbon, hydrogen and oxygen. The remaining 1.6 kilograms becomes water, which may be excreted in urine, faeces, sweat, breath, tears and other bodily fluids, the authors report. By tracing every atom's pathway out of the body, the authors discovered that when 10 kg of fat are fully oxidised, 8.4 kg departs via the lungs as carbon dioxide (CO2). "These results show that the lungs are the primary excretory organ for weight loss," they add. (This calculation ignores fat that may be excreted as ketone bodies under particular (patho)physiological conditions or minor amounts of lean body mass, the nitrogen in which may be excreted as urea) 
 
 Lifting the veil on weight loss At rest, an average 70 kg person consuming a mixed diet (respiratory quotient 0.8) exhales about 200 ml of CO 2 in 12 breaths per minute.4 Each of those breaths therefore excretes 33 mg of CO 2, of which 8.9 mg is carbon. Replacing one hour of rest with exercise that raises the metabolic rate to seven times that of resting by, for example, jogging, removes an additional 40 g of carbon from the body, raising the total by about 20% to 240 g. 
 
 But the authors point out that, for comparison, a single 100 g muffin represents about 20% of an average person's total daily energy requirement. "Physical activity as a weight loss strategy is, therefore, easily foiled by relatively small quantities of excess food," they say. "Our calculations show that the lungs are the primary excretory organ for fat," they conclude. Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of “eat less, move more.” We recommend these concepts be included in secondary school science curriculums and university biochemistry courses to correct widespread misconceptions about weight loss.
– 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.)