Sydney: Despite a worldwide obsession with diets and fitness regimes, many health professionals cannot correctly answer the question of where body fat goes when people lose weight, a UNSW Australia study shows. 
 
 The most common misconception among doctors, dieticians and personal trainers is that the missing mass has been converted into energy or heat. 
 
 "There is surprising ignorance and confusion about the metabolic process of weight loss," says Professor Andrew Brown, head of the UNSW School of Biotechnology and Biomolecular Sciences. 
 
 "The correct answer is that most of the mass is breathed out as carbon dioxide. It goes into thin air," says the study's lead author, Ruben Meerman, a physicist and Australian TV science presenter. 
 
 In their paper, published in the British Medical Journal today, the authors show that losing 10 kilograms of fat requires 29 kilograms of oxygen to be inhaled and that this metabolic process produces 28 kilograms of carbon dioxide and 11 kilograms of water. 
 
 Mr Meerman became interested in the biochemistry of weight loss through personal experience. 
 
 "I lost 15 kilograms in 2013 and simply wanted to know where those kilograms were going. After a self-directed, crash course in biochemistry, I stumbled onto this amazing result," he says. 
 
 "With a worldwide obesity crisis occurring, we should all know the answer to the simple question of where the fat goes. The fact that almost nobody could answer it took me by surprise, but it was only when I showed Andrew my calculations that we both realised how poorly this topic is being taught." 
 
 The authors met when Mr Meerman interviewed Professor Brown in a story about the science of weight loss for the Catalyst science program on ABC TV in March this year. 
 
 "Ruben's novel approach to the biochemistry of weight loss was to trace every atom in the fat being lost and, as far as I am aware, his results are completely new to the field," says Professor Brown. 
 
 "He has also exposed a completely unexpected black hole in the understanding of weight loss amongst the general public and health professionals alike." 
 
 If you follow the atoms in 10 kilograms of fat as they are 'lost', 8.4 of those kilograms are exhaled as carbon dioxide through the lungs. The remaining 1.6 kilograms becomes water, which may be excreted in urine, faeces, sweat, breath, tears and other bodily fluids, the authors report. 
 
 "None of this is obvious to people because the carbon dioxide gas we exhale is invisible," says Mr Meerman. 
 
 More than 50 per cent of the 150 doctors, dieticians and personal trainers who were surveyed thought the fat was converted to energy or heat. 
 
 "This violates the Law of Conservation of Mass. We suspect this misconception is caused by the energy in/energy out mantra surrounding weight loss," says Mr Meerman. 
 
 Some respondents thought the metabolites of fat were excreted in faeces or converted to muscle. 
 
 "The misconceptions we have encountered reveal surprising unfamiliarity about basic aspects of how the human body works," the authors say. 
 
 One of the most frequently asked questions the authors have encountered is whether simply breathing more can cause weight loss. The answer is no. Breathing more than required by a person's metabolic rate leads to hyperventilation, which can result in dizziness, palpitations and loss of consciousness. 
 
 The second most frequently asked question is whether weight loss can cause global warming. 
 
 "This reveals troubling misconceptions about global warming which is caused by unlocking the ancient carbon atoms trapped underground in fossilised organisms. The carbon atoms human beings exhale are returning to the atmosphere after just a few months or years trapped in food that was made by a plant," says Mr Meerman, who also presents the science of climate change in high schools around Australia. 
 
 Mr Meerman and Professor Brown recommend that these basic concepts be included in secondary school curricula and university biochemistry courses to correct widespread misconceptions about weight loss among lay people and health professionals. 
 
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 Media Contacts 
 
 Ruben Meerman 
 
 rubenmeerman@me.com 
 
 Professor Andrew Brown 
 
 + 61 (2) 9385 2005 
 
 aj.brown@unsw.edu.au 
 
 Deborah Smith 
 
 UNSW Science media officer 
 
 + 61 (2)9385 7307 
 
 + 61 (0) 478 492 060 
 
 deborah.smith@unsw.edu.au ||||| Widespread misconceptions about losing weight led Ruben Meerman and Andrew Brown at the University of New South Wales to calculate how we "lose weight." 
 
 Human fat cells store triglyceride, which consists of just three kinds of atoms; carbon, hydrogen and oxygen. Shedding unwanted fat requires unlocking the atoms in triglyceride molecules by a process known as oxidation. 
 
 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). The remaining 1.6 kg becomes water (H2O). 
 
 Their analysis shows that the inhaled oxygen required for this metabolic process weighs nearly three times more than the fat being 'lost'. To completely oxidise 10 kg of human fat, 29 kg of oxygen must be inhaled producing a total of 28 kg of carbon dioxide and 11 kg of water. 
 
 "None of this biochemistry is new, but for unknown reasons it seems nobody has thought of performing these calculations before," say the authors. "The quantities make perfect sense but we were surprised by the numbers that popped out." 
 
 "These results show that the lungs are the primary excretory organ for weight loss," they add. "The water formed may be excreted in the urine, faeces, sweat, breath, tears, or other bodily fluids and is readily replenished." 
 
 "The exhaled carbon can only be replaced by eating food or consuming beverages such as milk, fruit juices or sugar-sweetened drinks," the authors say. 
 
 At rest, an average 70 kg person exhales around 200 ml of CO2 in 12 breaths per minute. Each of those breaths therefore excretes 33 mg of CO2, of which 8.9 mg is carbon. By simply exhaling 17,280 times, an average person therefore loses at least 200 grams of carbon every day and roughly a third of that weight loss is achieved during eight hours of sleep. 
 
 "Keeping the weight off simply requires that you put less back in by eating than you've exhaled by breathing," explain the authors. 
 
 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.'" The authors recommend these concepts be included in secondary school science curriculums and university biochemistry courses "to correct widespread misconceptions about weight loss." 
 
 ### ||||| 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. We encountered widespread misconceptions about how humans lose weight among general practitioners, dietitians, and personal trainers (fig 1⇓). Most people believed that fat is converted to energy or heat, which violates the law of conservation of mass. 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. We present a novel calculation to show how we “lose weight.” 
 
 Fig 1 Responses of a sample of doctors, dieticians, and personal trainers to the question “When somebody loses weight, where does it go?” (Correct answer CO 2 ) 
 
 Weight we want to “lose” Excess carbohydrate or protein in the diet is converted to triglyceride and stored in the lipid droplets of adipocytes. Excess dietary fat needs no conversion other than lipolysis and re-esterification. People who wish to lose weight while maintaining their fat-free mass are, biochemically speaking, attempting to metabolise the triglycerides stored in their adipocytes. The chemical formula for an average triglyceride molecule can be deduced from fatty acid composition studies. In 1960, Hirsch and colleagues published data that yield an “average fatty acid” with the formula C 17.4 H 33.1 O 2 .1 This 50 year old result is in remarkable agreement with more recent data.2 Three “average fatty acids” esterified to the glycerol backbone (+3C, +6H) give an “average triglyceride” with the formula C 54.8 H 104.4 O 6 . The three most common fatty acids stored in human adipose tissues are oleate (C 18 H 34 O 2 ), palmitate (C 16 H 32 O 2 ), and linoleate (C 18 H 32 O 2 ),1 2 which all esterify to form C 55 H 104 O 6 . 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. 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. To calculate these values, we traced every atom’s pathway out of the body. The carbon and hydrogen atoms obviously depart as CO 2 and H 2 O, respectively. The fate of a triglyceride molecule’s six oxygen atoms is a conundrum solved in 1949 by Lifson and colleagues.3 They used labelled heavy oxygen (O18) to show that the oxygen atoms of body water and respiratory carbon dioxide are rapidly exchanged through the formation of carbonic acid (H 2 CO 3 ). A triglyceride’s six oxygen atoms will therefore be shared by CO 2 and H 2 O in the same 2:1 ratio in which oxygen exists in each substance. In other words, four will be exhaled and two will form water. 
 
 Novel calculation The proportion of a triglyceride molecule’s mass exhaled in CO 2 is the proportion of its molecular weight (daltons) contributed by its 55 carbon atoms plus four of its oxygen atoms: (661 Da (C 55 )+64 Da (O 4 ))/(861 Da (C 55 H 104 O 6 ))×100=84% The proportion of mass that becomes water is: (105 Da (H 104 )+32 Da (O 2 ))/(861 Da (C 55 H 104 O 6 ))×100=16% These results show that the lungs are the primary excretory organ for weight loss (fig 2⇓). The water formed may be excreted in the urine, faeces, sweat, breath, tears, or other bodily fluids. Fig 2 When somebody loses 10 kg of fat (triglyceride), 8.4 kg is exhaled as CO 2 . The remainder of the 28 kg total of CO 2 produced is contributed by inhaled oxygen. Lungs are therefore the primary excretory organ for weight loss. (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. In a day spent asleep, at rest, and performing light activities that double the resting metabolic rate, each for 8 hours, this person exhales 0.74 kg of CO 2 so that 203 g of carbon are lost from the body. For comparison, 500 g of sucrose (C 12 H 22 O 11 ) provides 8400 kJ (2000 kcal) and contains 210 g of 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 39 g of carbon from the body, raising the total by about 20% to 240 g. 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. Our calculations show that the lungs are the primary excretory organ for fat. 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. 
 
 Notes Cite this as: BMJ 2014;349:g7257 
 
 Footnotes Competing interests: I have read and understood BMJ policy on declaration of interests and have no relevant interests to declare 
 
 This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.
– 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.)