Want to lose fat? Just breathe it out

When we lose weight, most of it is… Well, just exhaled. In a recent study published yesterday (Dec. 16th) in the British Medical Journal, a group of researchers from the University of New South Wales (Sydney, AU) explain where the extra fat goes when we lose weight, and in the process they also clarify some of the most common misconceptions.

When we lose weight most of the mass is breathed out as carbon dioxide (credit: www.bodyscience.com.au)

When we lose weight, most of the mass is breathed out as carbon dioxide (credit: www.bodyscience.com.au)

Trainers, dieticians and even doctors might tell you that when you lose weight the lost mass has been converted into energy or heat – actually more than 50% of the 150 ‘experts’ interviewed erroneously stated that. This is not actually the case as it would – quite problematically – violate the Law of Conservation of Mass. The body stores the excess of proteins or carbohydrates in fat, more specifically in triglyceride molecules which consist of carbon, hydrogen and oxygen. To lose weight, these triglycerides must break up into their building blocks during a process called oxidation, where molecules of oxygen are consumed while producing carbon dioxide (CO2), water (H2O) and other waste products. “There is surprising ignorance and confusion about the metabolic process of weight loss” – Professor Andrew Brown involved in the study commented – “The correct answer is that most of the mass is breathed out as carbon dioxide. It goes into thin air”.

In the paper, the authors also did some simple maths about it. To burn 10 kilograms of fat, a person needs to inhale 29 kg of oxygen. The chemical process of burning that fat produces 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” – authors Ruben Meerman and Andrew Brown said. During weight loss, roughly 84 percent of the fat that is lost turns into carbon dioxide and leaves the body through the lungs, whereas the remaining 16 percent becomes water, according to the study. “These results show that the lungs are the primary excretory organ for weight loss. The water formed may be excreted in the urine, feces, sweat, breath, tears or other bodily fluids, and is readily replenished” – the researchers concluded.

The authors also clarified that breathing more would not cause weight loss, but only hyperventilation. They also tackled the question of whether weight loss increases global warming – it does not. The carbon atoms we exhale are returning to the atmosphere after just a few months or years trapped in food that was made by a plant.

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Carlo Bradac

Carlo Bradac

Dr Carlo Bradac is a Research Fellow at the University of Technology, Sydney (UTS). He studied physics and engineering at the Polytechnic of Milan (Italy) where he achieved his Bachelor of Science (2004) and Master of Science (2006) in Engineering for Physics and Mathematics. During his employment experience, he worked as Application Engineer and Process Automation & Control Engineer. In 2012 he completed his PhD in Physics at Macquarie University, Sydney (Australia). He worked as a Postdoctoral Research Fellow at Sydney University and Macquarie University, before moving to UTS upon receiving the Chancellor Postdoctoral Research and DECRA Fellowships.

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