WHERE does the fat go when people lose weight? It seems a simple question, but it is one many health professionals can't correctly answer.
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A UNSW study shows the most common misconception among doctors, dietitians and personal trainers is that the missing mass has been converted into energy or heat.
"The correct answer is that most of the mass is breathed out as carbon dioxide," said study lead author Ruben Meerman, a physicist and Australian TV science presenter.
In their paper published in the British Medical Journal, the authors show that losing 10kg of fat requires 29kg of oxygen to be inhaled, and that this metabolic process produces 28kg of carbon dioxide and 11kg of water.
Mr Meerman became interested in the biochemistry of weight loss through personal experience.
"I lost 15kg in 2013 and simply wanted to know where those kilograms were going," he said.
"After a self-directed, crash course in biochemistry, I stumbled onto this amazing result."
He was surprised that almost nobody could answer the question of where fat goes.
"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," said UNSW School of Biotechnology and Biomolecular Sciences Professor Andrew Brown.
"He has also exposed a completely unexpected black hole in the understanding of weight loss among the general public and health professionals alike."
The authors say if you follow the atoms in 10kg of fat as they are "lost", 8.4 of those kilograms are exhaled as carbon dioxide through the lungs. The remaining 1.6kg becomes water, which may be excreted in urine, faeces, sweat, breath, tears and other bodily fluids.
"None of this is obvious to people because the carbon dioxide gas we exhale is invisible," Mr Meerman said.
More than half the 150 doctors, dietitians and personal trainers surveyed thought the fat was converted to energy or heat. Some respondents thought the metabolites of fat were excreted in faeces or converted to muscle.
One of the most frequently asked questions encountered by the authors 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," Mr Meerman said.
"The carbon atoms human beings exhale are returning to the atmosphere after just few months or years trapped in food that was made by a plant."
