Despite a global obesity crisis and a diet industry of £2bn in the UK alone, the majority of people do not know where fat goes when you lose weight.
Researchers at the University of New South Wales (UNSW) also found that 50% of doctors, dieticians and personal trainers could not correctly answer this question.
Most people said fat was converted to energy or heat – but this is not the case.
Lead author Ruben Meerman, a physicist and Australian TV science presenter, said: "The correct answer is that most of the mass is breathed out as carbon dioxide. It goes into thin air."
Published in the BMJ, authors found that losing 10kg of fat requires 29kg of oxygen to be inhaled, which produces 28kg of CO2 and 11kg of water.
Researchers found that if you follow the atoms in 10kg of fat they as they are lost, 8.4kg is exhaled as CO2 through the lungs. The rest becomes water, excreted in sweat, breath, urine and other bodily fluids.
"I lost 15kg 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," Meerman said.
"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 [scientists] my calculations that we both realised how poorly this topic is being taught."
Andrew Brown, from UNSW, said: "There is surprising ignorance and confusion about the metabolic process of weight loss.
"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.
"He has also exposed a completely unexpected black hole in the understanding of weight loss amongst the general public and health professionals alike."
Meerman said that the lack of awareness is likely to do with CO2 being invisible when we exhale.
He also said breathing more will not make you lose more weight – it will result in hyperventilation instead.
Researchers also confirmed that weight loss will not increase 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," Meerman said.