When You Lose Weight, Where Does It Go?

In the midst of a global obsession with dieting lies an oft-pondered question that not even many healthcare professionals can answer correctly: Where do the lost pounds go? A new study from the University of New South Wales says the mass is exhaled as carbon dioxide.

"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.

Losing 10 kilograms of fat necessitates inhaling 29 kilograms of oxygen, according to Dr. Brown and his research team, who say this process produces 28 kilograms of carbon dioxide and 11 kilograms of water.

The authors traced the atoms in 10 kilograms of fat during the weight loss process and said 8.4 of them were exhaled through the lungs as carbon dioxide, becoming part of the atmosphere.

The rest of the weight -- 1.6 kilograms to be exact -- becomes water that leaves the body in the form of fluids such as urine, feces, sweat, breath and tears.

The authors interviewed 150 health professionals including doctors, dietitians and personal trainers, 50 per cent of whom had mistakenly thought the fat was converted into energy or heat.

"This violates the Law of Conservation of Mass," says co-author Ruben Meerman, a physicist and Australian TV science presenter. "We suspect this misconception is caused by the energy in-energy out mantra surrounding weight loss."

Other troubling misconceptions revealed themselves, for some respondents thought that the fat's metabolites were excreted in feces or converted to muscle.

A frequently asked question was whether breathing more can cause weight loss, the answer to which is no, say the authors.

Breathing more than the body's metabolic rate requires leads to hyperventilation, which can cause dizziness, palpitations and loss of consciousness, according to the authors.

Still others were curious to know whether weight loss could contribute to global warming.

"This reveals troubling misconceptions about global warming which is caused by unlocking the ancient carbon atoms trapped underground in fossilised organisms," says Mr Meerman, who also presents the science of climate change in high schools around Australia. "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."

The authors, whose study was published in the British Medical Journal, recommend including its basic concepts in secondary school curricula.

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