Surprising study: Where fat goes when you lose weight

Where the fat goes when you lose weight

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.

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"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," - Ruben Meerman

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"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.

According to their paper, published in the British Medical Journal, 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.

Exposed: Black hole of understanding in weight loss

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.

The authors met when Mr Meerman interviewed Professor Brown in a story about the science of weight loss for the Catalyst science programme 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."

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.

Breathe the fat out

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 become 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.

"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.

Could breathing more often help you lose weight?

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.

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