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This Is Where Body Fat Actually Goes When You Lose Weight

No, you don't pee it out.

17 FEB 2018

Despite society's obsession with weight loss, a study has revealed that, surprisingly, most health professionals don't actually know what happens to fat when we "lose it".

The research conducted by a team of Australian researchers back in 2013 calculated exactly what happens to our fat when we shed kilos, and revealed that doctor's leading theories are wrong - we don't convert our missing mass into heat or energy. We actually breathe it out.


Their results, published in the British Medical Journal, reveal that 22 pounds (10 kg) of fat turns into 18.5 pounds (8.4 kg) of carbon dioxide, which is exhaled when we breathe, and 3.5 pounds (1.6 kg) of water, which we then excrete through our urine, tears, sweat and other bodily fluids. 

"The correct answer is that most of the mass is breathed out as carbon dioxide. It goes into thin air," said lead author of the paper Ruben Meerman, a physicist and TV presenter.

Meerman first became interested in the biochemistry of weight loss when he dropped 33 pounds (15 kg) - but when he asked doctors where this weight went, he was surprised by the fact no one could tell him.

After surveying 150 doctors, dieticians and personal trainers, he discovered that more than half thought that fat was converted into heat or energy as we break it down. 

But, as a physicist, Meerman knew that this would violate the Law of Conservation of Mass.

We put on weight when excess carbohydrates and proteins that we've eaten are converted into triglycerides (compounds made up of carbon, hydrogen and oxygen) and are then stored in lipid droplets inside fat cells.


To lose weight, you need to break down those triglycerides to access their carbon.

The results showed that in order to completely breakdown 22 pounds (10 kg) of human fat, we need to inhale 64 pounds (29 kg) of oxygen (and somewhere along the way, burn 94,000 calories). This reaction produces 62 pounds (28 kg) of CO2 and 24 pounds (11 kg) of water.

"Our calculations show that the lungs are the primary excretory organ for fat," the team writes in the paper.

However, they couldn't work out exactly what was happening to the fat cells in this reaction. After months of research, Meerman discovered a formula from a paper published in 1949 that solved the problem - it showed that oxygen atoms are shared between the carbon and hydrogen in fat at a ratio of 2:1 (forming carbon dioxide and water)

This allowed them to come up with the final figure of 84 percent of a fat molecule's atoms being exhaled as carbon dioxide, and the remaining 16 percent ending up as water.

F2.largeMeerman and Brown, BMJ (2014)

But unfortunately this doesn't mean that simply breathing deeply will help us lose weight - we still need to do the exercise to unlock the carbon and break down the fat in the first place.

"You can only breathe so many times a day; on a day of rest, you breathe around 12 times a minute so 17,280 times you'll breathe in a day and each one takes 10 milligrams of carbon with it, roughly," Meerman told ABC Science. "So there's your limit on how much you're going to lose in a day with no exercise."


But will all this fat we're breathing out will contribute to climate change? The short answer is no.

"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," said Meerman in the release.

The funny thing about all of this is that, although most health professionals don't know what happens to fat when we lose weight, most Veritasium viewers probably do. In fact, as this mind-blowing video below explains, not only do we lose our "lost" fat through breathing, if we were in a closed system, we would actually become trees.

A version of this article was originally published in December 2014.


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