When You Lose Weight, Where Does the Fat Go? It's All Hot Air

When it comes to losing weight, fat seems to simply disappear. In fact, despite the worldwide obsession with diets and fitness, many don't know exactly where body fat goes. A common misconception is that it's converted to heat or energy, but that's not the case.

"There is surprising ignorance and confusion about the metabolic process of weight loss," said Andrew Brown, one of the researchers, in a news release.

So where does all of your fat go when you lose weight? Most of the mass, apparently, is breathed out as carbon dioxide. In fact, losing 10 kilograms of fat requires about 29 kilograms of oxygen to be inhaled. This process then produces 28 kilograms of carbon dioxide and 11 kilograms of water.

"I lost 15 kilograms in 2013 and simply wanted to know where those kilograms were going," said Ruben Meerman, one of the researchers. "After a self-directed, crash course in biochemistry, I stumbled onto this amazing result. 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 Andrew my calculations that we both realized how poorly this topic is being taught."

Following the atoms in 10 kilograms of fat as they are "lost," about 8.4 kilograms are exhaled as carbon dioxide through the lungs. The remaining 1.6 kilograms becomes water, which then is excreted. Surprisingly, though, very few know about this particular conversion. In fact, researchers found that more than 50 percent of the 150 doctors, dieticians and personal trainers that were surveyed thought that fat was converted to energy or heat.

"This violates the Law of Conservation of Mass," said Meerman. "We suspect this misconception is caused by the energy in/energy out mantra surrounding weight loss."

The findings reveal the importance of telling people exactly where weight "goes" once it's lost. With a world obsessed with diet, it's crucial to disseminate accurate information about weight loss.

The findings are published in the journal BMJ.

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