What Happens to Body Fat When You Lose Weight?

Some types of bodily functions are easy to understand. We exert energy; we sweat. We drink fluids; we pee. We eat solid food; well, you know the rest. (Don't worry, this isn't one of those gross anatomy lessons.)

When you lose body fat, where does it go? (Image: stevecoleimages/iStock/GettyImages)

But something that's been a bit of a mystery is the ultimate destination of fatty tissue after weight loss. In other words, where does your body fat go after you spend hours pounding the pavement or pedaling your butt off at an indoor cycling class?

When it comes to losing weight, we are sometimes so focused on the digits on the scale that we don't think about the physical space that fat takes up until it's gone and our skinny jeans are fitting better. The long-debated question of the fate of fat cells recently gained some traction — and maybe some answers — in a 2014 article written by two Australian researchers that appeared in the British Medical Journal.

After losing nearly 30 pounds, Ruben Meerman, lead study author and an Australian physicist, began to ponder where those pounds had actually gone. Had they evaporated into thin air or come out as waste?

Dispelling the Most Common Fat-Loss Misconception

What's most surprising is that this type of research "had, for some reason, never been done," Meerman said in a Skype interview. The idea that we consume solids and liquids and expel gases is an unusual idea to many, he said. "We can't see CO(2), so it's hard to think of it as having substance."

Even experts are unaware of this, he said, citing a survey that showed most doctors and dietitians believe that we expel fat through heat or energy. That notion is a common fallacy, he said, and one that he believes could negatively impact someone's weight-loss journey.

"I think it is detrimental to have this misconception for why someone is trying to lose weight — a mystery of where it is going," Meerman said. "I think it's an impediment for people successfully losing weight."

So he sought to calculate how much of the fat became carbon dioxide and how much became water to answer the question: Is it possible that we exhale fat?

Tracking Down Lost Fat

Now for a little chemistry: The human body is made up of carbon, hydrogen and oxygen atoms, so fat would logically be made up of a combination of these atoms too, he said. So if we just take more breaths, will we lose weight? Not exactly. Simply breathing more doesn't mean losing more weight, he said.

"To get rid of 10 kilograms (about 22 pounds) of fat, you'd have to inhale 29 kilograms (nearly 64 pounds) of oxygen to make this chemistry happen," Meerman said. Once inhaled, Meerman said, that metabolic process produces 28 kilograms (nearly 62 pounds) of carbon dioxide and 11 kilograms (about 24 pounds) of water. "You can't do that in one breath. It's going to take time," he said.

From that data, Meerman wanted to know how much of that 10 kilograms of fat would be expelled through the lungs and how much would become water. His calculations indicated that 84 percent of fat becomes carbon dioxide and the other 16 percent becomes water. He even determined the precise chemical formula for fat — C(55)H(104)O(6). So yes, you "exhale 84 percent of the fat you lose," Meerman said.

Meerman had wanted to do a story about his discovery for a TV show called "Catalyst" in Australia, but a physicist with a background in laser engineering wasn't exactly considered an expert in biochemistry. He needed a biochemist to validate what he found. He initially turned to Professor Andrew Brown, head of the School of Biotechnology and Biomolecular Sciences at the University of New South Wales, but Brown was in Finland on sabbatical.

It wasn't until a year later in 2014 that Brown and Meerman finally connected after researcher Claire Smith approached Brown for an interview for the ABC (Australian Broadcast Company) science show about 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," Brown said in a press release for the study. "He has also exposed a completely unexpected black hole in the understanding of weight loss amongst the general public and health professionals alike."

A day after they shot the segment, Brown asked Meerman to co-author a scientific paper about what Meerman had found. The British Medical Journal published the findings in December. "There is surprising ignorance and confusion about the metabolic process of weight loss," Brown said.

So, why is breathing so important? "Because if you don't do it you die," Meerman says with a laugh. "The 'breathing out' part is ultimately the last step. Weight loss is not caused by breathing. It's just the final step."

What's does this mean for you? Well, for one thing, it doesn't mean you should invest a lot of time in crazy breathing techniques, thinking you'll expel more fat. But it should be an encouragement when you hit the gym for an intense sweat session.

You increased breathing rate during Zumba class or a weight-lifting workout not only means you're working harder, burning calories and building endurance and strength, it also means you're helping your body get rid of that fat you're burning.

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