Where Does The Fat Go When You Lose Weight?

·Senior Editor

When you drop pounds, what happens to the fat you lose? It can’t just disappear into thin air … or can it? (Photo: Getty Images)

When Australian scientist Ruben Meerman lost 30 pounds last year, one question kept bugging him: Where did the fat go?

The answer might seem obvious: It was burned up, as we say — which implies that it was transformed into heat or energy. And when Meerman polled 150 doctors, dietitians, and personal trainers, that’s what a majority of the experts said, too.

But Meerman knew that couldn’t be true; turning fat into heat would violate a basic principle of chemistry, the law of conservation of mass. Only nuclear reactions turn matter into energy, and, as he says, “humans are not nuclear reactors.”

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The fat must have turned into something else, which was then expelled from the body. But what did it turn into? And where did it end up?

To find out, Meerman conducted some simple but novel calculations. Chemically speaking, fat loss (technically called fat oxidation) occurs when the triglycerides that fill fat cells are converted into carbon dioxide and water. Scientists have known this fact for years. But no one had looked into what exactly happens next. So using the standard formula for fat oxidation, Meerman tracked the path of every atom in a triglyceride molecule through and out of the body.

His discovery: A full 84 percent of fat is exhaled through the lungs as carbon dioxide. The rest becomes water, which is excreted via urine, sweat, tears, and any number of various bodily fluids.

Of the small sample of health professionals surveyed, zero doctors or personal trainers knew that fat became carbon dioxide. Three out of 50 dietitians answered the question correctly. “I was deeply shocked by this gap in the knowledge,” Meerman tells Yahoo Health.

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The study was recently published in the journal BMJ and coauthored by biochemist Andrew Brown, PhD, a professor at the University of New South Wales in Australia.

The research also clarifies the common saying “energy in/energy out.” The maxim refers to the idea that to maintain weight, you need balance the number of calories you consume with the calories your body uses to fuel exercise and other daily functions. Technically, “carbon in/carbon out” would be more precise, according to Meerman’s findings. “For weight loss, carbon is the key element to be excreting from the body,” he says. 

On a typical day, the average person breathes out about half a pound of carbon through the lungs via carbon dioxide. When you eat food, you replace some of the exhaled carbon atoms. “If the atoms ingested and digested equal the number exhaled, your weight won’t change. If you eat less than you exhale, you’ll lose weight. If you exhale less than you ingest, you’ll gain weight.”

If you’re trying to shed pounds, don’t worry too much about all of this talk of carbon atoms and oxidation. Balancing the calories you eat with the calories you expend is a sound strategy. In fact, it’s what Meerman did when he trimmed down last year.

“I was drinking three cappuccinos made with full cream milk each day, and I’d hit 40 years of age,” he says. “Your metabolism slows a bit as you get older so you can’t consume as many calories and expect to stay slim. So I cut out those coffees, started monitoring and counting the calories I was eating plus how many I was burning.”

By tracking his diet, he lost the extra padding around his belly, and now he finally knows where it went: into thin air.

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