Where ‘Lost’ Fat Really Goes When You Exercise

“If you would get exercise, go in search of the springs of life.” —Henry David Thoreau, “Walking”

In A Nutshell

It’s a common misconception that the fat we lose while dropping weight is merely converted to energy or sweated out, but in actuality we mostly breathe out the fat in the form of carbon dioxide. This is because, when broken down, fat recombines into predominantly carbon dioxide and a lesser amount of water. The carbon dioxide escapes through our lungs while the small amount of leftover water is excreted through bodily fluids.

The Whole Bushel

As any struggling dieter knows, there’s nothing quite like the thrill of seeing the number on the scale dip progressively lower. But amid the excitement of dropping pounds, we may find ourselves curiously wondering, “Where, exactly, did the weight go?” After all, people frequently say that they’ve “lost” weight, yet no one really discusses where the fat goes. Our grade school teachers told us that matter can neither be created nor destroyed, so we know it can’t just disappear. Do we sweat it out? Is it converted to energy? Or do we simply flush it down the toilet? It turns out, none of those common misconceptions are entirely correct. According to a paper recently published in the British Medical Journal, the majority of our “lost” fat is breathed out as carbon dioxide.

Ruben Meeman, a physicist and the lead researcher on the paper, became interested in the subject after losing some weight himself. Of course, with his inquisitive and scientific nature, he wasn’t content to just speculate about where his fat went—he had to get to the bottom of it. So, along with a team of fellow researchers, he set out to track what happens to fat on the atomic level as our bodies metabolize it.

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As Meeman explains in his paper, when fat is broken down and its chemical bonds are separated, heat and fuel are released, which then power the muscles and energize the body. However, even though the bonds are broken, the constituent parts of the fat (the atoms) still remain. Bodily fat, or triglycerides, are made up of only three kinds of atoms, including carbon, hydrogen, and oxygen. Once a triglyceride is broken down, those atoms recombine into two new molecules: water (H20) and carbon dioxide (CO2). Surprisingly, the CO2 molecules outnumber the H20 molecules by about 4:1, which means that most of our broken down fat leaves our bodies through our breath while the small amount of water leftover is excreted through urination, sweat, tears, and other bodily fluids.

Lost “water weight” is easily put back on just by drinking fluids, but the only way to restore exhaled carbon is by eating more food. This fact, much to the chagrin of lazy dieters everywhere, reinforces the notion that the only way to truly lose weight is by eating less and exercising more. And no, Meeman says we can’t simply breathe more to initiate weight loss, as that will only lead to hyperventilation.

Interestingly, this science explains what happens to bodily pounds that seem to mysteriously vanish while we sleep. Around one-third of all carbon lost in a day is exhaled while we sleep and we also lose a small amount of water through respiration and sweat. Thus, even if we step on the scale before our inaugural morning bathroom visit, we might still weigh less than the night before.

Incidentally, most doctors, dieticians, and personal trainers have no clue what really happens to burned off fat, since, when surveyed, the majority of them said the fat was just converted to energy or heat. Obviously, even health professionals have a lot to learn about fat loss.

Show Me The Proof

ScienceDaily: When you lose weight, where does the fat go?
BBC News: Fat ‘breathed out’ of body via lungs, say scientists

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