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What Happens to Fat When You Lose It?

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You know how weight loss works, right? Burn more calories per day than you take in. Burn upwards of 500 calories a day and you'll lose a pound a week. Simple math. But where does that fat actually go?

If you don't know, don't feel bad: A new study in the British Medical Journal surveyed doctors and biochemistry students and found that even they weren't quite sure. Researchers also surveyed family doctors, nutritionists and personal trainers, an alarming high number of whom thought that fat was converted to energy or heat. But think back to high school chemistry: Matter can neither be created nor destroyed—so this principle wouldn’t hold up, despite how easily we throw around the idea of energy in/energy out.

To illustrate how fat leaves the body, researcher Ruben Meerman traced every atom’s pathway out of the body, and then developed a formula which showed that 80 percent of fat leaves the body through the lungs. So you’re actually releasing most fat through carbon dioxide via your breath—the other 20 percent is released through water in the form of sweat, urine, tears, and other bodily fluids.

Turns out the fitspo trainers love to use (and apparently believe, based on this study), "Sweat is fat crying!" is only a tiny bit true. And all that heat building up in your body? It's just a byproduct of the chemical reaction. But hey, if the phrase works to inspire you, go with it!

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