Where Does The Fat Go When You Lose Weight?

After successfully shedding body fat, we’re often too busy basking in sweet satisfaction to question where that fat actually went. Did it transform into muscle? End up in the toilet? Seep out of our pores as sweat?

If you’re suddenly curious (and stumped), don’t worry: A 2014 survey found that 98 percent of health professionals don’t know where that fat goes either.

Most health experts surveyed assumed that fat we ‘lose’ is just transformed into heat, hence why we often talk about it as something we ‘burn off’—but it doesn’t just zap into thin air!

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Think back to high school chemistry. You probably learned about something called the ‘law of conservation of matter.’ It means that mass cannot be created or destroyed, so fat can’t just disappear, explains Spencer Nadolsky, M.D., diplomate of the American Board of Obesity Medicine and author of The Fat Loss Prescription.

After losing 33 pounds, a physicist-turned-media-personality named Ruben Meerman wanted to get to the bottom of where those pounds actually disappeared to, so he teamed up with lipid (a.k.a. fat) researcher Andrew Brown of the University of New South Wales to investigate.

Meerman and Brown’s study, which was published in The BMJ, looked at the chemistry of what happens to a triglyceride a.k.a. body fat molecule (it looks like this: C55H104O6+78O2) when it’s oxidized or broken down to be used for energy. It’s a complicated process, but that process creates two by-products that explain where our fat goes when we lose it: carbon dioxide (CO2) and water (H2O).

When the researchers measured what happened to 22 pounds-worth of triglycerides ‘lost,’ they found that about 18.5 pounds-worth of carbon dioxide were exhaled through the lungs, while the rest exited the body as water, whether in sweat, urine, or another bodily fluid. So even though we don’t quite breathe or sweat little fat particles, we do excrete the by-products produced when our body breaks down body fat, explains Pennsylvania-based family medicine physician, Rob Danoff, D.O., M.S., F.A.C.O.F.P, F.A.A.F.P.

(Just in case you’re wondering, the carbon dioxide you breathe out doesn’t harm the environment. The researchers encountered that question a lot…)

Related: Why Cardio Is NOT The Best Way To Lose Weight

While this study doesn’t really give us any new information about how to lose weight, it does help us understand how losing weight works—and it’s actually pretty fascinating, right?

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