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Do You Know Where Your Lost Fat Goes When You Lose Weight?

Tim Boyer's picture
lost fat in weight loss

Here’s a weight loss brain tickler that a high percentage of doctors and dieticians got wrong in a recent survey: When you lose weight, where does the fat actually go? Read on and discover the correct answer to this fat-burning question and impress your friends the next time you talk about dieting and how it works.


According to a recent issue of The British Medical Journal (BMJ), two researchers performed a small survey among people who should know the answer to the titled question about where fat goes when a person loses weight. The survey sampled 50 doctors, 50 dieticians and 50 personal trainers to see just how knowledgeable they are when it comes to a relatively simple metabolic process that our nation is obsessed with—the burning off of fat.

Surprisingly, an exceptionally high percentage of the surveyed experts got it wrong. The most common wrong answer was that fat is converted to energy or heat. This in turn was followed by other wrong answers such as that fat is metabolized and eventually excreted as fecal matter; or, is converted to muscle. Only a few dieticians answered correctly.

The short answer to this question is that we essentially exhale out our lost fat.

The longer version of the answer takes us out on a little drive down memory lane in college for those of us who took Biochemistry 101.

The most common fat we store in our bodies is a triglyceride that has the chemical formula C55H104O6+78O2. When fat is oxidized (burned off) we learned from the relatively simple equation―C55H104O6+78O2 —> 55CO2+52H2O+energy―that fat is converted into carbon dioxide, water and energy.

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Extrapolating from this chemical reaction, the authors calculated that in order to lose 10 kilograms (22 pounds) of fat, a person would have to inhale 29 kilos (~64 pounds) of oxygen and then produce 28 kilos (~62 pounds) of carbon dioxide and 11 kilos (~24 pounds) of water as the fat is melting away.

On the mass level, this translates to that of the fat lost during weight loss, 84% winds up as exhaled carbon dioxide (CO2) that is released into our atmosphere and 16% as water (H2O) that is lost as urine, feces, sweat, breath, tears, or other bodily fluids. In other words, as the authors of the study succinctly put it, “Lungs are therefore the primary excretory organ for weight loss.”

As an example of what this means when it comes to dieting, the authors provide further example of just how much fat we burn at rest; how much we burn with some added activity; and, how that even just a small amount of extra food such as a single muffin can thwart weight loss.

The authors concluded that, “…losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of ‘eat less, move more.’ We recommend these concepts be included in secondary school science curriculums and university biochemistry courses to correct widespread misconceptions about weight loss.”

Image Source: Courtesy of BMJ

Reference: “When somebody loses weight, where does the fat go?” BMJ Published 16 Dec. 2014