Where Does Weight Go When You Drop Pounds? The Answer Will Surprise You.

By Joelle Renstrom | 6 years ago

weightloss-578x807When it comes to the science of weight loss, most people are concerned about calorie consumption vs. calorie burning ratios, or whether 100 calories of carbs is different, or worse, than 100 calories of protein. One question I’ve never thought to ask when it comes to losing weight is where that mass actually goes. In a recent paper published in the British Medical Journal, researchers from the University of New South Wales show that most people, even folks whose business is to know about all aspects of weight loss, don’t actually know where the weight goes. The answer is most of it escapes our body ascarbon dioxide when we exhale.

If you think about it, there are many possible answers to the question of where our lost weight goes. Maybe it turns into muscle. Maybe we sweat, poop, or pee it out. Maybe it gets converted to energy or heat. I’ll admit that before I read the study, I probably would have gone with that answer. I wouldn’t be alone, either. According to the study, half of the 150 doctors, nutritionists, and exercise gurus surveyed by the researchers thought the same thing. But hey, it’s not like they’re scientists or anything…oh wait….

The problem with this theory is a little law called the Conservation of Mass, which dictates that while matter can change from a solid to a liquid, or a liquid to a gas, the amount of mass stays the same—in other words, mass can be rearranged or change form, but it can’t be created or destroyed. Ruben Meerman, the lead author of a study, “lost 15 kilograms in 2013 and simply wanted to know where those kilograms were going.” So he studied up on biochemistry and then decided to trace every atom of lost weight to see where it went.

In the study, he followed the atoms of 10 kilograms of lost fat. 1.6 kilograms were converted into water, and then expelled in ways people expect: sweat, tears, urine, feces, etc. But 8.4 kilograms, 84% of the total weight lost, exits the body as carbon dioxide exhalation. The authors also calculated that losing 10 kilograms of fat requires the inhalation of 29 kilograms of oxygen, and produces 11 kilograms of water and 28 kilograms of carbon dioxide.

Of course, that prompted people to start asking whether one can lose weight by breathing more, which would be the easiest and cheapest slim-down plan ever. But alas, no. That would just cause hyperventilation, which as far as anyone knows isn’t an effective weight loss technique. People also wanted to know whether the carbon dioxide emitted via weight loss could contribute to global warming. No need to worry about that. Unlike the ancient carbon trapped in fossils buried under Earth’s surface, when humans exhale, they’re returning carbon dioxide that was previously in the atmosphere.

Given the misconceptions regarding the biochemistry of weight loss, the researchers hope their findings will become part of curricula for doctors and dieticians, as well as for those counting calories.

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