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Where Does Fat Go When You Lose Weight? Mostly Into Thin Air

By Richard Knox

A couple of years ago, Ruben Meerman took off 40 pounds. And that got him wondering: What exactly happened to all that fat?

Conventional wisdom was that he “burned” it off. Or sweated it off. Or excreted it. None of that satisfied Meerman, who has a physics degree and makes his living explaining science to schoolkids and for the Australian Broadcasting Corporation.

So Meerman tackled the problem and eventually came up with a surprising answer: Most of the lost fat disappears into thin air.

More specifically, 84 percent of those fat molecules get exhaled as colorless, odorless carbon dioxide. The other 16 percent departs the body as H-2-O — plain old water.

Meerman says the discovery “got me really excited because I'd stumbled onto a gap in the knowledge. It struck me as remarkable that no one had thought this was interesting enough to pursue.”

The British Medical Journal thought so too. It has published a paper, co-authored by biochemist Andrew Brown of the University of South Wales, in its annual Christmas issue, which features off-beat (but peer-reviewed) research.

Weight Loss Realism

Meerman hopes the work will dispel misconceptions held by health professionals as well as the general public. And, he hopes it will provide a helpful dose of realism to counter the impossible expectations millions have about weight loss.

If people understand where the fat goes (and how), they’ll get “why there’s a limit to how quickly you can lose weight,” Meerman said in a Skype interview from Sydney. “And if you understand the limit, you won’t be so quickly depressed if you don’t lose 20 pounds in the first two weeks.”

First, the misconceptions. Meerman and Brown surveyed 150 professionals — split equally among family doctors, dietitians and personal trainers — about where they think the fat goes during weight loss.

By far the most common answer was that the fat was transformed into energy or heat — that is, "burned off." About two-thirds of doctors thought so. A slightly higher proportion of dietitians did too, and about 55 percent of personal trainers.

But that would violate the Law of Conservation of Mass. It’s a basic precept of chemistry, formulated in 1789 by the French scientist Antoine Lavoisier, which holds that mass is neither created nor destroyed in chemical reactions. The total mass at the end must equal the mass at the starting point — even if matter is quite transformed in the process, from solid to liquid or gas.

The Energy Of A Bomb

Meerman points out that if fat were transformed into pure energy during weight loss, the results would be cataclysmic.

Einstein’s famous equation says so: energy equals mass times the speed of light squared.

“If you lost 10 kilograms [22 pounds] of fat by turning it into pure energy,” Meerman says, “by Einstein’s equation it would generate about 20 kiloton-equivalents of TNT — almost precisely the energy released by the bomb dropped on Nagasaki. Humans just don’t do it that way.”

Thankfully not.

So how does it work? According to Meerman and Brown, like this:

Fat in our bodies is stored as a chemical called triglyceride. The average triglyceride molecule is made up of 55 carbon atoms, 104 hydrogen atoms and 6 oxygen atoms. “That triglyceride stuff is what you’re trying to get rid of,” Meerman says.

To do that, your body has to draw on the stored energy in triglyceride. That requires breaking it down into its elements — a process called metabolism. As long as you keep ingesting more of those elements than you immediately need — especially carbon atoms in food – that won’t happen.

Losing Carbon

But if you eat fewer carbon atoms than your body needs to metabolize for fuel, triglycerides will be broken down and the fat cells that contain them will shrink away...slowly.

The rate-limiting step in this process is the amount you can breathe out, in the form of oxygen and carbon dioxide (two carbon atoms plus one oxygen).

“With each breath, you lose about 10 milligrams of carbon — a tiny little bit,” Meerman says. “In a full day, where you’re spending the day at rest, you’ll breathe about 17,280 times — 12 times a minute on average.”

If you divide that 24 hours into equal periods of sleep, sitting at your desk, and light activities that double your resting metabolic rate, Meerman says you’ll lose about 203 grams of carbon in the form of carbon dioxide. That’s seven ounces, or a little less than half a pound.

“That’s the limit on how much you can lose,” Meerman says. “If you don’t eat a single thing that day, you’ll precisely lose that much.”

But of course, you will be eating. So you won’t be losing — unless you eat less carbon than you exhale.

“If you’re a 150-pound person and you go for a one-hour jog, you can increase the amount of carbon you exhale that day by about 20 percent,” he says. “But you can put that much into your body by eating a good-sized muffin. It’s just so easy to overeat.”

Of course, we know that weight loss is possible — you just have to eat less and exercise more, to shift the balance toward carbon depletion. Knowing the chemistry may help demystify the process.

Targeted Tweaking

But what about diets that promise to accelerate the process by tweaking the proportion of proteins, carbohydrates and fats you eat? The Meerman/Brown analysis accounts for that too.

The low-carb Atkins diet, for instance, nudges your metabolism toward a little more carbon depletion by converting more triglycerides into water-soluble chemicals called ketones.

“So you can urinate out a little more carbon than you would on a normal diet and get a little more acceleration in the amount of weight you lose,” Meerman says. “But there will be people who get quite depressed when they don’t get carbohydrates. There’s no one-size-fits-all solution.”

Meerman avoids giving any specific dietary advice. He just wants people to understand the biochemical rules of the game so they’ll have more patience with the process and stick to their carbon-depletion regimen, whatever one they choose.

Global Warming?

By now, some of you might be wondering: If enough people lose weight — breathing out lots more carbon dioxide in the process — will that contribute to global warming? It’s a question Meerman says he’s been getting whenever he talks about his discovery.

The answer is no, don’t worry.

The carbon we carry in our cells is all of pretty recent vintage. It ultimately came from the atmosphere, transformed by plants and sunlight into foodstuffs that were consumed by us and the animals we eat (many of us, anyway). When we die — or lose weight — it gets returned to the atmosphere and recycled endlessly.

“But the carbon we humans release into the atmosphere by burning coal and petroleum is ancient carbon that’s been locked up underneath the Earth’s crust,” Meerman says. “The difference is, our carbon’s going back into the atmosphere no matter what. But if you don’t touch fossil fuels it’s stored away and not going back into the atmosphere.”

In other words, the Earth’s atmosphere can tolerate all the weight loss we can possibly muster.

Richard Knox Twitter Senior Correspondent, CommonHealth
Richard Knox is a senior correspondent for WBUR's CommonHealth.

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