When You Lose Weight, You're Literally Disappearing Into Thin Air

When You Lose Weight, You're Literally Disappearing Into Thin Air

The Question: According to my nutritionist, I’ve lost 15 pounds of fat this year. Where did it all go?

The Answer: Our most predominant metaphors for weight loss are words like “melt” or “burn.” But if you consider the actual biochemistry of the weight loss process, it would be more accurate to say that when you lose fat, you’re literally disappearing into thin air.

If this is news to you, don’t worry -- you’re not alone. Physicist and Australian TV science presenter Ruben Meerman recently surveyed a group of 150 doctors, nutritionists and physical trainers to ask them what they thought happened to body fat in the weight loss process. Their answers: fat is converted to energy or heat (which violates the law of conservation of mass, notes Meerman), fat turns into muscle (physically impossible) or fat gets pooped out.

They were all wrong.

In truth, human fat cells are made up of triglycerides, which are molecules made of carbon, oxygen and hydrogen. When you metabolize fat in your body, the waste products are released as carbon dioxide when you exhale, and water when you excrete body fluids in urine, sweat or tears. Metabolizing those triglycerides involves a complex set of biochemical steps, but the co-authors summarize the process with this formula: CHO+78O→55CO+52HO+energy.

But Meerman and Brown took the calculations one step further and found that 84 percent of the fat lost is through our breath, while 16 percent of it is lost via water.

"Our calculations show that the lungs are the primary excretory organ for fat,” wrote Meerman and biochemistry professor Andrew Brown at the University of New South Wales in their study. "Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of 'eat less, move more.’"

"None of this biochemistry is new, but for unknown reasons it seems nobody has thought of performing these calculations before,” they added in a press release. "The quantities make perfect sense but we were surprised by the numbers that popped out." Their findings were published in the typically quirky Christmas issue of the BMJ.

But just because fat leaves the body as air, it doesn’t mean that weight loss is as easy as breathing. Nor does it mean that upping your aerobic exercise will make you breathe harder and thus exhale more fat. The authors note that their calculations show that “physical activity as a weight loss strategy is, therefore, easily foiled by relatively small quantities of excess food.”

But anyone who’s ever struggled with weight loss already knew that!

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