How Fat Is Lost from the Body

human body systems — (Image credit: alxhar/Shutterstock.com)

When you lose weight, where does it go? Turns out, most of it is exhaled.

In a new study, scientists explain the fate of fat in a human body, and through precise calculations, debunk some common misconceptions. Fat doesn't simply "turn into" energy or heat, and it doesn't break into smaller parts and get excreted, the researchers say.

In reality, the body stores the excess protein or carbs in a person's diet in form of fat, specifically, as triglyceride molecules, which consist of just three kinds of atoms: carbon, hydrogen and oxygen. For people to lose weight, their triglycerides must break up into building blocks, which happens in a process known as oxidation.

When a triglyceride is oxidized (or "burned up"), the process consumes many molecules of oxygen while producing carbon dioxide (CO2) and water (H2O) as waste products. [8 Strange Things Scientists Taste and Eat]

So, for example, to burn 10 kilograms (22 lbs.) of fat, a person needs to inhale 29 kg (64 lbs.) of oxygen. And the chemical process of burning that fat will produce 28 kg (62 lbs.) of carbon dioxide and 11 kg (24 lbs.) of water, the researchers calculated.

"None of this biochemistry is new, but for unknown reasons it seems nobody has thought of performing these calculations before," study authors Ruben Meerman and Andrew Brown of the University of New South Wales in Australia, said. "The quantities make perfect sense but we were surprised by the numbers that popped out."

The researchers showed that during weight loss, 84 percent of the fat that is lost turns into carbon dioxide and leaves the body through the lungs, whereas the remaining 16 percent becomes water, according to the study published today (Dec. 16) in a special Christmas issue of the medical journal BMJ.

"These results show that the lungs are the primary excretory organ for weight loss. The water formed may be excreted in the urine, feces, sweat, breath, tears or other bodily fluids, and is readily replenished," the researchers said.

The calculations also show the frightening power of, for example, a small muffin over an hour of exercise: At rest, a person who weighs 154 pounds (70 kg) exhales just 8.9 mg of carbon with each breath. Even after an entire day, if this person only sits, sleeps, and does light activities, he or she exhales about 200 grams of carbon, the researchers calculated.

A 100 g muffin can cover 20 percent of what was lost.

On the other hand, replacing one hour of rest with exercise such as jogging, removes an additional 40 g of carbon from the body, the researchers said.

Even if one traces the fates of all the atoms in the body, the secret to weight loss remains the same: In order to lose weight, one needs to either eat less carbon or exercise more to remove extra carbon from the body.

Email Bahar Gholipour. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.

Bahar Gholipour is a staff reporter for Live Science covering neuroscience, odd medical cases and all things health. She holds a Master of Science degree in neuroscience from the École Normale Supérieure (ENS) in Paris, and has done graduate-level work in science journalism at the State University of New York at Stony Brook. She has worked as a research assistant at the Laboratoire de Neurosciences Cognitives at ENS.