When you lose weight, where does it go? Have you ever wondered what happens to the fat when you successfully take off a few pounds?

Australian researchers were concerned that misinformation about weight loss contributes to some people's lack of dieting success. Some people believe, for example, that the extra fat is converted into energy or heat. This can't be true, the researchers said, because it violates a law of physics about the conservation of mass.

So they set out to find what happened to the fat and discovered that you literally breathe it away. Although it's not quite so simple, as dieters know all too well.

The inhaled oxygen required for oxidation weighs almost three times as much as the fat being ‘lost.’

The lungs are the primary route by which we excrete weight loss. Fat from food is stored in the body in adipocytes, or fat cells, as molecules called triglycerides. These triglyceride molecules are made up of carbon, hydrogen, and oxygen. To lose unwanted fat, your body must break down triglycerides into their components by a process known as oxidation.

When 10 kilograms — roughly 22 pounds — of fat are fully oxidized, 8.4 kilograms are exhaled as carbon dioxide (CO2) and 1.2 kilograms are excreted as water in urine, feces, sweat, tears, or other bodily fluids. The exhaled carbon is replaced when you eat or drink something.

“None of this biochemistry is new, but for unknown reasons, it seems nobody has thought of performing these calculations before,” the University of New South Wales researcher Ruben Meerman and Andrew Brown, a professor in the School of Biotechnology and Biomolecular Sciences, said in a press release.

“The quantities make perfect sense, but we were surprised by the numbers that popped out.”

Meerman and Brown calculated that the inhaled oxygen required for oxidation weighs almost three times as much as the fat being “lost.” For example, to oxidize 10 kilograms of fat, a person must inhale 29 kilograms of oxygen. This will yield 29 kilograms of CO2 and 11 kilograms of water.

At rest, the average 70-kilogram (about 154 pounds) person exhales 200 milliliters of CO2 in 12 breaths per minute. With each breath, they excrete 33 mg of CO2, of which 8.9 mg is carbon. So by exhaling 17, 280 times every day, the average person loses at least 200 mg of carbon daily, and about a third of that loss occurs during eight hours of sleep.

To keep weight off you only need to take in less carbon in by eating than you exhale while breathing, Meerman and Brown said. They calculated that replacing one hour of rest with a physical activity such as jogging raises the metabolic rate to seven times the resting rate. This jump in metabolism removes another 40 grams of carbon from the body, so 240 grams of carbon are lost that day.

But here’s the rub. This weight loss is easily undone by relatively small amounts of excess food, said Merman and Brown. They point out in their paper that a 100 g (about 3.5 ounces) muffin represents about 20 percent of a person’s daily energy requirement. So couch potatoes stay on the hook. You still have to exercise.

“Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of ‘eat less, move more,’” Meerman and Brown said. They suggested these concepts be included in secondary school science curriculums and university biochemistry courses “to correct widespread misconceptions about weight loss.”

The study is published in the British Medical Journal.