Where does the fat go when you lose it?

You’ve lost a pound of fat. Congrats, that’s not easy to do during the holidays. But where exactly does it go when you manage to get rid of it?

First, some possible answers:

A. The fat fairy came and took it. That’s why you always weigh less in the morning.

B. You converted it to heat and radiated it into the atmosphere.

C. It’s not really lost, it’s just delayed in Cleveland.

D. You released it as carbon dioxide and water through your lungs.

E. You melted it and excreted it in your urine and feces.

If you didn’t answer D, don’t worry too much. Neither did a bunch of doctors and biochemistry students whom Ruben Meerman queried before writing about all this in a short paper released in the British Medical Journal recently.

“We’re going to remove the mystery,” Meerman said in an interview from Sydney, Australia, where he lives. “Right now, most people, including doctors, have got an idea that’s scientifically incorrect. It’s literally impossible to do what they think is happening.”

Meerman is a former physicist who abandoned that career to take up “science communication,” including work for a popular Australian television show, “Catalyst.” Last year, he lost some weight and began to think about what happens on a molecular level to the kilograms of fat he was shedding.

“I had a little bit of understanding you can’t just turn fat into heat,” he said, though that turned out to be a popular answer when he started asking the question.

Meerman teamed up with Andrew J. Brown, a professor at the University of New South Wales, to write a simple explanation of the process, their small contribution to the war on obesity. To understand it, we unfortunately have to take you back to the dark days of high school or college biochemistry, the Krebs Cycle and how our cells create energy. Also, remember this one important principle: Matter cannot be destroyed. It can only be altered.

OK, here we go: “Complete oxidation of 10 kg of human fat requires 29 kg of inhaled oxygen producing 28 kg of CO2 and 11 kg of H2O,” Meerman and Brown wrote.

“This tells us the metabolic fate of fat but remains silent about the proportions of the mass stored in those 10 kg of fat that depart as carbon dioxide or water during weight loss. To calculate these values, we traced every atom’s pathway out of the body.”

There’s more, but you get the picture. And yes, there is heat released, especially when you exercise, but that is a byproduct of the chemical reaction, not the conversion of fat.

Here’s the bottom line: “Our calculations show that the lungs are the primary excretory organ for fat. Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of ‘eat less, move more.’ We recommend these concepts be included in secondary school science curriculums and university biochemistry courses to correct widespread misconceptions about weight loss.”

Meerman hopes that possession of that knowledge will prompt people to exercise more, increase the number of times they exhale and excrete more converted fat. And perhaps there’ll be a little more realistic about what is possible and how fast. And maybe children will start to get it, too.

“When you do the math and you look at the metabolic reasons for that, it makes nothing but sense,” he said.