Where does fat go when you lose it?

Mostly into the air. When your body uses fat for energy, it breaks it down into carbon dioxide and water. Of every 10 pounds of fat, about 8.4 pounds leave through your lungs as carbon dioxide you breathe out. The other 1.6 pounds become water, which leaves the usual ways.

What people guess

Most people guess fat turns into energy, or that it leaves as sweat or in the toilet. Energy is part of it: breaking fat down is how your body powers itself. But the atoms in the fat have to go somewhere, and most of them go out with your breath.

A burning log works the same way. Most of it goes up the chimney as gas, and very little is left in the fireplace. Fat leaves you like that, through your lungs.

The numbers

In 2014, Ruben Meerman and Andrew Brown of UNSW Sydney worked out the chemistry in the BMJ. To burn 10 pounds of fat, you breathe in 29 pounds of oxygen. That makes 28 pounds of carbon dioxide and 11 pounds of water. Of the original 10 pounds of fat, 8.4 leave as carbon dioxide and 1.6 as water. The paper worked in kilograms, and the ratio is the same in any unit.

What it means for you

Breathing faster won’t help. You only breathe out the carbon from fat your body has already broken down for energy, and that happens when you use more energy than you eat. Sweating won’t help either, since sweat is mostly water you replace when you drink. See does sweating more burn more fat.

So the way to send more fat out through your lungs is the plain one: eat a little less than you burn, move more, and keep at it for months.