You’ve lost weight. Where does the fat go, though? Source: Supplied

Neither do most doctors, a study has found.

The correct answer is that 84 per cent is breathed out as carbon dioxide.

The rest disappears as water, in the form of urine, perspiration, tears and so on.

“It goes into thin air,” says Ruben Meerman, a physicist and lead author of the study published in the British Medical Journal. Meerman, an Australian TV science presenter, and University of NSW’s Professor Andrew Brown report for the first time precisely where fat goes when it is “lost”.

In their paper they show that for every 10kg of fat lost, 8.4kg will be exhaled as carbon dioxide and the remainder will become water.

A woman before and after her diet. Source: Getty Images

“Basically in fat there are three types of atoms — carbons, hydrogens and oxygens,” says the head of UNSW’s School of Biotechnology and Biomolecular Sciences.

“You know that the carbons have to come out as carbon dioxide and the hydrogens have to come out as water.

“And then Ruben went back to a paper from 1949 which did some clever biochemistry to show you how the oxygen is partitioned between the carbon dioxide and the water.” As part of his study, Meerman also surveyed 150 doctors, dietitians and personal trainers and discovered most of them incorrectly thought that fat is lost by being converted to energy or heat.

Only three of the health professionals knew the correct answer. One of the most frequently asked questions Meerman encountered during the study is whether simply breathing more can cause weight loss.

“The answer is no, unless you’re moving more, that’s called hyperventilation,” he says.

Healthy eating looks appealing. Source: Getty Images