Despite generally being good at helping clients banish excess fat, a surprisingly high number of personal trainers and health professionals have little or no idea of where it actually goes.

That is the finding of a new study from the University of New South Wales, which examined the biochemistry of weight loss and the ignorance that surrounds it.

The most common misconception among health professionals is that the missing mass has been converted into energy or heat. More than half of the 150 doctors, dieticians and personal trainers who were surveyed thought the fat was converted to energy or heat.

"There is surprising ignorance and confusion about the metabolic process of weight loss," said professor Andrew Brown, head of the UNSW School of Biotechnology and Biomolecular Sciences.

"The correct answer is that most of the mass is breathed out as carbon dioxide. It goes into thin air," said the study's lead author, Ruben Meerman, a physicist and Australian TV science presenter.

In the paper When somebody loses weight, where does the fat go, published in the BMJ, the authors show that losing 10 kilograms of fat requires 29 kilograms of oxygen to be inhaled. This metabolic process produces 28 kilograms of carbon dioxide and 11 kilograms of water.

"None of this is obvious to people because the carbon dioxide gas we exhale is invisible," added Meerman.

"This violates the Law of Conservation of Mass. We suspect this misconception is caused by the energy in/energy out mantra surrounding weight loss."

The paper’s authors note that there is a “surprising unfamiliarity” about basic aspects of human biology. They recommend that the concept should be included in secondary school curricula and university biochemistry courses to correct the widespread misconceptions about weight loss.