Into thin air:

The correct answer is that most of the mass is breathed out as carbon dioxide. It goes into thin air.

Ruben Meerman, a physicist, and Brown write in the British Medical Journal that losing 10 kilograms of fat requires 29kg of oxygen to be inhaled.

This metabolic process produces 28kg of carbon dioxide and 11kg of water.

Meerman became interested in the biochemistry of weight loss through personal experience. This is what happened:

"I lost 15kg in 2013 and simply wanted to know where those kilogrammes were going. After a self-directed, crash course in biochemistry, I stumbled onto this amazing result. With a worldwide obesity crisis occurring, we should all know the answer to the simple question of where the fat goes. The fact that almost nobody could answer it took me by surprise, but it was only when I showed Andrew my calculations that we both realised how poorly this topic is being taught."

The authors met when Meerman interviewed Brown in a story about the science of weight loss for the Catalyst science program on ABC TV in March this year.

"Ruben's novel approach to the biochemistry of weight loss was to trace every atom in the fat being lost and, as far as I am aware, his results are completely new to the field," Brown says.

"He has also exposed a completely unexpected black hole in the understanding of weight loss amongst the general public and health professionals alike."

If you follow the atoms in 10kg of fat as they are "lost," 8.4 of those kilograms are exhaled as carbon dioxide through the lungs. The remaining 1.6kg becomes water, which may be excreted in urine, feces, sweat, breath, tears, and other bodily fluids.

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

More than 50% of the 150 doctors, dietitians, and personal trainers who were surveyed thought the fat was converted to energy or heat.

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

And people say America doesn't care about global warming.  If Americans 14 & older lost our average 20 lbs of excess fat, there would be an additional 11,365,992,582 lbs. of CO2 in the atmosphere.  Kyoto treaty and Jack LaLanne be damned!Fascinating numbers.

Wow, that's mind boggling.

Not sure what's the lesser bad since all of those people will eventually die anyway.

P.S. Yeah science!