Why weight loss is a lot of hot air

Ever wonder where the kilograms you burned to get a better holiday body actually went?

This was the question an Australian science television reporter and a professor asked. What happens to fat when it is broken down?

Andrew Brown, head of the School of Biotechnology and Biomolecular Sciences at the University of New South Wales, and TV presenter Ruben Meerman found that when people lose weight the fat is broken down into about 10% water and 90% carbon dioxide.

Water leaves the body through urine, sweat and tears. Carbon dioxide is breathed out.

Their discovery was published in the British Medical Journal yesterday.

Meerman, a physicist, lost 15kg last year. He wondered what exactly had happened to the fat cells so he did "a self-directed, crash course in biochemistry".

Then he "stumbled onto this amazing result".

Human fat cells store triglyceride, which consists of three atoms: carbon, hydrogen and oxygen.

Fat is burned when the bonds between the atoms in triglyceride molecules are unlocked by oxidation.

The authors traced the pathways the atoms took out of the body and found that when 10kg of fat is oxidised, 8.4kg departs from the lungs as carbon dioxide and 1.6kg becomes water.

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

The finding does not change the way people lose weight - which is by burning more energy than is consumed.

Meerman said the finding was important because it reveals why there is a limit to how much weight a person can hope to lose in a day, week, month or year.

He told The Times: "At present, people do not understand this limit and set themselves unrealistic weight-loss goals and inevitably fail. The limit is set by the number of times we exhale per day.

"You can increase the number of times you exhale per minute by exercising but, for most working people, one hour of exercise is about the limit."

Breathing more while sitting down will not help shed kilos - but can cause hyperventilation.