Melbourne: Ever wondered where the fat goeswhen we lose weight? Most of it is breathed out as carbon dioxide, according to a new study.

Lungs are the primary excretory organ for weight loss, scientists have found. Ruben Meerman and Andrew Brown at the University of New South Wales set out to calculate how we "lose weight".

Human fat cells store triglyceride, which consists of just three kinds of atoms; carbon, hydrogen and oxygen. Shedding unwanted fat requires unlocking the atoms in triglyceride molecules by a process known as oxidation. By tracing every atom's pathway out of the body, the researchers discovered that when 10 kg of fat are fully oxidised, 8.4 kg departs via the lungs as carbon dioxide (CO2). The remaining 1.6 kg becomes water (H2O). The analysis showed that the inhaled oxygen required for this metabolic process weighs nearly three times more than the fat being 'lost'.

To completely oxidise 10 kg of human fat, 29 kg of oxygen must be inhaled producing a total of 28 kg of carbon dioxide and 11 kg of water. "None of this biochemistry is new, but for unknown reasons it seems nobody has thought of performing these calculations before. The quantities make perfect sense but we were surprised by the numbers that popped out," the study authors wrote in the journal BMJ. "These results show that the lungs are the primary excretory organ for weight loss. The water formed may be excreted in the urine, faeces, sweat, breath, tears, or other bodily fluids and is readily replenished," they added. "The exhaled carbon can only be replaced by eating food or consuming beverages such as milk, fruit juices or sugar-sweetened drinks," the authors said.

At rest, an average 70 kg person exhales around 200 ml of CO2 in 12 breaths per minute. Each of those breaths therefore excretes 33 mg of CO2, of which 8.9 mg is carbon. By simply exhaling 17,280 times, an average person therefore loses at least 200 grams of carbon every day and roughly a third of that weight loss is achieved during eight hours of sleep. "Keeping the weight off simply requires that you put less back in by eating than you've exhaled by breathing," explained the authors.

Replacing one hour of rest with exercise that raises the metabolic rate to seven times that of resting by, for example, jogging, removes an additional 40 g of carbon from the body, raising the total by about 20 per cent to 240 g.

But the authors pointed out that, for comparison, a single 100 g muffin represents about 20 per cent of an average person's total daily energy requirement. "Physical activity as a weight loss strategy is, therefore, easily foiled by relatively small quantities of excess food," the authors said.