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 oxidized, 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 oxidize 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.
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.
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 percent to 240 g.
"Physical activity as a weight loss strategy is, therefore, easily foiled by relatively small quantities of excess food," the authors said.

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