"These results show that the lungs are the primary excretory organ for weight loss."
They estimate that an average person loses at least 200g of carbon every day and roughly a third of that occurs as we sleep.
Replacing one hour of rest with moderate intensity exercise, such as jogging, removes an additional 40g of carbon from the body, raising the total by about a fifth to 240g.
So to keep weight off you need to balance what you eat against what you burn off and exhale.
"Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often-heard refrain of 'eat less, move more,'" say the researchers.
Duane Mellor of the British Dietetics Association likened fat metabolism to burning petrol in a car - it makes heat and drives movement, but also creates and releases waste.
"The atoms left after breaking down fat for energy are like the exhaust fumes," he said.
Dr Tom Barber, associate professor of endocrinology at Warwick University and University Hospitals Coventry and Warwickshire, said the work was interesting and novel, and busted the misperception that fat is simply burned off as energy - something that even many doctors think.
"But it does not change the health message that we need to do exercise to keep fat off," he said.