More than 80% of body fat leaves the body through exhaling — making the lungs the primary organ through which we lose weight, researchers say.
Humans have a type of fat in the blood called triglyceride, which consist of three kinds of atoms: Carbon, hydrogen, and oxygen.
Shedding unwanted fat requires unlocking the atoms in triglyceride molecules through a process known as oxidation.
Scientists at the University of New South Wales in Australia found that when 10kg of fat are fully oxidised, 8.4kg leaves the body through the lungs as carbon dioxide. The remaining 1.6kg becomes water.
The oxygen that is required for this process weighs nearly three times more than the fat being “lost”, so to completely oxidise 10kg of human fat, 29kg of oxygen must be inhaled, producing 28kg of CO2 and 11kg of water.
Authors Ruben Meerman and Andrew Brown said: “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. The exhaled carbon can only be replaced by eating food or consuming beverages such as milk, fruit juices, or sugar-sweetened drinks.”
At rest, a person who weighs 70kg exhales around 200ml of CO2 by taking 12 breaths a minute.
So, by breathing out 17,280 times a day, they will lose at least 200g of carbon, with around a third of that weight-loss achieved during eight hours of sleep.
However, to keep the weight off requires putting less back in through eating than is exhaled by breathing — which might be tricky when the turkey and stuffing sandwiches come out on St Stephen’s Day. Going for a run for an hour would help remove an additional 40g of carbon from the body, the researchers say, raising the total loss by around 20%, to 240g.
However, that can be wiped out by a single 100g muffin, which represents around 20% of an average person’s total daily energy requirement.
Prof Brown and Mr Meerman said: “Physical activity as a weight-loss strategy is, therefore, easily foiled by relatively small quantities of excess food.
“Our calculations show that the lungs are the primary excretory organ for fat. Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of ’eat less, move more’.”
The research was published in the Christmas edition of the British Medical Journal.