Say you've lost weight since this time last year. Where did your former pounds go?
If you're like the majority of physicians, personal trainers and dietitians surveyed in a recent study, you'll answer that the fat you lost was converted to heat or energy. The truth, according to Australian researchers writing in The BMJ , is that you exhaled as carbon dioxide most of the weight you lost.
The researchers note that, in biochemical terms, people trying to lose fat are attempting to metabolize the triglycerides stored in fat cells. The researchers further note that, for triglyceride molecules to be metabolized, they must be broken into their constituents of oxygen, carbon and hydrogen via oxidation.
By tracing these atoms' departure from weight-losing bodies, the researchers determined that the bulk leave as exhaled carbon dioxide. In someone losing 10 kilograms (22 pounds), 8.4 kilograms are exhaled as carbon dioxide, according to the researchers' calculations. The remaining 1.6 kilograms become water and "may be excreted in the urine, feces, sweat, breath, tears, or other bodily fluids," the researchers write.
Thinking that weight lost is converted to heat or energy "violates the law of conservation of mass," the researchers write.
"Our calculations show that the lungs are the primary excretory organ for fat," the researchers conclude. "Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of 'eat less, move more.'"