When you lose weight, have you ever wondered where the fat goes? Most people believe that fat is converted to energy or heat, which violates the law of conservation of mass. According to researchers from the University of New South Wales in Australia, majority of the fat is exhaled or breathed out as carbon dioxide during weight loss (BMJ 2014;349:g7257).
Professor Andrew Brown and Ruben Meerman stated that excess carbohydrates or protein in the diet is transferred to triglyceride and placed in the lipid droplets of adipocytes. Extra dietary fat does not need conversion other than lipolysis and re-esterification (BMJ 2014; 349:g7257). When people desire to lose weight while keeping their fat-free mass, they are attempting to metabolize those stored triglycerides.
The complete oxidation of a single triglyceride molecule involves many enzymes and biochemical steps and the atoms that make up this formula are carbon, hydrogen and oxygen. Researchers wanted to study the fat storage that leaves as carbon dioxide or water during weight loss so they traced every atom’s pathway out of the body (BMJ 2014;349:g7257). The results showed that the lungs are the primary excretory organ for weight loss and when someone loses 10 kg of fat (triglyceride); 8.4 kg is exhaled as carbon dioxide. The rest of the 28 kg total of CO2 produced is contributed by breathing in oxygen.
When resting, the average 70 kg person consuming a mixed diet exhales 200 ml of carbon dioxide in 12 breaths per minute. At one day of sleeping, resting, and performing light activities that double the resting metabolic rate, each for 8 hours, a person exhales 203g of carbon from the body. If you replace just one hour of rest with exercise, you are increasing the metabolic rate to seven times that of by resting, for instance, jogging eliminates an additional 39g of carbon from your body, increasing the total by nearly 20% to 240g (BMJ 2014;349:g7257). “Physical activity as a weight loss strategy is easily foiled by relatively small quantities of excess food”, says Professor Brown and Meerman.
The main purpose of this study was to demonstrate that the lungs are the primary excretory organ for fat. Weight loss requires unleashing the carbon stored in fat cells, which reinforce the misconception of “eat less, move more.” The researchers recommend these concepts to be included in secondary school science curriculums and university biochemistry courses to correct widespread misconceptions or myths about weight loss.
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Source: BMJ 2014; 349:g7257