Where does the fat go when you lose weight?

Lungs are the primary excretory organ for weight loss. – Photo: R. Prasad
Many won’t know the answer to this question and there is no reason to feel embarrassed as general practitioners, dietitians and personal trainers themselves were not sure of the answer.

A study undertaken by two researchers and published in the journal BMJ found widespread misconception about the fate of fat among doctors, dietitians and personal trainers.  The misconceptions ranged from “fat is converted to energy or heat, metabolites of fat are excreted in the faeces or converted to muscle.” Nearly 70 per cent of 50 doctors and 50 dietitians thought fat is converted into energy, which violates the law of conservation of energy.

The correct answer is: lungs are the primary excretory organ for weight loss!

Let’s see how this is true. We know excess carbohydrate and protein consumed are converted to triglyceride and stored in fat cells (adipocytes).  When more energy is used than consumed, the body dips into the fat reserve to release energy. Biochemically speaking, the triglycerides stored in the fat cells are metabolised when we lose weight.

The fat cells release more triglycerides when energy expenditure is more than energy intake and results in fat cells shrinking in size.  But the fat cells never completely disappear and that is the reason why they easily bulge in size when energy intake is more than energy spent.

The triglycerides are metabolised by combining with oxygen. Broadly speaking, the process can be summarised as:

Triglyceride (C55H104O6) + 78O2 → 55CO2 + 52H2O + energy

Though energy is also produced when fat is burned, the proportion of carbon dioxide and water produced is much higher. According to the BMJ paper, when 10 kg of fat (triglyceride) is burnt, 8.4 kg of it is exhaled as carbon dioxide and 1.6 kg is lost as water.

Energy lost
Responses of a sample of doctors, dieticians, and personal trainers to the question “When somebody loses weight, where does it go?” (Correct answer CO2). – Image: BMJ
“These results show that the lungs are the primary excretory organ for weight loss,” the paper notes. “The water formed may be excreted in the urine, faeces, sweat, breath, tears or other bodily fluids.”

Ruben Meerman, a physicist and Australian TV science presenter and the first author of the paper lost 15 kilograms in 2013 and simply wanted to know where those kilograms were going. “After a self-directed, crash course in biochemistry, I stumbled onto this amazing result,” he told University of New South Wales.  “The fact that almost nobody could answer it took me by surprise, but it was only when I showed Andrew Brown, [head of the School of Biotechnology and Biomolecular Sciences, UNSW] my calculations that we both realised how poorly this topic is being taught.”

Lifting the veil on weight loss, the authors point out that a person weighing 70 kg spending a day asleep, at rest, and performing light activities would exhale 0.74 kg of carbon dioxide; the carbon lost from the body is 203 g.

By exercising for one hour such that his metabolic rate is seven times that of resting would allow the person to remove an additional 39 g of carbon. So the total carbon removed from the body works out to 240 g.


Alas, eating a single 100 g muffin would meet 20 per cent of his daily energy requirement. “Physical activity as a weight loss strategy is, therefore, easily foiled by relatively small quantities of excess food,” they underline.



One thought

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