Food / Healthy People/ Groups / Lifestyle / Physical / Research

What happens to “burned” fat?

When you burn fat, what exactly happens to it?

Does it shrink into smaller little balls of fat?

Does it literally turn into muscle?

When I asked a students around campus, the response to the question was met with an overwhelmingly amount of “I have no idea”s and “I really don’t know”s.

Well, when I stumbled across a recent study suggesting that we actually breathe our fat out, I was stunned, and yet it’s true. The primary organ for fat loss is in fact the lungs. The mass of our fat is actually breathed out as carbon dioxide.

Believe it or not, your lungs are primarily responsible for fat loss

Believe it or not, your lungs are primarily responsible for fat loss

The study began with an Australian TV personality who wanted to know where his fat went when he lost a substantial amount of weight. He teamed up with Andrew Brown from the University of New South Wales, who calculated the proportion of the mass stored in fat that exits as carbon dioxide and as water when we lose weight. Basically, we breathe most of our fat out as carbon dioxide and the little remaining fat is excreted in our urine, sweat, tears and other bodily fluids.

In order to verify this study, Tonic approached Dr Adrienne Edkins, a senior lecturer in biochemistry at Rhodes University, who wrote the following contributory response to the study:

“The article by Meerman and Brown examines the myths about what happens to fat when it is metabolised. This is an important topic, particularly in the current climate of fad diets. Over 50% of people questioned (including GPs, dieticians and personal trainer) thought that fats were converted to heat or energy. However, this assumption cannot be true as it is contrary to the law of conservation of mass. This law states that in order for fat to be ‘lost’ it must actually be converted into a different form that can then be excreted from the body. That alternate form is in fact, carbon dioxide.  Fats are made up of a combination of carbon, hydrogen and oxygen atoms. Through a series of complex chemical reactions in the body, fat is combined with large amounts of oxygen to be converted into carbon dioxide and water. However, this does not mean that this reaction does not result in the production of energy. This metabolism is induced by aerobic exercise and, in addition to the carbon dioxide/water, it also releases of energy (in the form of the chemical ATP). The difference is that the energy is generated by the breaking of the bonds in the fats, not by conversion of the fats directly to ATP. This may be the reason behind the misconception that fat is lost as energy. However, the carbon, hydrogen and oxygen that make up the fat molecule do not physically become part of the ATP molecule. The authors used fats containing labelled atoms that they could trace in the body. Using this approach they showed that carbon, hydrogen and oxygen that make up the fats are physically converted to carbon dioxide and water. The energy released by breaking of the bonds in the fat is used to convert ADP and phosphate into ATP – which then provides energy for the body to do work. Therefore, the conversion of fat to carbon dioxide generates energy, but the molecules from the fat molecule do not physically become part of the ATP molecule (energy molecule). The authors showed using a series of biochemical calculations that metabolism of fat requires a large amount of oxygen and that more carbon dioxide than water is formed. For example, the metabolism of 10 kg of fat gives rise to 8.4 kg (84%) of carbon dioxide and 1.6 kg (16%) of water. The water excreted in urine or sweat is the minor component. The major component is carbon dioxide, which is exhaled via the lungs and lost into the air. Therefore we breathe out the breakdown products of fats. This study not only dispels the myths about where fat goes when it is lost, but it also sheds light on the process of losing weight. The fact that the majority of fat is lost as carbon dioxide means that in order to lose weight, you need to mobilise the carbon stored in your fat cells. Routine daily activities will mobilise carbon, but exercise will be required to increase this level. One hour of exercise will increase the loss of carbon (excreted as carbon dioxide) by about 20% (1/5) compared to an hour of rest. However, this is a relatively small amount of carbon- roughly equivalent to the amount in a 100g muffin. This means that exercising will not result in weight loss if you are over-eating even by a small amount. Therefore it is likely that the approach of a combination of a balanced diet and exercise is the best approach to losing weight – i.e. use more carbon than you consume. This is very topical in the current environment of obesity, inactivity and extreme/fad diets.”

Edkins not only confirms the study, but also suggests the importance of this information with regards to effective and healthy weight loss. This study throws fad diets and preferential approaches to weight loss, such as choosing exercise or dieting but not both, out the window, proving that the best way to lose weight to exercise enough and to eat healthily and within a certain amount.


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