"When somebody loses weight, where does the fat go?"
This is the title of a short article in the British Medical Journal that I came across on a listing service. I couldnt help myself. I had to read the article.
It is not very long - only two pages in length. It is also not very complex having been written for a generalist audience by professor Andrew Brown, head of the University of New South Wales biotechnology and biomolecular sciences and Ruben Meeman, a physicist and Australian TV science presenter.
But it does make a point.
It is Browns and Meemans contention that most people, including medical professionals, have misconceptions about how people lose weight and where that weight goes.
They surveyed 150 professionals with the question: When somebody loses weight, where does it go?
Approximately 65 per cent of the doctors, 70 per cent of the dieticians, and 52 per cent of the personal trainers answered: Energy/heat.
It sounds like a reasonable answer, because we all know we get our energy from food. Excess weight is just stored food and it should result in more energy, right?
Except that is not where the weight goes.
Chemistry will tell you that atoms are indestructible and to a first approximation this is true. The atoms in the fat are still there.
Further, it is the atoms in the fat that provide the weight.
But fat isnt converted to energy. Rather, metabolizing fat into its final end products results in the release of energy. In this case, the final end products are carbon dioxide and water which are the same end products from most of our metabolic pathways.
Brown and Meeman worked through the balanced chemical reactions to illustrate just where the atoms end up. They traced the route for the excretion of each of atom and to do this they needed to start with a typical triglyceride or fat molecule.
In general terms, a triglyceride is three fatty acid molecules bonded to a glycerol molecule through ester linkages.
Further, the average formula for a fatty acid in the human body was worked out by Hirsch and co-workers back in 1960s. For the human body, an average fatty acid consists of 17.4 carbon atoms, 33.1 hydrogen atoms and two oxygen atoms.
Combine three average fatty acids with a glycerol (three carbon, eight hydrogen and three oxygen atoms) gives a chemical formula for an average fat or triglyceride molecule of 55.2 carbon, 101.3 hydrogen and six oxygen atoms. (Three water molecules are released during esterification.)
To simplify things a little, Brown and Meeman actually chose to use the three most common fatty acids in the human body: oleate, palmitate and linoleate.
This gives a triglyceride molecule with 55 carbon, 104 hydrogen and six oxygen atoms - which is very close to the average.
Combustion of this fat molecule with oxygen - which is metabolism at a molecular level - leads to the consumption of 78 oxygen molecules and the generation of 55 carbon dioxide and 52 water molecules along with a whole lot of energy. (It is a lot shorter to write it as a balanced chemical reaction!)
Yes, the burning of fat inside of the body does generate energy. But energy is not where the atoms or excess weight ends up. The energy is the consequence of the difference between the bond energies in the triglyceride and oxygen molecules and the bond energies in the carbon dioxide and water molecules.
Indeed, the metabolism of fat is a very energy rich process. One gram of fat produces about nine calories or 37 kJ of energy. This is why we have evolved to enjoying fatty food. It is a good source of energy for our bodies.
However, in our present quest, the question remains: what is the fate of the atoms? The carbon and hydrogen atoms from the fatty acids generate carbon dioxide and hydrogen directly, but the glycerol backbone is a little more complicated.
Careful analysis shows that four of the oxygen atoms in the triglyceride end up in carbon dioxide and two end up in water. The result of all this is that if we want to lose one kilogram of fat, we need to consume 2.9 kilograms of oxygen to generate 2.81 kilograms of carbon dioxide and 1.09 kilograms of water.
We breathe out the carbon dioxide. Yes, you are literally puffing the fat away as you exercise. About 84 per cent of the fat atoms are released in our breath.
The water molecules are excreted by several processes. Some are breathed out, some sweated away and the remaining molecules are excreted by our kidneys and bladder.
So, where does the fat go when we lose weight? It ends up as molecules of carbon dioxide and water which we excrete into our surroundings.
Something to consider if losing weight is your New Years resolution.
