‘YOU’VE LOST 2.1 KG OF BODY FAT, CONGRATUALTIONS!’ That’s awesome, thank you….but where did it go?’

I was asked this the other day in response to dramatic body fat reduction from a client on one of our nutrition programmes (they are fantastic, try one). Having some exercise physiology knowledge I explained that it is excreted by the body, mostly as heat through the sweat glands and skin. I was right…..but only partly.

We all talk about burning off fat, and it does burn in a way, going through a complex biochemical process. However, basic chemistry dictates that mass can’t be created or destroyed, so the atoms that made the triglycerides (fat cells) that filled up those love handles have got to go somewhere. As we look deeper into the physiology of substrate deletion, we learn our bodies have a type of fat in the blood called triglyceride. This fat is made up of three kinds of atoms – carbon, hydrogen and oxygen. Burning fat requires unlocking the atoms in triglyceride molecules by a process known as oxidation wherein an enzyme (hormone-sensitive lipase) breaks down the triglyceride into free fatty acids from where they can circulate in the bloodand enter muscle fibres. Interestingly, limited quantities of triglycerides are stored within the muscle….yep, you have fat in your muscles. Not just surrounding it.


Oxidisation, as you would expect, requires oxygen, acquired through breathing in and as it turns out (according to research conducted at the University of New South Wales) 84 percent of the result of oxidisation (breaking down carbon, hydrogen and oxygen) is carbon dioxide. CO2, the stuff we breathe out. Which makes sense.

So this is great news right. You breathe in Oxygen and breathe out fat. If this is the case we should all be ripped, shredded, lean and otherwise hugely desirable to the opposite (or same) sex…….but we aren’t are we?!

Why not?

Because oxidising 10 kilos of human fat requires inhaling 29 kilos of oxygen to produce 28 kilos of carbon dioxide and 11 kilos of water. Regularly breathing in 29 kilos of oxygen requires a level of training intensity and consistency that many people may not appreciate. If you attain it you will breathe out most of your fat with the remaining 16 percent water mass being lost through sweat, urine and tears which are the three ways I thought it all went. Wrong.



National Strength and Conditioning Association; Thomas R. EdD Baechle; CSCS (2008-06-25). Essentials of Strength Training and Conditioning, Third Edition (Kindle Locations 1430-1432). Human Kinetics. Kindle Edition.


When somebody loses weight, where does the fat go?
BMJ 2014; 349 doi: http://dx.doi.org/10.1136/bmj.g7257 (Published 16 December 2014) Cite this as: BMJ 2014;349:g7257