Huffin’ It


Where does the fat go when we lose weight?

In our brief discussion today, we’re going to consider the question of where the fat goes when we lose weight. A 2014 feature in the British Medical Journal looked at this question. Their own survey found a range of answers provided by doctors, dietitians, and personal trainers. Below is the figure from the paper showing the responses.

Meerman & Brown 2014 BMJ weight loss fat calories carbon hydrogen oxygen water burn

Energy/heat was the most common answer. When we talk about food and bodyfat, we use the word “energy” a lot. As the video on the journal’s page explains, we must remember that we’re not talking about “energy” in an Einstein nuclear bomb kind of way. We’re talking about “energy” in a biochemical kind of way. We actually use very little energy from the food we eat and the fat stored on our bodies.

Fat is made up of carbon, hydrogen, and oxygen molecules. As our body breaks down the fat molecules and utilizes the little energy that it does, the molecules are converted and expelled. Meerman & Brown (2014) calculated that we lose 16% of fat as water and 84% of it gets exhaled as carbon dioxide.

Below is another figure from the feature that describes the conversion of 22 lbs of fat.

Meerman & Brown 2014 BMJ weight loss fat calories carbon hydrogen oxygen water burn

16% of fat is sweated/pissed/evaporated out and 84% is breathed out.

In this example above, ~64 lbs of oxygen has to be inhaled!

Think about that when you’re working out and sweating or just hangry.

If you want to read a little more about this, the authors were interviewed for a brief news piece in Business Insider when the feature was published. Also, this feature generated a number of comments on the BMJ page. You can read through them here if you’re interested.

If you have any questions about this study or anything I said, please feel free to leave a comment. I will get back to you and others may have insight to offer, too. If you have any questions or topic suggestions that you would like answered as a post, then please email me at

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