Hang On A Minute… Where Did My Fat Go After A Workout?

You don't sweat it away. You don't burn it into nothing. So where does it actually go?

Previously, we discussed how our bodies generate biochemical energy from the carbohydrates and fats that we consume. These nutrients are eventually converted into acetyl-coenzyme A (acetyl-CoA), which enters the mitochondria to undergo a series of reactions that ultimately produce adenosine triphosphate (ATP), which is the energy currency that allows our muscles to contract.

However, the story does not end with ATP production.

As we discussed previously, the conversion of chemical energy into mechanical work is not perfectly efficient. A significant portion of the energy released during metabolism is dissipated as heat, which our bodies must continuously remove through processes such as sweating and evaporation.

But this raises another interesting question:

If carbohydrates and fats are being consumed during exercise, where does the fat actually go? Does it simply disappear? Does it turn into sweat? Does it get converted into heat? Or does it leave our bodies through another pathway entirely?

Surprisingly, the answer brings us back to the same biochemical machinery inside our mitochondria.

We’d have to look into the source of the acetyl-CoA, which happens to come from carbohydrates and fatty acids. Fatty acids are stored in our bodies as triglycerides, which consist of a glycerol backbone chemically bonded to three fatty acid molecules through ester linkages.

During exercise, these stored triglycerides must first be broken down to release fatty acids. These fatty acids can then undergo a series of reactions known as beta-oxidation, where they are progressively converted into acetyl-CoA before entering the mitochondria’s energy-generating pathways.

But this brings us back to our original question: Once the fat molecules have been broken down and converted into energy, where does the fat actually go?

Once fatty acids have been released from triglycerides and converted into acetyl-CoA, they enter the mitochondria where they undergo further oxidation through the TCA cycle.

This is where the fate of the carbon atoms within our fat molecules is determined. Fatty acids are molecules made up of carbon, hydrogen and oxygen atoms. The carbon atoms will ultimately be converted into carbon dioxide gas in the presence of oxygen, which is then transported to our lungs and exhaled from our bodies.

The hydrogen atoms, meanwhile, are transferred onto electron carriers such as NADH and FADH₂. These molecules bring electrons into the electron transport chain, where molecular oxygen eventually accepts these electrons and combines with hydrogen ions to form water.

So when we lose fat during exercise, we are not simply "burning it away". The carbon atoms that once formed our fat stores are literally leaving our bodies as carbon dioxide every time we breathe out.

That is one reason why we tend to take more frequent breaths during exercise. Our muscles require more oxygen to support the increased activity of the electron transport chain, while we also need to remove the increased production of carbon dioxide that results from the oxidation of nutrients.

We may also notice that our exhaled breath becomes more humid during exercise. This occurs because the air passing through our respiratory system becomes humidified by the moist surfaces within our lungs and airways. At the same time, the increased metabolic activity of our cells also contributes to the production of water through the electron transport chain, adding to the body’s overall water balance.

In other words, the same biochemical processes that allow us to extract energy from fat and carbohydrates also produce the carbon dioxide that we breathe out and the water that contributes to our body's water pool.

So if carbon dioxide leaves through our lungs, does that mean we literally breathe out our fat?

In a way, yes we do breathe out our fat, but not in the form that most people imagine. The fat molecules that are stored in our bodies do not simply travel to our lungs. Instead, their carbon atoms undergo a series of biochemical transformations inside our mitochondria, eventually becoming carbon dioxide that is transported to our lungs and exhaled.

The next time we breathe out during exercise, some of the carbon atoms that leave our body may have once been part of the fat stored around our waist, thighs or other parts of our body.

Unfortunately, we do not get to choose which fat stores get eliminated first. The body does not look at our preferred problem areas and decide, “let’s remove the fat from here today.”

This is why someone can exercise frequently and become generally leaner, yet still notice stubborn areas such as flabby arms or larger thighs that seem reluctant to change.

Ultimately, losing fat is not simply about exercising a particular body part. It is about consistently creating the conditions for the body to access and utilise its stored energy reserves, while providing the nutrients and recovery needed to maintain muscle mass and overall health.

There are no shortcuts to changing the composition of our bodies. The process is gradual, and the key is to keep going with consistency!

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