Where does all the fat go?

How do we actually lose weight through fat loss?
Ian Locke
Ian Locke
Mar 18, 2021

To lose weight, we can reduce the amount of water, carbohydrate, protein or fat stored in our body, but unless our athletic performance demands it, we’re typically going to want to prioritise losing fat.  That’s because:

  • We want to preserve protein (stored in our muscles) for aesthetic, mobility and biochemical reasons;
  • Water loss is only temporary;
  • We only have 400-800g of carbohydrate stored, so there’s not much to lose; and
  • Excess fat is more of a health concern than the rest.

What constitutes a “healthy” amount of fat varies wildly, depending on age, gender, genetics, activity levels, etc., and is estimated at 8-33% of bodyweight.  But regardless of where you’re starting from, if you lose body fat, where does it actually go?

This is not a stupid question

Where do you think the fat goes when you lose weight?  Typical answers include:

  • It’s converted to heat or other energy
  • It’s converted to muscle
  • We poo it out
  • We pee it out
  • We sweat it out

Back in 2014, a group of 150 doctors, dieticians and personal trainers were asked this question and only six knew the correct answer.

Drum roll please …

Where does all the fat go?

84% converted to carbon dioxide and lost through breathing


16% converted to water and excreted in urine, faeces, sweat, breath, tears, etc.

If you want to read about the science behind the process, it’s included as a post script to this article.

What does this mean for fat loss?

Our lungs are the most important organ for removing fat from the body.

Whilst we find it fascinating that our bodies covert something as solid and tangible as fat into water and gas, what does it actually mean for those who want to lose fat?

In a simple world, it means that if you’re not breathing more heavily, you’re not losing more fat.  To breath more heavily, you need to move more, move faster or find some other way to increase your breathing rate.

Or it means you need to eat less, so the fat stores used up by day-to-day living don’t get fully replenished after you breathe or pee them out.

Sadly, we’re not in a simple world where our bodies respond to changes as predicted.  “Eat less & move more” works to a point, but it’s easy to get caught in a cycle of chasing fat loss where we eat less and less and exercise more and more.  Soon enough, our bodies stop responding in a positive way and that’s not a healthy position to be in.

At Thrive, we turn this approach on its head and focus on encouraging bodies to function more effectively from the outset.  Get this right, and other goals will fall into place much faster, be that fat loss, muscle growth or athletic performance.

We only wish we could find a way to see the end products from fat loss leaving our body as we exhale.

Are you ready to thrive?

The sciency bit

Each molecule of stored body fat is made up three fatty acids attached to a glycerol backbone, forming a triglyceride.  Fat stays as a triglyceride until it needs to be transported through a cell wall, when it reverts to being three fatty acids.

Fatty acids are made up of carbon, hydrogen and oxygen atoms; glycerol is made up of carbon and hydrogen.  Each type of fatty acid has a different combination of atoms, but on average, a triglyceride will have 55 carbon, 104 hydrogen and 6 oxygen atoms.

When fatty acids are needed to create energy, they are transported to the mitochondria where they are converted into Acyl CoA and rely on a carnitine shuttle to move through into the mitochondrial matrix as Acyl Carnitine.

The fatty acid and carnitine molecules separate, allowing the fatty acid to be degraded through a series of steps.  It is now ready to have pairs of carbon atoms removed through beta oxidation to produce Acetyl CoA, which is then passed to the citric acid cycle to produce ATP.

It is the cleaving of a phosphate atom from ATP molecules which creates the energy spark that we depend on for life.

For this process to work, additional oxygen is required (it’s what’s called an “aerobic” process).  For every triglyceride, 78 molecules of oxygen are required and the overall chemical reaction is:

C55H104O6 + 78O2

55CO2 + 52H2O + Energy

Moving away from the molecular level, to lose 1kg of body fat, we need to breath in 2.9kg of oxygen.  This will produce 2.8kg of carbon dioxide and 1.1 litres of water.

If you want to follow more of the science, then check out this article.

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