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How Does Fat Loss Actually Occur?

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You’ve been on a fat loss shred for a while now and the weight is coming off nicely. You’re seeing more shape and tone and there’s even an ab muscle or two starting to poke through.

You’re checking yourself out in the mirror and happy with your progress.

And then a thought crosses your mind… what actually happens to fat when you lose it?

Does it disappear, turn to muscle, evaporate? It’s the kind of thing that’ll drive you crazy when you think about it too hard.

But in this article we take a look at the physiology of fat loss and answer that very question.

Read on to find out more…

How Did You Get That Excess Fat in the First Place?

There’s only one reason why you as a healthy person would start to get fatter – you take more calories into your body than you burn off.

Whether that’s because you eat lots of food, drink sugary drinks or are just really inactive, your body stores excess energy as fat or ‘adipose tissue’ because of energy balance.

What is energy balance?

When you eat more than you burn off your body has a surplus of energy. And in order to avoid that energy going to waste (as far as your body is concerned, you’re still a hunter-gatherer, unsure of where the next meal is coming from), it stores it for a rainy day.

This is what’s called a positive energy balance or calorie surplus, and the consequence is that you’ll start to get fatter based on how much surplus energy you eat and for how long the surplus occurs.

Fat loss occurs in a deficit

But when you eat less than your body needs each day, you’re left with an energy deficit – you’ve not got enough coming in to fuel your activities. Your body realizes that this is a perfect opportunity to make a withdrawal and tap into that stored energy to make up the difference. This is what’s called negative energy balance.

Calories count when it comes to fat loss

So the take home message here is that if you want to burn adipose tissue then you need to achieve a calorie deficit – negative energy balance. Because if you don’t, your body won’t open up your adipose cells and let that excess energy pour out.

Woman stood on weighing scales and holding a donut

Key Point: Energy balance is the most important factor of whether you gain or lose fat.

What’s The Deal With Fat?

Your fat cells operate like an ATM. Too much energy and you deposit your savings with the clerk behind the desk. But when you need more energy you queue up, make a withdrawal and get those savings taken out and spent.

Excess calories, regardless of whether they are from carbs, fats or proteins, are shuttled into your lipocytes (fat cells) where they sit as a reservoir of excess energy until it is needed. As a result, your lipocytes swell and you start to get fatter.

That excess energy is stored as little lipid droplets (triglycerides) and the lipocyte cell is a nice little home for it to kick off its shoes and put its feet up.

The laws of thermodynamics

If you think back to science class in high school you’ll have probably gone over the laws of thermodynamics in a fair bit of detail. And remember the first law – energy can’t be created or destroyed, only transferred. Calories are no different.

And as a type of energy, fat simply can’t just disappear. It has to go somewhere.

So you don’t ‘lose’ fat then?

Not really. From a thermodynamics point of view, when you’re actively trying to ‘lose fat’ you’re simply trying to metabolize the triglycerides from their stored source into energy.

And once they’re broken down they can be used as energy, before leaving the body.

Dark-haired woman in activewear holding a tape measure around her waist. She is happy that she's lost weight

How Does Fat Loss Occur?

So we know that energy can’t be created or destroyed. And that means that lipocytes can’t simply disappear.

So what does happen to it when you start to drop body fat?

After adulthood you’ve got the same number of lipocyte cells for life. They develop in number during childhood and then stay consistent through your adult years.

It’s just whether or not they’re full of fat is what makes the difference to your physique.

Firstly, your lipocyte cells don’t go anywhere. They stay right there, they just open the floodgates to let the energy inside them escape when it’s needed.

But when triglycerides are released into the bloodstream, they are transported to whenever they’re needed for energy – your biceps, your pecs, your heart, and so on.

So where does the fat go?

According to recent research, most of it is eliminated from the body when you breathe. That’s right, you breathe your fat out.

A study published in the British Medical Journal [1] reported that when triglyceride cells were tracked around the body, when oxidized, 84% of it left the body via the lungs – it was simply breathed out.


Well, triglycerides are made from three separate atoms. And when you want to use it as fuel it has to be broken down or ‘hydrolyzed’. The complete hydrolysis of triglycerol involves many different chemical reactions, but once complete, you’re left with (as well as other things) hydrogen, carbon and oxygen.

And it is through the process of breathing, that these byproducts leave the body – through water vapor and carbon dioxide.

And the remaining 16% fat? It was excreted from your body via urine, tears, sweat and other bodily fluid.

Can’t you just breathe more to lose fat then?

If only that were true. But unfortunately, breathing is only the mechanism of elimination. To break into those fat cells in the first place you’ve still got to achieve that oh so important energy deficit.


  • You can only lose fat if you are in a calorie deficit.
  • Excess energy is stored in your lipocyte cells as triglycerol.
  • When you use stored fat as energy, you are left with carbon, hydrogen and oxygen.
  • Your lungs are the major organ that gets rid of body fat through breathing.
  • Improving your physique requires unlocking the carbon stored in fat cells

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  1. Meerman, R et al. When somebody loses weight, where does the fat go? BMJ. 2014; 349: 7257