How can I burn fat? How does the body turn fat into energy? What happens to the fat after it is used? And how can I make my body burn up the fat so I can get lean?

Read on to discover the basic science of how fat burning really happens…

Ever noticed that people with careers related to medicine or biology tend to be leaner? Understanding how fat is stored and burned by the body makes it much easier to know how to control our body fat levels.

Of course, it’s a very complex process and most of us don’t have the time or inclination to take a degree in human biology just to be lean. But that’s OK, you don’t need to understand it on a deep level, just the basic principles will be helpful enough.

The problem is it’s almost impossible to find an article which explains them in a way most ordinary people can understand… But I’ve got you!

How did this fat end up on my body?

When people talk about wanting to lose weight, what they really mean is they want to shed fat, which means they want their body to burn off its fat stores.

Think about it – no one wants to make their bones thinner, or for their vital internal organs to shrink down and it’s very rare for someone to want to lose lean muscle tissue. The only reason we want the scale to tell us we weigh less is because we think this will indicate that we are carrying less fat on our bodies.

So wanting to “lose weight” usually really means wanting to have less fat.

Seems simple enough. But how does that work? How does the body “burn off” fat – and where does burned fat go?


Microscopic fat cells.

Food contains energy, when you eat you’re taking in energy (usually measured in calories).

You also use up energy throughout the day, even doing simple things like walking around the house or pushing a trolley down the aisle at the grocery store. Working out uses more energy, but works in the same way. As you move, your body is powered by the energy you’ve taken in – the same way any machine uses fuel.

Both eating well and exercising play a part in how much fat we have on our bodies: It’s all a matter of taking in energy and using it up.

As you probably already know, if you take in more energy than you use your body stores this as fat, if the food you eat brings in less energy than you use, you’re at a deficit and your body takes fuel from the fat stores and you get leaner.

One of the reasons so many people are carrying more fat nowadays is because today’s processed foods are so energy-packed that it has become really easy to take in more energy than our body uses, which means we end up with a surplus.

But how does it really work? How can a pretty cupcake or delicious-looking slice of pizza end up turning into body fat? To get your head around the answer, you first need to understand where the energy you take in via food actually goes.

Energy comes into the body via food & if not immediately needed it's stored as fat Click To Tweet

What happens to the energy you take in?

The energy contained in the foods and drinks you consume either goes straight into your bloodstream for immediate use – like a fast injection of fuel into our “engines”. This only happens for the energy we need at that moment. Otherwise it will be used to replenish energy stores in places like your muscles and liver or, once those stores are full, it gets stored as energy in your fat cells.

It’s important to note that any given moment, you’re both storing and using energy at the same time. We always need some fuel just to stay alive. But the less active you are and the more energy you take in via food, the more of a surplus of energy you have in your system and the more energy your body will store for later in your fat cells.


Where does fat go when you lose it?

Now, your body will first use the energy stored in your bloodstream and muscles before it starts using significant amounts of the energy stored as fat. As far as your body is concerned, fat is it’s stored energy – like the money in your bank account. The energy in your bloodstream and muscles (called glycogen) is the energy for immediate use – like cash in your pocket.

The body is going to use up the “pocket cash”, glycogen, before it goes to “the bank”, fat stores, to withdraw saved energy.

This is pork fat, but human body fat looks very similar.

(The image above is pork fat, human body fat looks very similar.)

Once your body starts tapping into fat stores, the more energy you use, the more fat you lose. But the storage cells don’t actually disappear, only the fat they contained is removed, so they shrink down.

This is good news if you want to get slimmer – shrink down enough of those fat cells and your body will get slimmer and leaner.

Fat cells don’t get “burned”, only the energy they contain

But those storage cells are still there and next time you take in more energy than is immediately required they’ll fill up (and you’ll fill out) again.

So, to make this shrinkage happen we need to get the body to first use up glycogen energy in the bloodstream. After this, if it still needs it extra energy it will start to use the stored energy in our fat cells.

This is why dieting or fat loss plans of any kind usually involve taking in less energy than you use (no matter how it’s dressed up). When enough energy leaves your cells for your body to use over a longer period of time, your fat cells start to “empty.”

Although we use the term “fat burning” the body doesn’t literally burn it like the combustion engine in a car, but those fat stores do get used up as fuel in a slightly similar way.

So where does used fat go?

The science behind this process is sort of complicated. But just understand that eventually the fat stored in your cells gets broken down into its components – carbon, hydrogen and oxygen.

What your body does not use of those chemicals, it gets rid of. You either breathe it out (exhaling carbon dioxide) or excrete it as water (through your urine, sweat, etc.).

So fat sort of does get burned off when you lose weight. Just not in the way many people assume.

We all know that this process can be tricky to achieve and there’s growing evidence that, although “Calories in Vs Calories out” is an important equation, there are hormonal processes (mostly related to insulin) which can also impact on the body’s tendency to store or release fat at different times.

I’ll talk more about this in an upcoming post, because it is related to how I have been able to use intermittent fasting to shed fat without losing muscle tissue.


Will I lose muscle if I lose fat?

Losing muscle tissue is a worry for a lot of people when they are working on shedding fat.

Once the energy supplies in your blood start to run out your body can also turn to your muscles as an energy source, which can lead to muscle tissue also being “burned up” as fuel. This is the last thing you want as not only will it weaken you, it’ll cause your metabolism to slow down, meaning your body burns less energy in the future.

But it’s not as common as many people fear and eating plenty of protein-rich foods (meat, fish, eggs, etc.) will help avoid it.


How can I burn fat?

Empty fat cells.

Empty fat cells shrink, making our bodies slimmer and firmer.

I’ll share more about my approach in a post soon, but the most important thing for you to understand really is that a diet of mostly natural, unprocessed foods and moving your body regularly is what your body needs to stay lean, fit and healthy.

Keep in mind that you need to move in order use up significant amounts of energy. Eating a healthy diet and staying sedentary probably won’t help you burn much fat.

On the flipside, you also can’t just workout and fill your body up with processed and/or energy-dense food and expect your body to use up your fat stores. It simply won’t happen.

Healthy portions of protein, fat and carbs along with a well planned, progressive exercise program, is the most effective way to burn fat and slim down.

Does that take effort? Of course. But there is NOTHING more worth investing effort in than the condition of your body – there is no aspect of your life which is not improved by you being healthy.

For optimal fat loss you need BOTH a healthy diet and an effective exercise program Click To Tweet

When you really understand how fat loss works, you understand how exercise helps get us lean.



[Post revised and text updated: 17th October 2019]