Burning Fat Vs Burning Carbohydrate: A Flawed Argument

The title of my PhD thesis is “Metabolic Flexibility and Endurance Performance”. The term Metabolic Flexibility is one that not a lot of people have heard in regards to the Fat Vs Carbohydrate debate. Here I’d like to introduce the reader to why you can’t use carbohydrate or fat, you must and always will use both.

As humans, we generate energy from the three macronutrients that make up the foods we eat; fat, carbohydrate and protein.

Fat is the most abundant, and at 9 Kcal per gram, fat is the most energy dense. It is fat that we rely on to store our bodies excess energy. For this reason, the energy we can get from fat is almost unlimited, even in the leanest athlete. Carbohydrate is limited in its availability. The average individual can store approximately 500g of carbohydrate in their muscles and liver in the form of glycogen. Once this glycogen store is gone we can no-longer use carbohydrate for energy unless we ingest carbohydrate via food or fluid, i.e. gels, bars and sports drink. Protein is made up of amino acids and is predominantly used for building enzymes, structural proteins and metabolic molecules. Very little protein is used for energy. If protein is used for energy it is the last resort.

The main takeaway here is that fat is unlimited, and carbohydrate is limited.

When we exercise, fat and carbohydrate are burned as fuel in the muscle to power muscular contractions (movement). When the amount of muscular force is low (jogging) the majority of energy comes from oxygen and fat (aerobic metabolism). As our muscular force output increases (sprinting, uphill running) we require more muscle fibres, and the more muscle fibres we use the more fuel we need and the faster we need it.

Muscle fibres are activated in the same pattern every time: type 1 (fat burning) first, type 2 (carb burning) second. This phenomenon is known as Henneman’s size principle. In very basic terms, it works like this: as we walk down the street we use some of our type 1 muscle fibres while burning predominantly fat, as we begin to speed up from a walk to a slow run we start to use all our type 1 muscle and are now burning as much fat as possible. This is the aerobic ‘sweet-spot’ often represented as ‘zone 2’. It is at this intensity we begin to see the difference a low carbohydrate diet can make on an individuals ability to burn fat. It is at this point your dietary carbohydrate intake has the most impact.

If you begin to run uphill you now have to recruit type 2 fibres because you have no more type 1 fibre left. The type 2 fibres are going to produce the muscular force required to run up the hill. If we only relied on our type 1 fibres we would be reduced to a walk because those fibres aren’t capable of producing the force required to run up the hill.

Type 2 fibres are very bad at burning fat and rely almost exclusively on carbohydrate. Even though you are now using type 2 fibres to run up the hill, you are still burning the same amount of fat because you’re still using the type 1 muscle fibres that you initially recruited, but you’re also using the type 2 fibres for the added force need to run uphill. In this scenario, you are using both type 1 and type 2 muscles fibres and you are burning both fat and carbohydrate, not one or the other.

Now consider the three stages above, walking, slow running, and uphill running in terms of your carbohydrate intake.


Walking down the street has a low energy requirement due to the low amount of muscular force required. Regardless of your diet, you will be burning fat, so while you’re walking there is no need for carbohydrate. You could walk for hours and not need a gel or sports drink.

Easy Running

However, running down the street at a slow pace uses a reasonable amount of energy and requires a moderate level of muscular force. The energy required for this muscular force will come from fat and carbohydrate, and how much of each will depend on your diet. An individual who eats a carbohydrate-rich diet will use predominantly carbohydrate because it’s a more metabolically efficient fuel to burn because it’s smaller and less complex than fat. Think of the comparison between gas and coal. If you eat very little carbohydrate in your diet your type 1 muscle fibres can’t rely on carbohydrate as a fuel and therefore must adapt to burning fat. In this situation, you will be burning predominantly fat.

Running UpHill

Running uphill uses a lot of energy very quickly and requires a large level of muscular force. In order to generate this force, we need to recruit our type 2 muscle fibres which burn carbohydrate. In this situation, it doesn’t matter how much carbohydrate you eat because those type 2 fibres will burn carbohydrate whether you eat it or not.

We now know that slow running is going to burn predominantly fat for low carbohydrate athletes and predominantly carbohydrate for high carbohydrate athletes. We also know that running uphill is going to use almost exclusively carbohydrate regardless of your diet.

50km Trail Runner

Let’s break this down for a high carbohydrate runner competing in a 50km trail run. The runner is going to be running most of the 50km at a slow pace. Due to their diet, we know that this runner is going burning carbohydrate at while running slowly. However, we also know that carbohydrate is limited in its availability. If the runner was to only drink water and not ingest in carbohydrate they would probably run out of carbohydrate (glycogen) after 3hrs. But that’s if they’re on the flat. If this runner chooses to run up a hill rather than walk they will be using their type 2 muscles fibres and they will be burning even more carbohydrate. Now they may run out of carbohydrate at 2hrs.

What if this runner was on a low carbohydrate diet? during their slow running, we know they would be burning predominantly fat, therefore their risk of running out of carbohydrate (glycogen) would be very low. Yes, the low carbohydrate runner will still be using carbohydrate if they choose to run up the hill but that is the only time they will use carbohydrate, so their glycogen will last a lot longer than the high carbohydrate athlete. The low carbohydrate athlete will still need to have eaten carbohydrate before the race because they are still using the carbohydrate dependent type 2 muscles fibres to run up the hills.

What about carbohydrate supplements?

The high carbohydrate athlete isn’t complete doomed because they can always supplement their store carbohydrate with gels, bars, and sports drinks. However, the high carbohydrate runner must supplement to ensure they don’t run out of energy. The low carbohydrate runner, on the other hand, does not have the same requirement for carbohydrate as the high carbohydrate runner and therefore doesn’t need to rely on ingesting carbohydrate constantly throughout the race. The low carbohydrate runner is therefore at less risk of running out of energy.

Metabolic flexibility

The two carbohydrate runner can be considered metabolically flexible because they can burn fat when the intensity is low and carbohydrate when the intensity is high. The high carbohydrate athlete is considered metabolically inflexible because they must always burn carbohydrate.

Hopefully, you can now see that even if you completely cut carbohydrate out of your diet you may improve your metabolic flexibility or “sweet spot” but you will not eliminate your need for carbohydrate. The need for carbohydrate is always present, especially during hard exercise. Having the metabolic flexibility to alter how your muscle generates energy based on the demands of the body is important. By removing carbohydrate from your diet you will definitely become better at burning fat and so long as you allow yourself a carbohydrate load before a hard training session or race you will also become better at switching to carbohydrate when it’s needed.


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