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The myth of switching to fat metabolism

There are countless times I have watched sports on TV and commentators provide their insights on the regulation of fuel use. “And then you switch to fat metabolism” is one of the commonly heard catch phrases. Last weekend this happened again when I was watching cycling. But it is a myth that we have a switch that allows us to select different fuels. How does it work? Please pass this one on to TV commentators.

Pathways of energy metabolism

Energy for muscle contraction

When we exercise the muscle contracts and we need energy for this. The energy is provided in the form of adenosine triphosphate or ATP. The amounts of ATP stored in our body are very small and therefore it is crucial to have systems in place that can regenerate ATP very quickly.

The 4 important “energy systems” are

  • Phosphocreatine (PCr)

  • Glycolysis and lactic acid formation

  • Carbohydrate oxidation

  • Fat oxidation

Aerobic vs anaerobic?

Glycolysis is often referred to as anaerobic carbohydrate metabolism and oxidation is the aerobic component. The misunderstanding (and another myth) is that glycolysis happens when there is no oxygen in the muscle cell. Although the supply of oxygen and these metabolic pathways are coupled, when oxygen is measured in the cell, it is there, so the process is not anaerobic in that sense. But the pathways do not involve oxygen.

These systems vary in their speed with which they can generate ATP. They also differ in their capacity and some processes require oxygen.

During higher intensity exercise the demand for ATP is very high and although ATP levels never drop very much there can be a change in the ratios of ATP and the breakdown products ADP and AMP (where ATP is broken down and we lose one or two phosphate molecules). These are signals to speed up energy production. It is, for example, a trigger for glycolysis to speed up. This means more and more rapid carbohydrate use.


The end product of glycolysis is pyruvate and this is the substrate (fuel) for aerobic metabolism. However, if too much pyruvate is available and cannot be removed fast enough by aerobic metabolism this would eventually slow down glycolysis. Therefore, some pyruvate is turned into lactic acid, so that pyruvate concentrations stay low and glycolysis can proceed. The production of lactic acid is thus helping us, not harming us! (even though the production of lactic acid will eventually result in an acidification of the muscle which may have effects on muscle function and pain sensations we experience).

Why there is no "switch" to fat metabolism

Aerobic metabolism is really fuelled by acetyl-CoA, a metabolite that can be derived from carbohydrate or from fat. In the mitochondria, the energy factories of the cell, pyruvate is turned into acetyl-CoA and this will then be used in the so called Tricarboxylic Acid Cycle or TCA cycle (Krebs cycle).


Acetyl-CoA can also come from the breakdown of fatty acids (fat) through a process called fat oxidation. Once carbohydrate and fat are broken down to acetyl-CoA the processes do not distinguish anymore between acetyl-CoA from carbohydrate and acetyl-CoA from fat.


So, if there was a switch this switch would have to exist before carbohydrate or fat is turned into acetyl-CoA and this simply doesn’t exist. These pathways are always active and acetyl-CoA is always produced from both sources.


So, if there was a switch this switch would have to exist before carbohydrate or fat is turned into acetyl-CoA and this simply doesn’t exist.


However, sometimes pyruvate is produced at high rates because the intensity is higher and more pyruvate will be converted into acetyl-CoA. This is mostly driven by activation of glycolysis. In other conditions glycolysis proceeds slowly because there is less demand for ATP and there may be a large supply of fatty acids. In such cases more acetyl-CoA will come from fatty acids.

Sources of carbohydrate or fat for metabolism

So far, we talked about carbohydrate and fat, but there are different sources of carbohydrate and different sources of fat. Some carbohydrate is stored in the liver, some may come from our nutrition and some is stored in the muscle. The liver and nutritional glucose will be transported via the blood to the muscle. We can distinguish these sources in studies and we can see that with increasing intensities the contribution of both plasma glucose and muscle glycogen increase, but by far the largest increase and contribution is from the readily available muscle glycogen.


Contribution of different fuel sources during exercise

Fat comes from different sources too. Some fat is stored in the muscle. This is called intramuscular fat or intramuscular triglycerides (IMTG). Even though endurance athletes may be lean they actually have slightly larger IMTG stores in their muscle than less trained individuals (a topic for a future blog). Fat can also come from adipose tissue and be transported to the muscle as fatty acids in the blood. These fatty acids are then taken up in the muscle and can undergo beta oxidation in the cell. Fat from food is probably a less important energy source because it reaches the circulation very slowly and when it reaches it, it is in the form of large fat particles that need to be broken down to fatty acids first before they can be used. 

Exercise intensity regulates fat metabolism

Fat oxidation from blood fatty acids and IMTG increase from low to moderate intensities and decrease when the exercise intensity increases from moderate to high. Peak fat oxidation can often be observed around 65% VO2max, although this is highly individual as we discussed in previous blogs about FatMax (Finding your fat burning zone; What is FatMax?).

With this in mind, one cannot ‘switch’ from carbohydrate to fat burning. Both processes continue, but the magnitude of contribution changes. Here are two summaries to replace “And then you switch to fat metabolism” that you can now use when watching endurance events:

  • The contribution of fat and carbohydrate to provide energy changes during the race, depending on the intensity.

  • As the intensity decreases, there may be a transient increase in fat metabolism to provide energy.

Conclusions on fat metabolism

So, in conclusion:

  1. There are essentially 4 energy systems

  2. They are all used at the same time all of the time

  3. The main driving force for substrate use is the intensity of exercise and the demand for ATP

  4. Muscle glycogen can deliver energy via glycolysis and aerobically via oxidation

  5. Carbohydrate use (both from plasma glucose and muscle glycogen) increase gradually with increasing intensity

  6. Muscle glycogen is by far the most important substrate at higher intensities

  7. Fat oxidation increases from low to moderate intensities and decreases from moderate to high intensities

  8. Fat is derived from plasma (mostly from adipose tissues) as well as from stores inside the muscle (IMTG)

  9. There is NO SWITCH to go from carbohydrate to fat metabolism or the other way around.


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