Why high fat oxidation rates are not always desired
People keep seeking ways to increase fat oxidation rates. But are high fat oxidation rates always desired? Not really.

Athletes keep searching ways on how to increase fat oxidation rates during exercise. The first reason for such a goal is the belief that utilising the highest amounts of fat during exercise leads to fat loss. The second reason for trying to increase fat oxidation rates is to conserve glycogen stores for the final climb in a race. Although these two ideas seem logical, there are scenarios where we want to maximise carbohydrate and reduce fat utilisation.

Burning more fat during exercise does not equal fat loss

First things first. It is a widespread myth that utilising more fat during exercise would lead to a loss in fat mass. It is true, exercising at lower intensities will allow the body to utilise more fat during exercise. It is also true that exercising after an overnight fast will result in higher fat oxidation rates during exercise as compared to training with having a carbohydrate based breakfast. For couch potatoes this can even mean some positive health benefits. However, higher fat oxidation rates do not directly imply fat loss. As a matter of fat, it doesn’t imply fat loss at all. What actually matters is the energy balance at the end of the day. Have you ingested more energy than you have expended it? Then forget about the fat loss. It is as simple and as complicated as that. Everything else is considered science fiction. Not convinced? There is a wealth of evidence that high intensity interval training intervention can also lead to fat loss even-though during high intensity exercise predominantly carbohydrates are utilised. But this isn’t today’s topic…


There is probably no keynote of mine that does not contain this slide at some point:

Fats and carbohydrates can both be utilised during exercise and are both essential for athletes. The fat stores are much larger than the carbohydrate stores in the body. So, during exercise when there is a chance of depleting carbohydrate stores, it is of crucial importance for the athlete to be able to effectively use fats as well. I call this metabolic flexibility. It is about being able to utilise fats at low intensities while sparing carbohydrates and using them when they are required.

But why do we need carbs if there is enough fat?

Because fat utilisation is limited to the aerobic metabolism. In other words, we cannot synthesise ATP when exercising at high intensities due to a limited oxygen delivery to the mitochondria located within the muscle tissue. This means that at high intensities having sufficient carbohydrate stores and an ability to actually utilise carbohydrates is crucial. But this is not the question this article is trying to answer.

The real question is:

Why high fat oxidation rates are not always desired?

Oxygen transport to the muscles is limited. So we want to use it as efficiently as possible. This means producing as much ATP as possible for a certain amount of oxygen. And if we look at how much energy we obtain by using 1 Litre of oxygen with carbohydrates and fats, we see a pretty significant difference in favour of carbohydrates.

Put differently, we can produce more power or run at a higher pace when using the same amount of oxygen if we utilise carbohydrates and not fats.

So, when we want to go really fast we simply want to use carbohydrates!! It is thus not always about the fat…

Most athletes thus want to be metabolically flexible. However, there are certain types of races when it is almost impossible to finish the race glycogen depleted, such as time trials. This is when we want to sacrifice metabolic flexibility in favour of utilising more carbohydrates as this will enable athlete to be more efficient. Marginal gains? Perhaps. But this is what is decisive in the end of the race, right?

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