Determinants of Substrate Utilization

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Many factors determine the substrate utilization by our body during exercise. It is of great importance to understand them when it comes to designing training and nutritional strategies to improve and enhance performance. Here we will talk about those factors that have the greatest influence on whether fat or carbohydrates is the primary energy source.

Hey there again! In this article I would like to talk about one of the basic yet most important topics when it comes to designing either training or nutritional strategies in pursue of performance optimization. This is a must know among exercise physiologists and performance nutritionists.

I am talking about the substrate utilization during exercise and the factors that it can be affected by.

You don’t want to make your athlete improve his reliance on fat by cycling for 45 minutes with intervals at 90% of his VO2 max. Or planning a nutritional strategy of 90 g/hour of carbohydrates during a long, low-intensity run. It’s just not going to work. It would be like trying to freeze water in the oven.

Knowing what substrate provides the majority of the energy to the exercising muscles will allow us to adjust the training characteristics and the nutritional strategies so that the outcome of that training session is enhanced.

As a remainder, substrate utilization is measured in the laboratory as the ratio between the amount of carbon dioxide produced in metabolism and oxygen used, which is called the respiratory exchange ratio (RER). As you might know (and if you don’t, you can take a look at a previous article about the energy systems), the fatty acid molecules contain way more carbon than glucose. Because of this, oxidation of only fat brings the RER down to about 0.7, whereas oxidation of only glucose gives a RER of 1.00 or above (because of the carbonate buffer that neutralizes the acidosis during high-intensity exercise). Since our body always uses both substrates in different proportions, the actual value of RER during exercise will be in between both extremes.

Now that we know why this is essential for optimizing performance, and how it is measured, let’s get deep into this.

Substrate utilization determinants

The type of substrate that our body utilizes predominantly as an energy source will depend on several factors. Some of them can be controlled by the coach or the athlete itself, but some others cannot be modulated and we need to be aware of them in order to effectively predict the substrate utilization.

Be aware that, while many factors determine substrate utilization, I am just considering the ones that have a greatest impact on it from my understanding.

Exercise intensity

The intensity at which we exercise is, by far, the main element that determines whether our body is getting energy from fat or from carbohydrates.

We already know that the oxidation of fat requires way more steps and greater amount of oxygen than carbohydrate oxidation. This means that fats will provide energy at a slower rate than carbohydrates and their oxidation will be impaired when oxygen supply exceeds oxygen consumption, which occurs during high-intensity exercise. On the contrary, at high intensities require a higher rate of energy supply, which can be met by catabolizing carbohydrates instead of fat.

Therefore, fat is used as the main energy source at lower exercise intensities, whereas carbohydrates are the primary energy source for high-intensity exercise.

This can be clearly drawn and explained through the “crossover” concept established by Brooks in 1994 as seen in the following graph. The point at which the primary energy source turns upside down from fat towards carbohydrate as exercise intensity increases is called the crossover point.

The crossover concept
The crossover concept

The location of the crossover point in normal individuals is at around 60% VO2 max. However, this will also depend on other factors such as training state, nutritional state, fiber type composition, etc.

Exercise duration

The duration of the exercise bout also plays an important role in determining substrate utilization since glycogen depletion turns on the oxidation of fat. Therefore, as exercise goes on and glycogen is consumed (remember that even at low intensities, part of the total energy supply comes from carbohydrates), fat oxidation becomes more and more important.

Training state

Training can shift the crossover point towards higher intensities, thus relaying on fat up to a higher percentage of VO2 max and leaving reliance on carbohydrates to really high intensities.

With training, we can also improve our reliance on fat at low intensities, which will spare muscle glycogen, and enhance carbohydrate use at high intensities. This is called metabolic flexibility and will be examined in detail in future articles.

Fiber type composition

It is well known that slow-twitch fibers have a greater oxidative capacity than fast-twitch fibers. In consequence, people that contain a higher percentage of fast-twitch fibers will rely more on carbohydrates and less on fat. On the other side, people with greater percentage of  slow-twitch fibers will utilize greater amounts of fat.

The fiber type composition is determined genetically. However, it can be slightly modulated by training.

Environmental conditions

Different studies have examined the effects of both hot and cold temperatures on substrate utilization. The evidence currently says that heat increases the reliance on carbohydrates, but only at high intensities, whereas the effect is not that clear at lower intensities. Cold temperatures, instead, enhances fat utilization by the body as a primary energy source.

Pre-exercise nutritional state

The glycogen content (within the muscles) at the beginning of the workout also plays an important role in the same way that it does with exercise duration. Starting a workout with low glycogen levels will downregulate the carbohydrate use while enhancing the reliance on fat. Contrarily, high glycogen levels at the beginning of the workout will enhance carbohydrate oxidation and, therefore, reduce fat oxidation.

Intake during exercise

It is also well known that ingesting carbohydrate during the exercise bout enhances its use and significantly reduces the effect that duration of the workout has on glycogen depletion and, therefore, substrate utilization towards fat.

This is actually one of the most basic and essential strategies for optimizing performance though nutrition, and it has fascinating results. A noteworthy anecdote on this that clearly shows its importance, is that of Chris Froome during the 19th stage of the Giro d’Italia 2018. Froome successfully went solo 80K’s out to become leader. After the stage, the general manager of the Team Sky (now the “Team Ineos”) Dave Brailsford claimed that that day “was about fueling”, it “was about making sure that you can fuel a ride like this all the way through the end and it’s fundamental”.

Today was about fueling and today was about making sure that you can fuel a ride like this all the way through the end and it’s fundamental.

Dave Brailsford

Wrap up

Understanding how our body uses substrates and how its reliance on either fat or carbs is modulated is crucial for designing training and nutritional strategies to enhance and optimize performance. To do so, we use the ratio between the CO2 produced in metabolism and the O2 consumed to obtain the RER, which gives us an idea of the primary substrate that our body is using as an energy source.

Many factors determine the substrate utilization. Among them, only the ones with greatest impact on substrate use have been mentioned.

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