Effects of Carbohydrate Ingestion During Exercise

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Our society has often pushed us towards the search for new limits within different fields of our life, and exercise performance has not been an exception here. We are constantly seeking new strategies to become stronger, to improve our endurance, to get faster, and to ultimately break new records.

However, the interest in finding strategies to enhance athletic performance is not something new. In fact, it is well documented that back in the ancient Greece, Milo of Croton – one of the best wrestlers of the Olympic Games – consumed 9 kg (20 lbs) of meat and bread, and 10 liters (18 pt) of wine. As you see, the interest in improving athletic performance through nutrition is older than we could imagine.

This time I want to talk about how carbohydrates can optimize our performance during exercise. Particularly, I will address the effects of their ingestion on our organism during exercise.

Unlike many other practices, the ingestion of carbohydrates is not a trend, but one of the most studied and supported methodologies for improving exercise performance instead. In fact, there are very few performance-enhancing strategies whose evidence is so uniform and clear as carbohydrate feeding during exercise. At present, ingesting carbohydrates during exercise is one of the most popular practices among endurance athletes – we could even say that is part of their training program.

But before we dig in, let me just give you a brief overview of the path of carbohydrates through the science.

Historical overview of carbohydrates

In the 1900s research showed that carbohydrate and fat were important fuels for our body, and in the 1920s it was first recognized the importance of carbohydrates as a fuel source during exercise (Krogh and Lindhard 1920). In the 1960s, after the reintroduction of muscle biopsy by a group of Scandinavian scientists, muscle glycogen was shown to have a critical role during exercise (Bergström & Hultman 1966). But it wasn’t until the 1980s that that research convincingly demonstrated that carbohydrate ingestion during prolonged exercise could improve exercise capacity (Coyle et al. 1983).

This led to an era where more and more studies gave evidence for the ergogenic effect of carbohydrate ingestion during exercise, and this practice became more and more popular in many sports, especially in endurance sports. In parallel, a huge market of the so-called sport drinks grew up.

However, very few advances were made until 2004, which marked the starting point of a series of breakthroughs regarding carbohydrate feeding during exercise, such as the use of multiple transportable carbohydrates, which refers to the combination of different sugars – concretely, glucose and fructose – to enhance carbohydrate absorption and thus increase the ergogenic effect of carbohydrates. You can read more about this in a previous article about the absorption of carbohydrates.

Since then, carbohydrate recommendations during exercise have progressed from 30-45 g/h to the current guidelines developed by Asker Jeukendrup of 90 g/h during endurance events.

Carbohydrate ingestion and exercise performance

After more than 100 years of intense research on the relationship between carbohydrate ingestion and exercise performance, we have achieved a decent understanding of how carbohydrates influence our body, concretely our physiology and metabolism.

We are certain about the fact that ingesting carbohydrates during exercise can enhance our athletic performance in terms of both exercise capacity and power output. Not only that, but there is also evidence about a positive dose-response relationship between carbohydrate feeding and endurance performance – as we will see soon. In other words, it appears that the beneficial effects of carbohydrates on endurance performance increase as we increase the rate of their ingestion.

In this line, many world-class endurance athletes have carbohydrate intakes that far exceed the current recommendations of 90 g/hour during competitions, and they are getting fascinating results, such as the marathon world record by Eluid Kipchoge and the Giro d’Italia 2018 by Chris Froome. However, since we do not really understand how these high carbohydrate intakes are even possible, which show us again that science is behind the reality.

But, going back to our point, how do carbohydrates influence our body? What are the benefits? Where does the increase in performance come from? Let’s now address these questions as we currently understand them.

To make it easier though, I will split up the effects of carbohydrates into two main groups: metabolic and central effects.

Metabolic effects of carbohydrate ingestion during exercise

The effects that carbohydrate ingestion during exercise has on our metabolism are based on the substrate utilization during exercise and the different factors that may influence it. Therefore, understanding the regulation of the different energy substrates during exercise – you can review this on a previous post about substrate utilization.

We can briefly summarize the main factors that alter the substrate use during exercise in descending order of relative impact:

  1. Exercise intensity.
  2. Exercise duration.
  3. Endogenous substrates availability.
  4. Exogenous substrates availability.
  5. Training state of the athlete.
  6. External factors.

As you can see, the availability of both endogenous and exogenous substrates highly impact on the use of the different energy fuels, and this is something we can change through nutritional strategies such as carbohydrate feeding during exercise.

Therefore, high endogenous and exogenous carbohydrate availability can impact metabolism during exercise in a way that will enhance athletic performance during activities of moderate-to-high intensity. Here, we are actually talking about increasing the exogenous carbohydrate availability through carbohydrate feeding during exercise and the impact on exercise performance.

Before we move on to explaining the actual effects of carbohydrates, we should bear in mind that fatigue during endurance sports is associated with both muscle glycogen depletion and an inability to maintain euglycemia – normal blood glucose levels. Then, for carbohydrates to enhance exercise performance, they either delay fatigue by delaying both of the events just mentioned, or enhance the power output.

Now let’s see the actual benefits!

Increased oxidation of exogenous carbohydrates

It is well documented that ingesting carbohydrates during exercise – which provides our body with an energy substrate ready to oxidize – enhances the oxidation of the same ingested carbohydrates. In fact, it is shown a positive, linear dose-response relationship between the rate of carbohydrate feeding and the rate of exogenous carbohydrate oxidation, at least up to the current guidelines of 90 g/h, although there are some insights about this linearity for higher intakes. In other words, the more carbohydrates we provide our body with, the more are oxidized.

This is most likely due to a higher availability of blood glucose and lactate, which allows a greater uptake of them and thus a higher glycolytic flux. Ultimately, this would predispose our metabolism to oxidize more carbohydrates.

Besides, when energy substrate levels are high, the total energy expenditure is also high, which allows for increased power outputs to be achieved and sustained.

Sparing of endogenous carbohydrate stores

We just talked about how the energetic contribution of exogenous carbohydrates increases when we increase their availability by ingesting them. If part of the energy needed during exercise is provided by the carbohydrates we eat, less of the endogenous carbohydrate stores will have to be used in order to meet the total energy requirements. This basically means that we can spare our own carbohydrate stores and delay their depletion by providing external carbohydrates

Remember that two main carbohydrate stores exist: muscle glycogen and liver glycogen.

It has been suggested that carbohydrate feeding during exercise spares muscle glycogen and thus delays its depletion and the onset of fatigue. However, there is some controversy about it at present. On one side, many studies show that carbohydrate ingestion during exercise does not change the rate of muscle glycogen utilization. On the other side, some studies have demonstrated a reduction in muscle glycogen utilization in type I (slow twitch) fibers of the working muscles when consuming carbohydrates during running trials compared to those that drunk plain water.

This controversy may have arose as a result of differences in the methodologies used to study exercise performance parameters or in the type of exercise used (those that found a reduction in muscle glycogen utilization used running instead of cycling trials), or due to the fact that analysis of glycogen at the mixed muscle level might not be sensitive enough to detect changes that are restricted to one population of fibres. Anyway, further research must be done to confirm whether ingestion of carbohydrates during exercise spares muscle glycogen.

Unlike the effects of carbohydrates on muscle glycogen, it is well established that carbohydrate feeding during exercise progressively inhibits glucose release by the liver in a dose-response manner. It is believed that the increased levels of plasma insulin seen when carbohydrates are ingested during exercise mediates the reduced hepatic release of glucose, thus favouring hepatic glycogen sparing and exogenous glucose oxidation. This, together with the exogenous carbohydrate supply, ultimately leads to a better maintenance of euglycemia and a delayed onset of fatigue.

Inhibition of fatty acid oxidation

We mentioned that an increased exogenous carbohydrate availability enhances their oxidation and thus the glycolytic flux. In an older post we also saw how an increased glycolytic flux inhibits the oxidation of fatty acids (you can see it here).

Generally speaking, an enhanced glycolytic flux give rise to an increased Acetyl-CoA production and accumulation. This results in carnitine sequestration, which is a key piece for fatty acid transport into the mitochondria by CPT1. Also, Acetyl-CoA accumulation increases the production of Malonyl-CoA, which inhibits CPT1 activity and thus the transport of fatty acids into the mitochondria for further oxidation. Overall, a decrease in free carnitine and an increase in Malonyl-CoA levels following a high glycolytic flux inhibits free fatty acid oxidation.

Also, carbohydrate feeding during exercise attenuates the decrease in insulin and the increase in catecholamines observed during exercise, which negatively impacts on lipolysis, thus reducing plasma fatty acid availability and further reducing fatty acid oxidation.

Altogether, the ingestion of carbohydrates during exercise causes a shift towards carbohydrate utilization.

Increased bioenergetic efficiency

Through calorimetry studies, it is well known that carbohydrates are a more efficient substrate compared to fat in terms of energy released per litre of oxygen consumed.

To give you a better idea of this, the energy released per litre of oxygen when fat are the only substrate oxidized is 4.7 kcal, while if carbohydrates are the only substrate being burned the value rises to 5.05 kcal. This means that, at the same relative intensity, the power output or speed will be greater when the body uses carbohydrates instead of fat as energy fuel.

The shift towards carbohydrate oxidation during exercise when consuming carbohydrates will enhance our efficiency and enable us to achieve greater speed or power outputs at the same relative intensity. Isn’t it great?

Central effects of carbohydrate ingestion during exercise

Beyond the metabolic effects that we have just mention, carbohydrate ingestion during exercise has been shown to have other effects that are not, at least directly, related to metabolism, but to the nervous system instead.

Mouth-brain communication

Until now, we have talked about the benefits of carbohydrate ingestion during moderate-to-high intensity prolonged exercise. However, the benefits of carbohydrates are not restricted to endurance events. In this respect, carbohydrates have been shown to have ergogenic effects during relatively short (< 60 min) high-intensity exercise.

However, muscle and liver glycogen are not limiting factors of performance in such events. In addition, the amount of carbohydrates that can be absorbed in such a short period of time is small – it was estimated to be approximately 15 g – and would hardly have any impact on metabolism.

In fact, a study by Carter et al. showed that infusion of glucose did not improve performance as oral consumption of glucose did – a mouth rinse with a carbohydrate-containing solution was enough to positively impact performance. This demonstrated that the mechanism by which glucose improves performance during this type of exercise is not metabolic but rather central.

It is believed that this performance-enhancing effect of carbohydrates during short, high-intensity exercise occurs via a mouth-brain communication. It is suggested that carbohydrates are sensed in the oral cavity, which then stimulate the reward and/or pleasure centres in the brain. An important fact here is that this mechanism is triggered not only by sweet tasting – sugars – and non-sweet tasting – maltodextrin – carbohydrates, suggesting that it is not the taste what triggers these neural messages.

Then, in a practical level, oral exposure to carbohydrates via carbohydrate mouth rinse – with enough oral contact time, between 5 to 10 sec – provides an additional mechanism for enhancing performance.

Reduced rating of perceived exertion – RPE

As indicated by its name, the rating of perceived exertion (RPE) is a quantitative measure of perceived exertion – or feeling of effort – during physical activity. If you are familiar with RPE, you probably know that it is one of the best indicators of internal training load at present.

It is well established that both carbohydrate mouth rinse and their consumption result in a decrease in the perceived exertion, which in turn decreases the feeling of dizziness and weakness during exercise.

This enable the athletes to exercise at higher intensities with the same feeling of effort, or to exercise at similar intensities with a lower impact on the internal training load, which in turn improves recovery.

Wrap up

In conclusion, different mechanisms account for the enhanced performance observed when carbohydrates are ingested during exercise.

On one side, carbohydrate ingestion impacts on substrate utilization in different ways. It increases the oxidation of exogenous carbohydrates while sparing endogenous stores. In addition, it causes a shift towards carbohydrate utilization at the expense of fat as energy substrates, which results in an increase in bioenergetic efficiency.

But not all the effects of carbohydrates involve changes in metabolism. They also have central effects. In this line, it is well known that carbohydrates can enhance exercise performance during short events by stimulating the reward and/or pleasure centres in the brain. Besides, they reduce the perceived exertion during exercise, which in turn enhances the ability of the athlete to exercise at higher intensities.

I hope you enjoyed this blog and learnt something new and useful! Again, do not hesitate to contact me for any doubt or suggestion, or if you just want to talk about this!

Thanks for your time!

References

  1. Jeukendrup, Asker E. 2004. “Carbohydrate Intake during Exercise and Performance.” Nutrition 20 (7–8): 669–77.
  2. Jeukendrup, Asker. 2014. “A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise.” Sports Medicine 44: 25–33.
  3. Smith, Johneric W., Jeffrey J. Zachwieja, François Péronnet, Dennis H. Passe, Denis Massicotte, Carole Lavoie, and David D. Pascoe. 2010. “Fuel Selection and Cycling Endurance Performance with Ingestion of [13C]Glucose: Evidence for a Carbohydrate Dose Response.” Journal of Applied Physiology 108 (6): 1520–29.
  4. Tsintzas, Kostas, and Clyde Williams. 1998. “Human Muscle Glycogen Metabolism During Exercise.” Sports Medicine 25 (1): 7–23.

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