Carbohydrate – high-octane fuel for endurance athletes

ScienceViews 6. aug 2021 6 min Assistant Professor Kasper Degn Gejl Written by Kasper Degn Gejl

Our understanding of the role of carbohydrate in muscle function and performance has expanded significantly in the past 120 years. Several studies have shown why carbohydrate is essential for muscle function and athletic performance. Despite the lack of evidence, many coaches and athletes still swear that endurance athletes should eliminate or reduce carbohydrate intake and instead improve the body’s ability to use fat as fuel. However, the latest science underscores the importance of carbohydrate and especially muscle glycogen for endurance events.

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The Olympic Games are underway, and with the world’s best athletes gathered in one place, small margins will determine the distribution of medals in many disciplines. Several Olympic disciplines require high work intensity, and some over a long duration. For example, the winner of the men’s marathon crosses the finish line after about 2 hours of work at about 90% of the maximum heart rate, and the riders in the road cycling races are typically in the saddle for 5–6 hours, with a mix of light, moderate and very intense work periods along the way.

The composition of the athletes’ diet plays a key role especially in longer events, and the wrong nutritional strategy before and during an event can decisively hurt performance.

The endurance sports events naturally require a substantial amount of energy, and the athletes have two primary energy sources available – fat and carbohydrate. This has been known since about 1900, when Nathan Zuntz (1847–1920) demonstrated that fat is an energy source. Until then, it was actually believed that fat had to be converted into carbohydrate before the muscles could use it as an energy source. However, Zuntz also observed that after consuming a high-fat diet the oxygen required for physical activity increased.

Subsequently in 1920, August Krogh (1874–1949) and Johannes Lindhard (1870–1947) also observed that performance during high-intensity work deteriorated after consuming a high-fat diet. Thus, evidence established early that diet can alter the body’s choice of energy source and that the transition to a high-fat diet can affect performance.

Glycogen – a valuable but limited source of energy

The body’s largest carbohydrate store is the muscles’ storehouse of glycogen – long-branched chains of 10.000 glucose molecules. After the muscle biopsy was introduced, Jonas Bergstrøm and colleagues conducted a landmark study in 1967 that clearly demonstrated that endurance performance depends on the availability of glycogen in the working muscles.

This presents a considerable challenge for athletes because this carbohydrate store is small and is quickly depleted during high-intensity work. However, periods of endurance training and a high-carbohydrate diet in the days leading up to competition have been shown to increase the storage of muscle glycogen, and the muscles of endurance athletes in the Olympic Games are therefore better equipped to store glycogen than the muscles of less-trained individuals.

Thus, although it has been known for more than 50 years that a lack of muscle glycogen inhibits endurance performance, the underlying mechanisms are still only partly understood. In our research unit, we therefore try to understand how glycogen influences muscle cell function.

In several studies, we have investigated how the presence of glycogen affects several of the key mechanisms that ultimately lead to contractions and force generation in the muscle fibres. We have shown how energy-intensive components in the muscle fibre, such as the sodium-potassium pump, which ensures that the electrical signals from the nervous system can be transmitted to the muscle fibre, function optimally in the presence of glycogen.

Further, the results of other studies with well-trained endurance athletes have indicated that glycogen also promotes the non-energy-intensive and extremely important process in which muscle cells release calcium to proteins that can contract and thus initiate the mechanical contraction of muscle fibres.

Glycogen is thus more than just an energy source, and several studies have shown how abundant energy – in the form of ATP – does not optimally support the needs of muscle fibre without the presence of glycogen. Thus, great ability to store and break down glycogen seems to be important when a sport requires rapid and repeated muscle contractions.

Location, location, location

Further, our research has shown that glycogen should not solely be considered globally in the muscle cell. The sodium-potassium pumps, the calcium-release channels and the proteins that generate force in the muscle fibres are located in different places in the muscle cell, and the same applies to the glycogen molecules.

We hypothesise that the various glycogen pools are located strategically to locally support the energy-intensive processes in different places in the muscle cell. We think that this distribution of glycogen in the cell enables ATP to be produced much closer to the site where it will be used.

Therefore, if one of these specific glycogen pools is depleted, energy deficiency is likely locally and may inhibit important steps in activating the muscle cells and ultimately performance.

We have previously shown how the amount of glycogen in these specific pools is related to the function of the local processes that underlie the muscle’s ability to contract, and this again emphasises the importance of having glycogen available in different parts of the muscle cell.

Dietary manipulation – a future nutrition strategy for athletes?

As described above, today we know a lot about how people can use diet to manipulate the body’s choice of energy sources during prolonged endurance work. Nevertheless, the jury is still out on the optimal distribution of fat and carbohydrate in the diet of endurance athletes.

In connection with competitions at moderate and high intensity, the glycogen tank of athletes will continue to be emptied, and even if they consume carbohydrate during the competition, fat will become the dominant fuel at some point.

Unfortunately, fat is metabolized relatively slowly, and the ability to perform moderate- and high-intensity work therefore deteriorates significantly when the body’s carbohydrate stores are reduced to a critical level and athletes metaphorically hit the wall.

But what if athletes could improve their body’s ability to use fat as an energy source? Could they then conserve muscle glycogen and make it last longer, thereby performing better in the endurance disciplines?

Several inappropriate adjustments

After many years of training, Olympic endurance athletes have high capacity to metabolise fat as an energy source, but this capacity can actually be increased in just a few weeks by switching to a high-fat and low-carbohydrate diet – even among already highly trained elite athletes.

For this reason, many recreational athletes, elite athletes and coaches swear by a diet low in carbohydrate, and the discussion has sometimes been heated on social media and in scientific contexts about how relevant a high-fat diet is for endurance athletes.

Despite an improved ability to burn fat, however, identifying the literature and evidence showing that switching well-trained endurance athletes to a high-fat diet improves performance is very difficult.

In reality, the existing evidence shows that a high-fat diet does not improve endurance performance, and many studies have even shown impaired performance. The lack of effect on performance may result from several inappropriate adjustments that also come with a high-fat diet.

Possibly underestimated

As mentioned earlier, one factor is increased oxygen demand in the muscles when fat is the primary source of energy, but the ability to utilise carbohydrate and glycogen as an energy source is also reduced.

Reduced ability to store and break down glycogen should presumably impair function in essential parts of the muscle cell, which can thus help to explain the impairment of both the intensity of training and performance associated with a high-fat diet.

Reduced ability to work at high intensity is extremely critical in endurance sports competitions, with fluctuating and occasionally very-high intensity. Many people think that marathon runners and triathletes perform at relatively stable intensity, but the crucial periods in these sports, and endurance sports in general, are often high intensity.

I therefore think that some scientific contexts may even underestimate the negative effect of a high-fat diet on endurance performance, since most existing studies have evaluated endurance performance in tests with a fixed load and thus without the periodic shifts to high intensity that occur in sports such as cycling, triathlon and running.

Periodic training without carbohydrate – an appropriate compromise?

An alternative strategy frequently used by endurance athletes is periodic training with low carbohydrate and glycogen availability. With this strategy, athletes have some training sessions with high carbohydrate availability, thus ensuring the ability to train at high intensity, while other and less intense training sessions are performed with low carbohydrate availability to enhance the training effect.

One way of achieving this is to train twice each day: first with an intense workout that aims to reduce the amount of glycogen in the muscles, a recovery period without carbohydrate intake and then a workout that can then be completed with reduced glycogen availability.

This strategy is based on several studies over the past 20 years showing how the acute response to endurance training is enhanced when training takes place with low energy availability and completely or partly depleted glycogen stores.

This leads to increased metabolic stress manifested by increased upregulation of the muscle cells’ central energy sensor AMPK, which precisely activates signalling pathways that are especially important in achieving high-endurance muscles. This applies, for example, to the cell signalling that initiates the muscle cells’ new formation of mitochondria, which for endurance athletes is an especially important component of the muscle cells.

No evidence indicates that removing carbohydrate

Despite these promising findings, however, we recently concluded in a meta-analysis that no evidence yet indicates that this strategy enhances the performance of elite endurance athletes. Thus, periodically restricting carbohydrate intake several times a week for a longer training period does not improve performance compared with control groups training with high carbohydrate availability.

This inability to convert the otherwise promising short-term effects to improved long-term performance may result from the fact that the athletes in the short-term studies were exposed to an unfamiliar training and nutrition strategy and that the positive short-term effect therefore wanes as the athletes get used to it.

We have also observed that depleting the large muscle glycogen stores of athletes repeatedly in real-life training is difficult and that a lack of glycogen depletion can therefore explain the lack of training effect. Finally, the training itself may simply result in a maximal or near-maximal training response, which cannot be further enhanced by restricting carbohydrate intake.

This is supported by the marked improvements in performance observed in most of the control groups who trained with high carbohydrate availability.

So despite the possibility of increasing the muscles’ ability to use fat as an energy source with a long-term high-fat diet and despite promising short-term studies with periodic carbohydrate restriction, no evidence indicates that removing carbohydrate from the diet enhances endurance performance – neither long term nor periodically.

Thus, the endurance athletes at the Olympic Games and other athletes competing in long-distance events with high-intensity bursts should have carbohydrate both when they train and when they compete.

Kasper Degn Gejl is an Assistant Professor in the research unit for Muscle Physiology and Biomechanics, Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense. He is part of DEEPn – the Danish Elite Endurance Performance Network, one of the five research networks supported by the Novo Nordisk Foundation that Team Danmark launched to improve Danish athletes’ potential for success.

Further reading

Zuntz N. Über die Bedeutung der verschiedenen Nährstoffe als Erzeuger der Muskelkraft. Archiv für die gesamte Physiologie des Menschen und der Tiere. 1901;83:557–71.

Krogh A, Lindhard J. The relative value of fat and carbohydrate as sources of muscular energy: with appendices on the correlation between standard metabolism and the respiratory quotient during rest and work. Biochem J. 1920;14:290–363.

Bergstrom J, Hermansen L, Hultman E, Saltin B. Diet, muscle glycogen and physical performance. Acta Physiol Scand. 1967;71:140–50.

Ørtenblad N, Westerblad H, Nielsen J. Muscle glycogen stores and fatigue. J Physiol. 2013;591:4405–13.

Jensen R, Nielsen J, Ørtenblad N. Inhibition of glycogenolysis prolongs action potential repriming period and impairs muscle function in rat skeletal muscle. J Physiol. 2020;598:789–803.

Gejl KD, Hvid LG, Frandsen U, Jensen K, Sahlin K, Ortenblad N. Muscle glycogen content modifies SR Ca2+ release rate in elite endurance athletes. Med Sci Sports Exerc. 2014;46:496–505.

Burke LM. Ketogenic low-CHO, high-fat diet: the future of elite endurance sport? J Physiol. 2021;599:819–43.

Gejl KD, Nybo L. Performance effects of periodized carbohydrate restriction in endurance trained athletes – a systematic review and meta-analysis. J Int Soc Sports Nutr. 2021;18:37.

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