How many carbohydrates are needed during hard physical activity to maintain performance levels?
Written by: Hans Kristian Stadheim
Key points:
Carbohydrates are important for endurance performance, especially at high work intensity.
During long-term endurance activity (45 min +) where the relative work intensity is high (85% HRmax and above), the availability of carbohydrates to produce energy (ATP) can become a decisive factor in determining performance, as well as influencing training effect/adaptations.
Based on the scientific literature, current recommendations for carbohydrate intake are: 30g/h for activities lasting between 30-60 minutes, 60g/h for durations between 45-90 minutes, and for activities over 90 minutes, 90g/h.
Insufficient carbohydrate intake has been associated with a significant reduction in performance by as much as 30-40% if the body must switch to fat oxidation during high-intensity exercise.
Introduction
Athletic performance, including performance in cross-country skiing, is limited by several physiological, psychological, and biomechanical factors. In light of this, nutrition is one of several factors that are important to optimize in order to benefit from training (optimize recovery), as well as to maximize performance during high-intensity sessions and competitions (3). In particular, muscle access to “fast energy” (carbohydrates/glycogen) has been shown to be decisive for high endurance performance (8). Most people interested in cross-country skiing and endurance sports understand that carbohydrates are important for optimal performance. As a result, carbohydrate intake to optimize training performance has become almost mandatory in recent years, both during competitions and training (3,5,6,7).
Carbohydrates consumed before, during, and after training can all influence the quality of training and competition performance. This likely also has implications for the effect of physiological responses and adaptations. In this article, we will focus specifically on the intake of carbohydrates during (acute) competitions and training. The goal is to get the greatest possible benefit from your training or to bring out the maximal physiological potential during a given competition. While we will primarily focus on intake during (acute) activity, carbohydrate intake in the days leading up to, and after training/competitions will, in most circumstances, ensure muscle glycogen stores are maximized (9). Additionally, consuming carbohydrates 3–4 hours before training/competition is likely to increase muscle glycogen content. During activity, carbohydrate intake helps maintain blood glucose levels and/or provides fuel for oxidation. This often means that muscle and liver glycogen can be conserved.
Finally, post-exercise intake aims to replenish both muscle and liver glycogen stores (3). The speed of recovery post-training/competition depends on factors like timing and the amount of carbohydrates (and proteins). Fast and large amounts of carbohydrates can be essential for optimizing performance during a subsequent session or competition (8). Therefore, carbohydrate intake should be considered both before and after training/competition, though this article will focus on during.
It is also important to note that, as with most topics in exercise science and nutritional physiology, this is a complex puzzle depending on several individual factors. The recommendations in this article should be regarded as general advice based on existing scientific studies, as well as practical experience. We at Team Aker Dæhlie hope you find this content engaging and, if nothing else, that it inspires you to test these tips in training or competition with the goal of achieving optimal performance and potentially improving your training results.
General recommendations for carbohydrate intake
The Norwegian Directorate of Health recommends that the general population consume a balanced diet. Of the total energy intake, it is recommended to consume:
10-20% protein
25-35% fat
50-60% carbohydrates (1)
If you engage in physical activity/exercise, you likely need to adjust your total food intake (kcal), and the composition accordingly, to ensure the human body has the energy and nutrients it needs to function optimally. This is determined by factors such as the type of activity/exercise (intensity, quantity, duration), and how often it is performed. Olympiatoppen recommends:
5-7 grams of carbohydrates per kilogram of body weight per day - if you train for less than 1-1½ hours a day.
7-10 grams of carbohydrates per kilogram of body weight per day - if you train for more than 1½-2 hours daily.
10-12 grams of carbohydrates per kilogram of body weight per day - if you train 6-8 hours a day (2).
If you are aiming to perform well in sports or high-intensity endurance workouts, there is little doubt today that carbohydrates are important both before and, in many cases, during both training and competition (3,4,5,6,7). This applies to both the physical and mental factors that influence performance. But what exactly are carbohydrates?
What are carbohydrates?
Carbohydrates are found in most foods. Some foods with high (concentrated) amounts include grains, rice, pasta, and fruit. Less healthy products that contain carbohydrates include sweets, soft drinks, and cakes, to name a few. These "unhealthy" products have often been labeled "empty calories," which is not entirely correct. They actually contain a high amount of carbohydrates, and in many cases, concentrated, high-glycemic carbohydrates.
Figure 1: When we consume carbohydrates, our blood sugar increases resulting in secretion of insulin. Carbohydrates are essential for our ability to produce energy (ATP) during high-intensity endurance work. Image taken from: https://eurodiet.no/temaartikler/blodsukker-og-hormonbalanse/
On the other hand, "unhealthy" foods will, to a greater extent, lead to a sudden and rapid increase in blood sugar (as well as the release of insulin), which means the feeling of satiety will disappear more quickly, and people tend to eat more calories in total. This is often viewed as negative, unless you are physically active and need a lot of energy (kcal) (Figure 1).
This article will not discuss this phenomenon or the health issues related to this concept, but will instead focus on the actual importance of carbohydrates for high sporting performance. The concept of fast vs. slow carbohydrates is, in light of this, likely based on a general misconception in society. It is true that some carbohydrate-rich products are "fast" or "slow," but it would perhaps be more accurate to say this refers to how quickly the body can utilize the glycogen molecules to produce energy (ATP) and trigger the release of insulin. A general misconception among many is that these products contain less/more energy, but as mentioned above, this mainly relates to the feeling of satiety and how quickly our blood sugar rises (and the amount of insulin released). One gram (1g) of carbohydrates (regardless of type) provides 4.2 kcal of energy, but the type can be of great importance for physical performance during activity (4). When discussing carbohydrate intake during activity itself, how much carbohydrate is needed during intense physical activity to maintain performance, according to the scientific literature and research?
Carbohydrates and exercise performance
If you engage in high-intensity work (85% of HFmax and above) over a longer period of time (30 minutes or more), carbohydrates are by far the body's most important source of energy (3). To maintain a certain work intensity, for example, a specific speed (such as km/h or watts), there is a risk that the carbohydrate stores (glycogen) in the muscles and liver may run "empty" after 45-75 minutes. Recently, studies have also shown ergogenic effects of carbohydrates during shorter high-intensity exercise (approximately 20-40 minutes at over 80% of maximum oxygen consumption). In such cases, glycogen stores are likely not empty, but the amount of stored carbohydrates (glycogen) has dropped to levels that cause the body to reduce performance neurologically (8). Should this occur, one would reduce work intensity due to a shift in energy substrate oxidation, utilizing fat/lipids to form ATP. If this happens during high-intensity exercise, it is often referred to in endurance environments as "hitting the wall" or "running out of steam."
To prevent this from happening, it is common to recommend consuming various carbohydrate-rich products during both long-term endurance activity and high-intensity exercise, such as gels and sports drinks. The composition of these products varies, but they generally consist of 99% carbohydrates (glucose, maltodextrin, and fructose) and 1% salts (magnesium, calcium, sodium). The composition of these energy-rich products is largely based on scientific studies clearly showing that carbohydrate composition is critical for absorption during activity (5,6,7). Furthermore, it is quite clear that during activity, it is the "fast" and "unhealthy" carbohydrates that you want to consume, as these can be utilized by the body quickly and efficiently, raising our blood sugar levels, which is beneficial both physically and mentally during exercise.
In Team Aker Dæhlie, we use Maurten products to ensure we don't run out of energy during both competitions and training. This is crucial for us to be able to perform at the highest level. You can read more about some tips and tricks from them here:
https://www.maurten.com/events/ironman
How much carbohydrates are available in the body?
When we eat food, carbohydrates are broken down in the digestive system into small sugar molecules called glucose (except for dietary fiber). After this breakdown, glucose travels through the bloodstream to the cells, where it is used to create energy (ATP). Glucose is also stored as glycogen (long glucose chains) in the muscles and liver. We rely on these stores, especially when we need extra energy during training. A direct link has been shown between endurance performance and glycogen stores in the muscles (3,7,8), making sufficient carbohydrate intake essential for optimal training and competition performance. The cells in the body absorb and utilize glycogen (glucose) as needed. The higher the work intensity (higher % of HFmax), the greater the need for carbohydrates as an energy source (Figure 2).
Figure 2: A simplified presentation of the recommended intake of carbohydrates per hour for various work intensities and activity durations.
The body's total carbohydrate stores amount to approximately 300-600g (400-500g in muscles and 100-150g in the liver), which equates to around 1600-2500 kcal. This amount allows for high-intensity work for roughly 45-60 minutes (+/- 15 min) for most people. When these glycogen stores are depleted, the phenomenon of "hitting the wall" occurs. This can happen during both training and competition and is primarily due to glycogen stores being nearly exhausted. However, it's important to note that the body's glycogen stores never truly run completely empty during physical activity. Since glucose (glycogen) is vital for brain function, the body ensures that a small amount remains in the muscles and liver (3). This is part of why different individuals experience performance reduction with varying amounts of remaining carbohydrates.
Figure 3: Screenshot of a training session. Here we see that as work intensity increases (measured as % of HFmax), the need and consumption of carbohydrates (red line) rise compared to fat (yellow line). These values should be seen as estimates.
There are significant individual differences in how low a person's glycogen levels can go before "hitting the wall." However, general studies indicate this occurs when there are approximately 25-50g left in the liver and 50-100g in the muscles (6,7,8). This means the actual energy available for high-intensity work is around 250-300g of glycogen (if the stores are full), which corresponds to about 1000-1200 kcal, enough for around 30-45 minutes (+/- 15 min) of high-intensity work before glycogen is depleted. To maintain the same work intensity over this duration, more carbohydrates must be consumed. Otherwise, the body will switch to using fat/lipids as the energy substrate to form ATP. This shift requires a reduction in work intensity.
Keep in mind, the more oxygen consumed (VO2), the more kcal burned per minute. As a general rule of thumb, for every liter of oxygen consumed, approximately 5 kcal are burned, which corresponds to 1.2g of carbohydrates. This conversion factor allows you to estimate energy usage based on VO2 (8).
How much carbohydrates should one consume during high-intensity physical activity?
With the above in mind, the following recommendations are general guidelines. At Team Aker Dæhlie, we strongly encourage everyone to experiment and "research" how much carbohydrate intake works best for them individually. The amount of carbohydrates needed during physical activity depends on several factors, including:
Duration
Work intensity
The goal of the training session,
F.ex. which energy systems you want to stimulate/influence/train. For high-intensity exercise, the following intake is recommended:
30g/hour for activities lasting less than 30-60 minutes.
60g/hour for activities lasting 45-90 minutes.
90-120g/hour for activities lasting longer than 90 minutes.
Figure 4: Intake of different carbohydrate quantities based on intensity and duration (11).
The number of grams of carbohydrates one should consume is largely influenced by the intensity of the exercise (Figure 4).
Using the data from Figure 4 as an example, maintaining the highest heart rate value for 60 minutes would require approximately 20 kcal per minute, totaling 1200 kcal. This suggests that oxygen usage at this workload is approximately 4L per minute. If no carbohydrates are consumed during such an activity, performance could drop toward the end of the session.
However, research has not always shown that consuming exogenous carbohydrates during exercise improves performance. That said, few studies have observed a decrease in performance due to carbohydrate intake (11). In one study by Pettersson et al. on Swedish biathletes/skiers, consuming 130 grams of carbohydrates per hour over 120 minutes at an intensity of 70% VO2max, followed by an hour-long trial, did not show a significant improvement. These findings align with earlier studies on time trials lasting less than 25 minutes (8, 11).
There is some suggestion that periodized carbohydrate intake may benefit endurance athletes during training (9), although current evidence is limited. Nevertheless, consuming carbohydrates during training offers some advantages:
Increased "durability"/resistance to fatigue: Higher training volumes can likely be sustained with carbohydrate intake.
Proactive recovery: Carbohydrate consumption aids in recovery, allowing for continuity in training by maintaining fuller glycogen stores and enabling you to finish workouts in better condition (Figure 5).
Figure 5: Consumption of different energy substrates depending on whether (water/placebo) or carbohydrates were consumed during activity (10).
We hope the information in this article helps you perform better in both competitions and training!
Special thanks to Tobias Christensson from Maurten for his valuable input and review of the article!
Summary and "Take-Home Message"
Carbohydrates are crucial for endurance performance, especially during high-intensity work.
This is particularly important during prolonged endurance activity (45 minutes and beyond) where the relative work intensity is high (85% HRmax and above).
Based on scientific research, increasing carbohydrate intake is recommended according to the duration and intensity of the exercise.
Fast carbohydrates are the preferred choice, as they are absorbed quickly and effectively increase blood sugar levels.
General recommendations for maintaining performance quality during high-intensity exercise are:
30g/hour for activities lasting 30-60 minutes,
60g/hour for activities lasting 45-90 minutes,
90g/hour for exercises lasting longer than 90 minutes.
References
Helsedirektoratet Norge 2023 - https://www.helsedirektoratet.no/faglige-rad/kostradene-og-naeringsstoffer/inntak-av-naeringsstoffer
Olympiatoppen 2023 - https://olympiatoppen.no/fagomrader/idrettspsernaering/faktaark/
Jeukendrup Asker - A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise. Sports Medicine volume 44, pages25–33 (2014)
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Fielding RA, Costill DL, Fink WJ, et al. Effect of carbohydrate feeding frequencies and dosage on muscle glycogen use during exercise. Med Sci Sports Exerc. 1985;17:472–6.
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Stefan Pettersson, Fredrik Edin, Linda Bakkman, Kerry McGawley (2019). Effects of supplementing with an 18% carbohydrate-hydrogel drink versus a placebo during whole-body exercise in −5 °C with elite cross-country ski athletes: a crossover study. Journal of the International Society of Sports Nutrition volume 16, Article number: 46
Podlogar T and Wallis G.A (2022). New Horizons in carbohydrate reaserch and application for endurance athletes. Sports Medecine 52.