Ugent - The science behind carbohydrates and endurance performance

Carbohydrates and fats are the primary sources of energy for our skeletal muscles during prolonged exercise. As the intensity of the exercise increases, a greater proportion of energy is derived from carbohydrates. Therefore, having sufficient carbohydrate stores is crucial for optimizing sports performance during moderate to high-intensity efforts. The new Etixx sports drink PRO LINE High Carb Drink fits perfectly within this framework. With no less than 70 grams of carbohydrates per 500 ml, this drink provides a fast and efficient way to replenish carbohydrate stores during extended efforts.
In the blog below, Freek Van de Casteele from the Department of Movement and Sports Sciences at Ghent University delves deeper into how many carbohydrates we actually need to avoid "hitting the wall" and what ratio is most suitable.

How many carbohydrates do you really need? From 30 to 120 grams per hour

In the human body, there is a limited supply of carbohydrates stored under the form of glycogen in skeletal muscle and in the liver. In addition, there is a small amount of glucose in our blood. These endogenous carbohydrates, and in particular muscle glycogen, are sufficient to provide the necessary energy during short efforts of <1 hour. Interestingly, however, a mouthwash with a carbohydrate-rich drink does improve performance on efforts of ~1h (Carter et al., 2004) . This is because the oral perception of carbohydrate causes certain brain regions of reward and motor control to be stimulated (Chambers et al., 2009).

Effective carbohydrate intake has a clear performance-enhancing effect only during longer endurance exercise (Jeukendrup, 2008) . These exogenous carbohydrates are, on the one hand, an additional source of energy for the muscles and, on the other hand, they help keep blood glucose concentration stable and spare liver glycogen. There are studies showing that muscle glycogen is also spared, but this is often not confirmed. How much and which carbohydrates it is best to take in depends mainly on the duration of exercise. During efforts of 1 to 3 hours, it is recommended to take 30 to 60 grams of fast-oxidising carbohydrates per hour (Table 1) (Burke et al., 2011; Thomas et al., 2016) . Some examples of such carbohydrates include glucose, maltodextrin, which a chain of multiple glucose molecules, and glucose-fructose mixes. These can be ingested through daily foods or specific sports nutrition products. Bars, gels and drinks are equally effective because the form does not affect how much exogenous carbohydrate is oxidised and thus contributes to energy production in the muscle (Pfeiffer et al., 2010a, 2010b) . So the choice will mainly depend on how practical it is to consume a particular nutrient in your sport, the need to hydrate and your personal preference.

During efforts longer than 3 hours, the traditional guideline is to take up to 90 grams of carbohydrates per hour. Here, it is essential to consume a combination of glucose or maltodextrin and fructose. This is because the absorption of glucose and fructose from the small intestine is via different transporters. The glucose transporter (SGLT1) is saturated at a glucose intake of 60 to 70 grams per hour. Ingesting more glucose does not lead to higher burning of exogenous carbohydrates, because glucose simply cannot be absorbed faster. The addition of fructose does lead to higher burning of exogenous carbohydrates (Jentjens et al., 2004) , because it is transported via a different transporter (GLUT5). Combining glucose and fructose also effectively leads to improved performance. For example, in a study in which trained cyclists completed a 40 km time trial after 2 hours of cycling at moderate intensity, they cycled the time trial 8% faster when taking a mix of glucose and fructose in a 2:1 ratio than when taking glucose alone (Currell & Jeukendrup, 2008).
 

Table 1 Guidelines for carbohydrate intake during endurance exercise.

Based on Thomas et al ( .2016)

A glucose-to-fructose ratio of 2:1 or 1:0.8

It is clear that a mix of glucose and fructose is needed for optimal performance in endurance sports of >3 hours, but what is the best ratio? Initially, a glucose fructose ratio of 2:1 was the standard guideline, but there is research indicating that ratios approaching an equal amount of glucose and fructose, such as 1:0.8, may be even better (O'Brien et al., 2013; Rowlands et al., 2015) . This can partly be explained by the fact that a higher proportion of ingested carbohydrates can also be effectively used as energy. Moreover, the risk of gastrointestinal problems is slightly lower at a 1:0.8 ratio.

During sports competitions such as the Tour de France and Ironmans, elite athletes sometimes take in more than 90 grams of carbohydrates per hour. So the classic guideline of maximum 90 grams per hour may need to be revisited. First, it is important to know whether even higher carbohydrate intakes are at all feasible without developing gastrointestinal problems. For 120 grams per hour this seems to be the case during cycling, also for trained amateurs (Hearris et al., 2022) , but during running this is less evident. Indeed, a study of elite trail runners showed that two out of nine runners who ingested this much carbohydrate had to give up because of gastrointestinal problems during a mountain marathon (Viribay et al., 2020) . If you want to take in large amounts of carbohydrates during competitions, it may help to train your gastrointestinal system accordingly. This is called 'training the gut' and an example of this is to train immediately after a meal (Jeukendrup, 2017) . An intake of 120 grams of carbohydrates per hour in a glucose fructose ratio of 1:0.8 during 3 hours of cycling, provides a higher consumption of exogenous carbohydrates than 90 grams per hour in a 2:1 ratio (Podlogar et al., 2022) . However, there is currently little to no scientific evidence for a performance benefit at intakes of 120 grams compared to 90 grams per hour. What can be stated with certainty, however, is that for carbohydrate intakes exceeding 90 grams per hour, a ratio approximating an equal amount of glucose and fructose, such as 1:0.8, is recommended. This way, the capacity of the glucose transporter (SGLT1) is not exceeded and a greater proportion of the ingested carbohydrate is also effectively used as fuel.

Freek Van de Casteele - Department of Movement and Sports Sciences University of Ghent

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