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Nutrition and Performance in Sport

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Christophe Hausswirth

Chapter 1. Nutrition: from training to competition

Topic 1. Carbohydrate, sports drinks and performance: strategies for Olympic sports

Asker Jeukendrup et Lindsay Baker

Résumé

For Olympic endurance events lasting 30 minutes or longer carbohydrate intake during exercise is an important determinant of performance. Although this is true for most sports, this review will focus mostly on endurance activities and less on stop-and-go sports. An individualized nutritional strategy can be developed that aims to deliver carbohydrate to the working muscle at a rate that is dependent on the absolute exercise intensity as well as the duration of the event. Although it has been known since the 1980s that carbohydrate ingestion during exercise can improve endurance exercise performance, it is only in the past ten years that we are getting a better understanding of the optimal amounts and types of carbohydrates to ingest. Studies in the 1980s and 1990s demonstrated that carbohydrate ingested during exercise was oxidized at rates up to 60 g/h. Carbohydrates could be separated into two categories: slowly oxidized carbohydrates such as fructose, galactose and insoluble starch and rapidly oxidized carbohydrates such as glucose, sucrose, maltose and maltodextrins. However, even with very high-carbohydrate intake, exogenous carbohydrate oxidation did not exceed 250 kcal/h whilst energy expenditure during activities could easily amount up to 1000 kcal/h. It became obvious that intestinal absorption was the main limitation to providing exogenous carbohydrate to the working muscle. This can at least partly be overcome by making use of multiple transportable carbohydrates. Ingestion of these carbohydrates may result in higher intestinal absorption rates and has been shown to lead to higher rates of exogenous carbohydrate oxidation (up to 105 g/h), which can also result in better endurance performance. In the past few years, studies have emerged to demonstrate a dose–response relationship between exogenous carbohydrate oxidation rates and performance suggesting that higher intake rates result in superior performance. This is reflected in practical recommendation: carbohydrate intakes of 90 g/h for events longer than 2.5 h as a balance between practicalities and ergogenic effects. It also seems possible to increase the absorptive capacity of the intestine by adapting to a high-carbohydrate diet. Although there are still many unanswered questions, it is clear that there are ample opportunities to develop strategies that enhance the delivery of carbohydrates and thereby improve endurance performance. These strategies should also include aiming to maintain hydration status (avoid >2% loss in body mass) to contribute to the prevention of fatigue, especially when exercising in warm-hot environments.

Texte intégral

1. Introduction

1It has been known for some time that carbohydrate feeding during exercise can enhance exercise performance. Initially, it was concluded that carbohydrate could enhance performance lasting two hours or longer. More recently it was discovered that carbohydrate could also improve exercise performance during shorter higher intensity exercise, although the mechanisms were completely different. Because many Olympic sports are generally 30 minutes or longer, this chapter will discuss the evidence of performance effects as well as practical guidelines and the underlying scientific basis for this advice.

2. Physiological and nutritional demands of sport

2The physiological demands of a sport influence the nutritional demands. For most sports, provision of energy and maintaining hydration are probably the most important challenges. During most competitive situations, muscle glycogen and blood glucose are the primary substrates for the contracting muscle (Romijn et al. 1993). Fatigue during prolonged exercise is often associated with muscle glycogen depletion, diminished total carbohydrate oxidation and/or reduced blood glucose concentrations (Jeukendrup, 2004). Maintaining carbohydrate oxidation through carbohydrate feeding has proven to be an effective strategy to delay fatigue.

3In addition to carbohydrate depletion, dehydration can also impaire endurance performance (see Sawka et al. 2007, for review). Sweat losses occur because there is the need to dissipate the heat that is generated during exercise. Therefore, the nutritional challenge is to prevent major dehydration (>2%) and thereby contribute to the prevention of fatigue (Shirreffs et al. 2001). This recommendation is in line with the most recent guidelines by the American College of Sports Medicine (ACSM) stating that dehydration of more than 2% of body mass should be prevented but also warning against drinking in excess of sweating rate (Sawka et al. 2007) to prevent hyponatramia. Here we will focus mainly on carbohydrate intake during the events.

3. Carbohydrate ingestion during exercise and performance

4Although the exact mechanisms are still not completely understood, it has been known for some time that carbohydrate ingestion during exercise can increase exercise capacity and improve exercise performance (for reviews see Jeukendrup, 2008 and 2010). In general, during exercise longer than 2 h, the effects of carbohydrate are mainly metabolic in nature. However, carbohydrate ingestion during exercise has also been demonstrated to improve exercise performance when the exercise is of high intensity (>75%VO2max) and relatively short duration (~1 h). It has become clear that the underlying mechanisms for the ergogenic effect during this type of activity are not metabolic but may instead reside in the central nervous system. Carbohydrate mouth rinses have been shown to result in similar performance improvements (Jeukendrup et al. 2010). This would suggest that the beneficial effects of carbohydrate feeding during exercise are not confined to its conventional metabolic advantage but may also serve as a positive afferent signal capable of modifying motor output (Gant et al. 2010). These effects are specific to carbohydrate and are independent of taste (Chambers et al. 2009). Performance benefits from carbohydrate mouth rinses have been observed after an overnight fast as well as 2 h postprandially, albeit the magnitude of performance enhancement seems to be greater after an overnight fast (Lane et al. 2013; Fares et al. 2001). The receptors in the oral cavity have not yet been identified and the exact role of various brain areas is not clearly understood. Further research is warranted to fully understand the separate detection and transduction pathways for simple and complex carbohydrates and how these differ between mammalian species, particularly in humans. However, it has been convincingly demonstrated that carbohydrate is detected in the oral cavity by unidentified receptors and this can be linked to improvements in exercise performance (for review see Jeukendrup et al. 2010). New guidelines suggested here take these findings into account (Fig. 1).

Figure 1: Recommendations for carbohydrate intake during different endurance events. These guidelines are intended for serious athletes, exercising at a reasonable exercise intensity (>4 kcal/min). If the (absolute) exercise intensity is below this, carbohydrate intake should be adjusted downwards.

Figure 1: Recommendations for carbohydrate intake during different endurance events. These guidelines are intended for serious athletes, exercising at a reasonable exercise intensity (>4 kcal/min). If the (absolute) exercise intensity is below this, carbohydrate intake should be adjusted downwards.

5These results suggest that it is not necessary to ingest large amounts of carbohydrate during exercise lasting approximately 30 min to 1 h and small amounts or even a mouth rinse with carbohydrate may be sufficient to get a performance benefit (Fig. 1). In most conditions the performance effects with the mouth rinse were similar to ingesting the drink. Thus, there does not seem to be a disadvantage to ingesting the drink, although occasionally athletes may complain of gastrointestinal distress when taking on board a relatively large volume of fluid. Of course, when the exercise is more prolonged (2 h or more), carbohydrate becomes a very important fuel and it is therefore essential to ingest the carbohydrate instead of only rinsing the mouth with it. In addition, as will be discussed below, larger amounts of carbohydrate may be required for more prolonged exercise.

6Different carbohydrates ingested during exercise may be utilized at different rates (Jeukendrup, 2010); but until a landmark publication in 2004 (Jentjens et al. 2004) (discussed later), it was believed that carbohydrate ingested during exercise could only be oxidized at a rate no higher than 1 g/min (60 g/h), independent of the type of carbohydrate. This is reflected in guidelines published by the ACSM, which recommends that athletes should ingest between 30 and 60 g of carbohydrate during endurance exercise (>1 h) (Sawka et al. 2007) or 0.7 g/kg/h (Rodriguez et al. 2009).

7It seems that exogenous carbohydrate oxidation is limited by the intestinal absorption of carbohydrates. It is believed that glucose uses a sodium dependent transporter SGLT1 for absorption which becomes saturated at a carbohydrate intake around 60 g/h. When glucose is ingested at this rate and another carbohydrate (fructose) that uses a different transporter is ingested simultaneously, oxidation rates well above 1 g/min (1.26 g/min) (Jentjens et al. 2004) can be observed. A series of studies followed in an attempt to determine the maximal rate of exogenous carbohydrate oxidation. In these studies, the rate of carbohydrate ingestion as well as the types and combinations of carbohydrates was varied. All studies confirmed that multiple transportable carbohydrates resulted in (up to 75%) higher oxidation rates than carbohydrates that use the SGLT1 transporter only (for reviews see Jeukendrup, 2008 and 2010). Interestingly, such high oxidation rates could not only be achieved with carbohydrate ingested in a beverage but also as a gel (Pfeiffer et al. 2010) or a low-fat, low-protein, low-fibre energy bar (Pfeiffer et al. 2010). In addition, exogenous carbohydrate oxidation rates have been shown to be similar in cycling and running exercise that is performed at similar relative intensities (Pfeiffer et al. 2011).

4. Carbohydrate during exercise and performance: dose–response

8Very few well controlled dose–response studies on carbohydrate ingestion during exercise and exercise performance have been published. Most of the older studies had serious methodological issues that made it difficult to establish a true dose–response relationship between the amount of carbohydrate ingested and performance. Initially it was concluded that the athlete needed a minimum amount of carbohydrate (probably about 20 g/h based on one study) but it was assumed that there was no dose–response relationship (Rodriguez et al. 2009).

9Evidence is accumulating for a dose–response relationship between carbohydrate ingestion rates, exogenous carbohydrate oxidation rates and performance. In one recent carefully conducted study, endurance performance and fuel selection was measured during prolonged exercise while ingesting glucose (15, 30, and 60 g/h) (Smith et al. 2010). Twelve subjects cycled for 2 h at 77%VO2 peak followed by a 20-km time trial. The results suggest a relationship between the dose of glucose ingested and improvements in endurance performance. The exogenous glucose oxidation increased with ingestion rate and it is possible that an increase in exogenous carbohydrate oxidation is directly linked with, or responsible for, exercise performance.

10A large-scale multicentre study by Smith et al. (2010) also investigated the relationship between carbohydrate ingestion rate and cycling time trial performance to identify a range of carbohydrate ingestion rates that would enhance performance. In their study, across four research sites, 51 cyclists and triathletes completed exercise sessions consisting of a 2-h constant-load ride at a moderate to high intensity. Twelve different beverages (consisting of glucose: fructose in a 2:1 ratio) were compared, providing participants with 12 different carbohydrate doses ranging from 10 to 120 g carbohydrate/h during the constant load ride. At all four sites, a common placebo that was artificially sweetened, colored, and flavored and did not contain carbohydrate was provided. The order of the beverage treatments was randomized at each site (three at each site). Immediately following the constant-load ride, participants completed a computer-simulated 20-km time trial as quickly as possible. The ingestion of carbohydrate significantly improved performance in a dose-dependent manner and the authors concluded that the greatest performance enhancement was seen at an ingestion rate between 68 and 88 g carbohydrate/h. A meta-analysis showed similar results (Vandenbogaerde et al. 2011). Based on the studies mentioned above carbohydrate intake recommendations for more prolonged exercise (>30 min) can be formulated and are listed in newly proposed guidelines in Figure 1. The significant changes in the understanding of the role of carbohydrates during endurance exercise in recent years have allowed for more specific and more personalized advice with regard to carbohydrate ingestion during exercise than previous recommendations. For more detail on personalized nutrition for endurance athletes, the reader is referred to a recent review (Jeukendrup, 2010).

11Please note that these guidelines for carbohydrate intake during exercise are expressed in grams per hour of exercise and that these figures are not corrected for body mass. Since exogenous carbohydrate is independent of body mass or muscle mass, but dependent on absorption and to some degree the absolute exercise intensity (at very low absolute intensities, low carbohydrate intake rates may also restrict exogenous carbohydrate oxidation), the advice given to athletes should be in absolute amounts.

5. Training the gut

12Because the absorption of carbohydrate limits exogenous carbohydrate oxidation, and exogenous carbohydrate oxidation seems to be linked with exercise performance, an obvious potential strategy would be to increase the absorptive capacity of the gut. Anecdotal evidence in athletes would suggest that the gut is trainable and that individuals who regularly consume carbohydrate or have a high daily carbohydrate intake may also have an increased capacity to absorb it. A study by Lambert et al. (2008) found improved gut comfort after repeated sessions of drinking a glucose-electrolyte solution while running. Intestinal carbohydrate transporters may also be upregulated. By exposing an animal to a high-carbohydrate diet increased expression of transporter proteins for carbohydrates was observed (Ferraris, 2001). To date, there is, however, limited evidence in humans. A recent study by Cox et al. (2010) investigated whether altering daily carbohydrate intake affects substrate oxidation and in particular exogenous carbohydrate oxidation. It was demonstrated that exogenous carbohydrate oxidation rates were higher after the high-carbohydrate diet (6.5 g/kg/day; 1.5 g/kg provided mainly as a carbohydrate supplement during training) for 28 days compared with a control diet (5 g/kg/day). This study provided evidence that the gut is indeed adaptable and this can be used as a practical method to increase exogenous carbohydrate oxidation. We recently suggested that this may be highly relevant to the endurance athlete and may be a prerequisite for the first person to break the 2 h-marathon barrier (Stellingwerff et al. 2011). For a more in-depth discussion on the gut and nutritional strategies to reduce the risk of gastrointestinal problems, the reader is referred to a recent review (Oliveira et al. 2014).

6. Maintaining fluid balance during exercise

13Dehydration of >2% body mass has been shown to impair aerobic exercise and endurance performance (Sawka et al. 2007; Cheuvront et al. 2014). While the exact mechanisms are debated, one of the primary physiological effects of a body water deficit is increased cardiovascular strain. When dehydration occurs, plasma volume decreases in proportion to the decrease in total body water. As a result of the lower plasma volume, cardiac filling and stroke volume declines, leading to increased heart rate and increased perceived exertion during aerobic exercise (Montain et al. 1992; Gonzalez-Alonso et al. 2000). The magnitude of performance impairment with dehydration is dependent upon multiple factors, including the individual’s unique biological characteristics that determine their tolerance to dehydration, as well as the environmental conditions (Sawka et al. 2007).

14During exercise in a hot environment, high skin blood flow (i.e. redistribution of blood flow from the central to peripheral circulation) is required for heat dissipation. Thus, when heat stress is combined with dehydration there is greater cardiovascular strain resulting from the competition between the central and peripheral circulation for limited blood volume (Nadel, 1980). As determined in a series of carefully conducted studies by the U. S. Army Research Institute of Environmental Medicine, the decrement in aerobic performance with dehydration (3 to 4% of body mass) and heat stress can be estimated as a function of skin temperature (Sawka et al. 2012). These studies determined that, starting at a skin temperature of ~27 to 29°C, the percentage decrement in aerobic time trial performance declines linearly by ~1.3% for each 1°C rise in skin temperature (Kenefick et al. 2010; Castellani et al. 2010). In short, hyperthermia (particularly hot skin) exacerbates the performance decrement for a given level of dehydration. Accordingly, athletes should consider the environmental conditions as well as individual sweating rate when developing hydration strategies for training and competition (Kenefick et al. 2010). Skin temperature increases in proportion to ambient temperature and humidity and is modified by convective cooling from air movement and the cooling effect of sweat evaporation during exercise. More detailed discussions on this topic can be found elsewhere (Sawka et al. 2011).

15To prevent impairments in performance associated with dehydration, perhaps the best advice is for endurance athletes to weigh themselves to assess fluid losses during training and racing and drink enough to limit body mass losses to 2%. In the absence of such planning, concrete advice on fluid intake needs is difficult to give as differences between individuals, race distances, course profiles, and environmental conditions will confound any suggestions. Another possible hydration strategy is for athletes to simply drink according to thirst sensation (i.e. ad libitum fluid intake). This strategy has been advocated by some based on 1) recent studies reporting no time-trial performance enhancement from ingesting fluid at a rate above that of ad libitum intake (Goulet, 2011; Dion et al. 2013) and 2) observations that top finishers of endurance events often accrue significant dehydration during races (Zouhal et al. 2011; Beis et al. 2012). However, more work is needed to determine the efficacy of this strategy as there are several potential limitations to consider. Thirst perception is complex and ad libitum fluid intake is dictated by a number of regulatory and non-regulatory factors. Thirst and ad libitum fluid intake may be unreliable in some individuals (Goulet, 2012) and can be blunted by psychological stress, social factors, or the flavour and palatability (affecting the athlete’s “acceptability” or “liking”) of the available beverage (for review see Baker et al. 2014). The stimulation of physiological thirst does not occur until after dehydration has accrued. And dehydration by ≥ 3% body mass has been associated with impaired gastric emptying of fluids (Rehrer et al. 1990; Neufer et al. 1989), thus waiting until the perception of thirst to drink could induce gastrointestinal distress. Additionally, in some situations, the athlete’s access to fluid may be limited or unpredictable. Thus, it stands to reason that athletes should go into competition with a plan on how to meet their hydration and carbohydrate needs, including what, how much, and when to drink/eat during exercise.

16Although hypertonic solutions tend to delay water absorption in the intestine (Rehrer et al. 1994) and energy density is perhaps the most important factor dictating gastric emptying rates (Noakes et al. 1991; Brouns et al. 1995), the use of multiple transportable carbohydrates can help to maintain high rates of gastric emptying and improve the delivery of fluid (Jeukendrup et al. 2010). Although it is difficult to draw firm conclusions, in almost every study we have also seen better tolerance of the drinks with multiple transportable carbohydrates compared with a single carbohydrate at these high intake rates (>1 g/min). The reader is referred to our recent review (Baker et al. 2014) for a more comprehensive discussion of the effects of beverage composition on fluid replacement during exercise.

17In summary, a balance must be struck between the goals of maintaining hydration status and providing carbohydrate to the working muscle. The rate of fluid absorption is closely related to the carbohydrate content of the drink with high-carbohydrate concentrations compromising fluid delivery, although multiple transportable carbohydrates can remove some of this impaired fluid delivery.

7. Acknowledgements

18Asker Jeukendrup is an employee of PepsiCo, Inc. and a visiting Professor at the Loughborough University. Lindsay Baker is an employee of PepsiCo, Inc. The views expressed in this article are those of the authors and do not necessarily reflect the official position or policy of PepsiCo, Inc.

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Table des illustrations

Titre Figure 1: Recommendations for carbohydrate intake during different endurance events. These guidelines are intended for serious athletes, exercising at a reasonable exercise intensity (>4 kcal/min). If the (absolute) exercise intensity is below this, carbohydrate intake should be adjusted downwards.
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