Carbohydrate Recommendations for Endurance Sport: A Literature Review

Introduction

Amateur sporting events have flourished in recent years with numerous events held throughout the UK. Many major cities now host a half or full-distance marathon. Participant numbers for these events increased every year up until 2016 (Anderson, 2020). Triathlon has continued to see a consistent growth of 3% since 2017, most notably due to increased female participation (Triathlon Industry Association, 2019).

Nutrition plays an integral part in any sporting event. As events become for the importance of adequate nutrition increases. For endurance sport, carbohydrate (CHO) provides the fuel to keep an athlete moving. The role of CHO in sport is well known. However, this is not to say the literature on this topic is complete. As humans develop a deeper understanding of their physiology, so grows their understanding of what is vital in sports performance.

Carbohydrate Loading

In a 1997 review, Hawley et al. (1997) demonstrated that super compensated muscle glycogen levels could increase performance compared to low or normal muscle glycogen by 2-3%. This improvement was only found in events of 90 minutes or more in duration, suggesting carbohydrate loading is of little value in events shorter than this. It is useful to see this observation in both men and women, as some later studies fail to report any conclusions in women. Although this information was published in 1997, Jeukendrup (2011) confirms that this study is still relevant today. Jentjens and Jeukendrup (2005) discovered that glycogen super-compensation could be achieved by athletes ingesting 10-12 g/kg/day on the day before a competition. Reducing exercise load in the days preceding an event helped ensure glycogen stores were maximised. The researchers concluded this might improve performance by 2-3% for events over 90 minutes in duration. The applicability of the results for female athletes is unclear as the study only included eight male participants.

Oxidation Rates

In 2003, Jentjens et al. demonstrated that ingested CHO could be oxidised at a rate greater than 60 g/hour if the source contained a mixture of glucose and fructose. The researchers lay out how the intestinal absorption rate limits exogenous CHO oxidation because glucose uses sodium-dependent transporter SGLT1 for absorption, which becomes saturated at an intake of around 60 g/hour. The authors note that previous study in children yielded contradictory results to their own. This finding is of relevance given the limited age range (29 􏰁 ±2.7 years) of this studies participants, raising questions over the age range these results might apply. Tappy and Rosset (2017) explain the advantages of combining fructose with glucose for athletes. Following absorption, fructose is converted into glucose or lactate in splanchnic fructose-metabolising cells such as the liver. The fructose derived glucose can be utilised via typical glycolysis pathways, whereas skeletal muscle cells can oxidise the lactate via a 'reverse Cori cycle'. Deriving energy through these pathways is not as efficient as directly consumed glucose. However, through the combined ingestion of glucose and fructose, athletes can provide their muscles with a larger fuel volume.

Smith et al. (2010) have also investigated absorption rates. Their study consisted of 12 cyclists required to complete a 2-hour constant load ride followed by a 20 km time trial. The all-male participants had a direct link between increased glucose ingestion and improved performance. Pfieffer et al. (2011) also observed these results in the real world. A link between 'Iron Man' triathlon finish times and CHO ingestion was demonstrated, with higher rates of quicker finish times and increased nausea and flatulence. This study relied on two self-reported questionnaires, which allowed participants to record their responses up to 2 days after the event—however, the researchers' employed techniques to enhance the data's accuracy. However, human recognition can be flawed. Severity scores of symptoms were self-reported on a 10 point scale; this would have allowed for a large degree of subjectivity in the reported scores between individuals.

Gut Training

In 2017 Jeukendrup described how increased dietary CHO consumption up-regulates the amount and activity of intestinal SGLT1 transporters. As these transporters appear to be the limiting factor in CHO oxidation, bolstering their function is likely to lead to performance gains during prolonged exercise. Interestingly, the reverse also seems accurate, with individuals who have reduced their CHO intake, through either total energy reduction or a specific low CHO plan, such as a ketogenic diet, reducing their ability to absorb CHO. The consequences of reduced SGLT1 may include slower gastric emptying and reduced intestinal absorption. These effects were linked to excessive feelings of fullness, discomfort and diarrhoea. Jeukendrup (no date) summarises these issues, pointing out that cycling stage racers are often described as professional eaters, consuming around 8000 kcal per day during the Tour de France (Saris et al., 1989).

In contrast, the author notes that running events are not as easy to fuel, and thus these athletes often have a lower tolerance of stomach fulness. Jeukendrup highlights that many in these sports feel their options are to accept a performance drop or suffer abdominal issues. However, the author believes a third option exists in 'gut training', yielding increased performance and minimising gastric distress. Jeukendrup (2017) closes by noting that there is currently a lack of evidence to make specific endurance athletes' recommendations. Current guidance is based on presumed best practice, suggesting 'gut training' take place weekly, with individuals mimicking their racing feeding regime during training sessions. To date, there appears to be no research examining amateur athletes' knowledge or practices of 'gut training'.

Osmolality

Endurance athletes face several practical considerations concerning refuelling strategies. Runners can only carry so much nutrition on their person, whereas cyclists can potentially carry sufficient supplies on their bike. Some events may offer nutrition products at aid stations, although the quality and quantity of products on offer can vary considerably. This approach leaves the athlete relying on a product they might not have tried before. Cox et al. (2010) investigated the carbohydrate intakes of elite triathletes, finding 84% of participants utilised a sports gel as their primary source of carbohydrate during races. In contrast to this, Sparks and Fletcher (2018) identified a preference for sports drinks, closely followed by gels, amongst amateur cyclists completing a 94.7 km road race. Numerous research papers highlight the importance of considering osmolality in sports nutrition products to promote performance whilst minimising gastric discomfort (Jeukendrup, 2017).

Zhang et al. (2015) conducted a detailed review of many gel products from the most popular brands to assist with athlete feeding strategies. The authors concluded that whilst most products contained a suitable amount of CHO to meet suggested delivery amounts of up to 60 g/hour, the osmolality varied more than any other quality, with 70% of gels tested having osmolality values above 2000 mmol/kg. These products will lead to hypertonic gastric contents at this concentration and without additional water consumption, likely resulting in delayed gastric emptying, reduced carbohydrate delivery to the small intestine, distention, and cramping. Many gel products do not note osmolality on their packaging and the need for dilution during consumption. The authors note that consuming isotonic gels should not require further fluid ingestion, improving control over gastric emptying and subsequent carbohydrate metabolism.

In contrast to gels, Zhang et al. note that most sports drinks are now isotonic, containing a carbohydrate concentration between 6-8%. These drinks allow for rapid gastric emptying and absorption through the small intestine. This study highlights the importance of carefully planning CHO and fluid intake to maximise the gut's absorptive capacity. It is unknown if amateur athletes know the potential issues surrounding hypertonic gastric contents and the resulting complications this can cause.

International Society of Sports Nutrition (ISSN)

From this evidence base, some formal recommendations have been developed. Generally, the recommendations are in agreement with each other, although some contain more detail than others. Writing on behalf of the ISSN, Kerksick et al. (2008) conclude that glycogen stores largely depend on an athlete's nutritional status and current training load. An endogenous CHO store typically lasts from 90 minutes to 3 hours, depending on the exercise's intensity. This review points out that as glycogen stores are used up, physical performance decreases, muscle tissue breakdown increases, and the immune system becomes depressed. To ensure glycogen stores are fully stocked, a high CHO diet is encouraged, defined as either 600-1000 g or ≈ 8-10 g/kg/day. During exercise, the authors suggest CHO availability becomes an excellent biomarker for determining performance, significantly when glycogen stores are depleted at the onset of activity. For activities lasting less than 60 minutes, exogenous CHO becomes an essential variable for maximising performance. Glycogen levels are ideally supported through ingestion of 30-60 g of CHO per hour, which can be made up of 1-2 cups of a 6-8% CHO solution taken every 15 minutes. Mixtures of glucose, fructose, sucrose and maltodextrin are recommended to support maximal CHO oxidation. However, caution is advised in consuming high concentration fructose mixtures as this has been associated with increased gastric discomfort.

The American Dietetic Association (TADA)

TADA (Rodriguez et al., 2009) summarise that a pre-event meal should contain 200-300 g CHO and be consumed 3-4 hours prior. Ideally, this will be low in fat and fibre, high in CHO and moderate in protein. During events lasting longer than 60 minutes, CHO should be consumed at a rate of 0.7 g per kg of body weight (BW) per hour, or 30-60 g per hour. This is particularly important if no food has been consumed before exercise. Many reviews have questioned why TADA has included a consumption rate based on body weight, as glucose absorption is generally accepted to be independent of body size. TADA suggests that intra-workout CHO is crucial for exercise in heat or humidity. CHO should ideally be consumed in a 6-8% solution. However, the form (gel, snack, drink) does not matter. Fuel should be primarily composed of glucose. Fructose has been found to cause diarrhoea in high concentrations. A mixture of the two is noted as being useful, although no specific ratio is mentioned.

International Olympic Committee (IOC)

Writing on behalf of the IOC, Maughan and Burke (2012) believe athletes preparing for events of more than 90 minutes should consume 7-12 g of CHO per kg of BW per 24 hours. CHO loading can be undertaken for activites over 60 minutes in length by consuming 10-12 g/kg BW per 24 hours for 36-48 hours prior. Additionally fuelling 1-4 hours before exercise should be at a rate of 1-4 g/kg BW for events of more than 60 minutes. Food should be low in fibre and fat. CHO rates should be tailored to individual tolerances to avoid gastrointestinal discomfort. Food should be low glycemic index if nothing is to be consumed during the event. For exercise lasting less than 45 minutes, nothing needs to be consumed during the event. For events lasting over 60 minutes, CHO should be ingested at a rate of 30-60 g/hour. For endurance events over 2.5 hours, CHO should be ingested at rates up to 90 g/hour. The authors (2012) suggest that athletes practice their nutrition plan before a competition. Generally, higher CHO intakes are associated with increased exercise performance. A glucose-fructose mixture is recommended to enhance oxidation rates.

Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine

The most recent guidance is provided in a joint statement written by Thomas et al. (2016). Unlike the previous guidelines, the authors suggest recommendations can be adjusted to individual needs by a sports dietitian. Daily CHO intake for endurance athletes is set at 6-10 g/kg BW per 24 hours, rising to 8-10 g for those undertaking 'extreme' events lasting longer than 4 hours. Suggested carbohydrate loading and pre-race window (1-4 hours) recommendations mirror the IOC guidelines, as do CHO intakes during activities. Compared to other guidelines, Thomas et al. provide more detailed guidance on the importance of minimising gastric discomfort. The authors suggest athletes choose CHO-rich food sources that are low in fibre and easily consumed to ensure gut comfort, pointing out there may be benefits in consuming small, regular meals. This point is concluded by suggesting the athlete practice their refuelling plan to ensure it does not induce gastric distress. The event's practical needs and individual's preferences are considered within recommendations, as these can vary considerably. The authors note that opportunities to consume food and drink vary according to each sport's rules and nature. Therefore a range of everyday dietary choices and specialised sports products may be useful. In line with other guidance, glucose-fructose mixtures provide the highest oxidation rate for CHO delivery during exercise.

Athletes' Understanding and Implementation

Athletes' awareness of and ability to implement these recommendations has not been established, especially among amateur athletes. The current body of evidence suggests that both professional and amateur athletes do not comply with CHO intake recommendations. Mason and Lamarche (2016) proposed little known about the dietary intake of non-elite multi-sport endurance athletes. Their study investigated the intake of CHO and protein. A total of 116 non-elite athletes filled in questionnaires to ascertain dietary intake. Results showed that athletes consume sufficient protein but not sufficient CHO. The researchers called for targeted nutritional education to improve intakes. Although the researchers did use a validated online food frequency questionnaire, data were collected retrospectively for the previous month, which may have introduced error due to inaccurate recall. Furthering this idea, Heikkila et al. (2018) proposed that both athletes and coaches' nutrition knowledge was inadequate. The study included 312 athletes and 94 coaches. Numerous topics were investigated, including nutrition recommendations for endurance sports. The results showed that coaches scored considerably better than athletes. The authors concluded that both groups would benefit from enhanced nutrition education. Athletes were only included in this study if they were between 16 and 20, whereas coaches could be any age. In contrast to the researchers' conclusions, this could suggest that a better understanding is attained with age than athletes lacking knowledge.

McLeman et al. (2019) investigated runners' knowledge and pre and post-event CHO and protein consumption practices. The authors suggested that runners would not meet guidelines, and knowledge would be inadequate; their results confirmed this suggestion. McLeman et al. hypothesised this is likely to extend to amateur athletes also. The authors conclude that protein consumption guidelines are well adhered to, but CHO intake is below recommendations. The paper summarises that a lack of access to nutrition specialists might contribute to these findings.

In contrast to previous research, participants were encouraged to complete the 24-hour pre-exercise recall in real-time, enhancing the studies validity. Sparks and Fletcher (2018) made similar inferences about amateur cyclists and found the majority had inadequate knowledge regarding CHO loading and usage habits. Interestingly the authors discovered that whilst the majority were aware of CHO loading, 16% chose not to due to fear of weight gain, and 12% due to previous gastrointestinal distress. Sparks and Fletcher suggest this indicates a need for further education on the correct method of implementing this strategy. The authors noted that half of the respondents were unaware of the CHO concentration of their chosen intra-race fuel, and less than half were aware of the types of CHO it contained. The study concludes that many athletes will experience inadequate CHO replacement, despite expectations to the contrary, due to an incomplete understanding of this topic. In contrast to the papers mentioned above, Sparks and Fletcher used a cross-sectional study to collect data a few days before the event. In essence, this allowed the researchers to ascertain what the participants expected to do, not what they did.

Sources of Nutritional Information

The preferred source of nutritional information for athletes appears to vary depending on their age, sex, geographical location and level of access to services. In 2012 Diehl et al. found that adolescent athletes primarily utilised their coaches. Study data were derived from a cross-sectional analysis of elite adolescent athletes with no clear description of how it might compare to amateur athletes. Abdullah and Mal-Allah (2013) identified that female athletes were aware of the benefits of 'human' (nutritionist and medical doctors) sources of nutritional knowledge. However, they preferred to access information from 'material' (magazine and internet) sources. The main reasons given were accessibility and availability. This study appears to be the only research that identifies 'why' athletes prefer a particular information source. However, none of the participants took part in endurance sports. Therefore, caution should be used in generalising these findings to athletes competing in these sports. Graham-Paulson et al. (2015) examined nutrition and supplement use of elite and non-elite athletes with an impairment. In contrast to the previous study, they found that a nutritionist/dietitian was the most used and trusted source of information, with elite athletes more likely to access nutrition support.

Denham (2017) conducted a review of studies that explore information sources used by athletes, finding that study participants from the USA preferred coaches and trainers, whereas those outside the USA tended towards doctors and nutritionists. The author did not comment on why this difference was seen. Doering at al. (2016) reported the primary source of nutrition information for amateur triathletes. The author found sports dietitians, magazines, friends and the internet were the most popular responses. As this study focused on post-exercise nutrition, the results may not transfer to pre and intra-workout knowledge and preferences. More recently, Kimmel et al. (2019) conducted a similar study focusing on student-athletes. The researchers identified the participants' primary source of nutrition advice as the internet. However, this study does not state how the results relate to specific sports, leaving questions over their applicability to endurance athletes. McLeman et al. (2019) note that most professional athletes are likely to cite nutrition specialists as their preferred source of information, opining that this is due to their enhanced access to these sources.

In contrast to this, amateur athletes reported the internet as their preferred source of information, followed by magazines and friends. The authors suggest that event organisers and governing bodies provide useful and accurate information to enhance the general public's understanding. To accomplish this, content creators will need to recognise what formats and platforms people prefer to utilise. If this can be achieved, accurate information will be easily accessible to amateur athletes from all demographics.

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