Relationship of Carbohydrate Intake during a Single-Stage One-Day Ultra-Trail Race with Fatigue Outcomes and Gastrointestinal Problems: A Systematic Review

This systematic review was carried out in accordance with the guidelines of the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA^®) [46] and the PICOS model [47] for definition of the following inclusion criteria: P (Population): “endurance and ultra-endurance runners”, I (Intervention): “CHO intake during SOUT”, C (Comparators): “international recommendations for CHO intake at events lasting more than 3 h”, O (Outcomes): “CHO intake, post-exercise recovery, internal load and GI problems“, S (Study design): “any type of design”. A systematic search of current scientific literature was undertaken to find up-to-date information about CHO intake consumed by ultra-trail athletes who compete in events of more than 4 h duration during SOUT races, as well as its relationship with exercise load, EIMD and GI problems.

A structured search was conducted of the Medline (PubMed), Scopus and Web of Science (WOS) database that includes other databases such as BCI, BIOSIS, CCC, DIIDW, INSPEC, KJD, RSCI and SCIELO, all of which are of high quality and guarantee suitable bibliographic support for this systematic review completed on 16 March 2021. The keywords provided in the full article were used in accordance with the following Boolean search equation: (“marathon running”[MeSH Terms] OR (“marathon”[All Fields] AND “running”[All Fields]) OR “marathon running”[All Fields] OR “marathon”[All Fields] OR “marathons”[All Fields] OR “marathoner”[All Fields] OR “marathoners”[All Fields] OR (“ultramarathon”[All Fields] OR “ultramarathoners”[All Fields] OR “ultramarathons”[All Fields]) OR “ultra-endurance”[All Fields] OR “ultra-trail”[All Fields]) AND (“food intake”[All Fields] OR “fluid intake”[All Fields] OR (“carbohydrate s”[All Fields] OR “carbohydrated”[All Fields] OR “carbohydrates”[MeSH Terms] OR “carbohydrates”[All Fields] OR “carbohydrate”[All Fields])) AND (“post-exercise recovery”[All Fields] OR (“recoveries”[All Fields] OR “recovery”[All Fields]) OR “gastrointestinal problems”[All Fields] OR “gastrointestinal events”[All Fields] OR “exercise induced muscle damage”[All Fields] OR “muscle damage”[All Fields] OR (“fatiguability”[All Fields] OR “fatiguable”[All Fields] OR “fatigue”[MeSH Terms] OR “fatigue”[All Fields] OR “fatigued”[All Fields] OR “fatigues”[All Fields] OR “fatiguing”[All Fields] OR “fatigueability”[All Fields]).

Relevant articles were also obtained using this equation by applying the snowball strategy, with all titles and abstracts from the search being cross-referenced to identify duplicates and any potentially missing studies. Titles and abstracts were selected for further review of the full text. The search for published studies was carried out independently by two authors (S.A. and J.M.A.), and any disagreements about all outcomes were resolved through discussion (J.C.G.).

The following inclusion criteria were applied in selecting studies for the articles found in the search: (I) studies that were carried out during a SOUT race; (II) reporting on CHO intake (quantity and/or time of intake); (III) inclusion of post-exercise recovery, internal load information and/or description of GI complications associated with CHO intake during SOUT events; (IV) inclusion of those performed on any number or type of athlete regardless of category, training status or gender; (V) languages restricted to English, German, French, Italian, Spanish and Portuguese.

The following exclusion criteria were applied to the experimental protocols of this systematic review: (I) ultra-trail events that exceed 24 h or ultra-endurance races on the road, (II) those performed by bike or some other specialty that is not agreed; (III) absence of information about CHO intake; (IV) that the study was conducted on participants with a pathological condition (IV) those that exclusively describe other nutrients during a marathon, ultra-trail or ultra-marathon. There were no filters applied to athletes’ level, race, gender, ethnicity, or age in order to increase the analytical power of the analysis itself.

Once the inclusion/exclusion criteria had been applied to each study, data on study source (including authors and year of publication), study design, supplement administration (dose and timing), sample size, participant characteristics (level, race and gender) and final outcomes of the interventions were mined independently by two authors (S.A. and J.M.A.) using a spreadsheet (Microsoft Inc, Seattle, WA, USA). Subsequently, disagreements were resolved through discussion until a consensus was reached or there was third-party adjudication (J.C.G.).

The literature studied information related to the CHO intake by ultra-endurance athletes during SOUT, as well as its effect on post-exercise recovery, exercise load and/or GI problems, using several outcome variables such as CHO intake (amount, type of CHO and nutritional strategies), fatigue (exercise internal load, EIMD, recovery) and upper and lower GI symptoms. The results could have been influenced by the type of event, amount of each supplement and duration of the intervention (races). Participant’s characteristics, such as age, gender, ethnicity, body composition, training level, differences in training, nutrition and health status and ethnicity, may also have influenced the results.

Methodological quality and risk of bias were assessed in accordance with qualitative studies via a McMaster Critical Review Form [48] by two authors independently (J.M.A. and S.A.), with disagreements being resolved by third-party evaluation (A.V.). There were 16 evaluated items (purpose, literature review, study design, blinding, sample description, sample size, ethics and consent, validity of outcomes, reliability of outcomes, intervention description, statistical significance, statistical analysis, clinical importance, conclusions, clinical implications, study limitations), which were rated as “1” if the criteria were fully met, “0” if they were not completely met, or “NR” in cases where information was not reported. Study scores were as follows: ≤8 points indicated poor quality; 9–10 points acceptable quality; 11–12 points good quality; 13–14 points very good quality and ≥15 points excellent quality. As such, 1 out of 8 studies included in this systematic review evidenced poor quality, 1 acceptable quality, 5 studies good quality and 1 very good quality (Table 1).

The initial literature search provided a total number of 93 relevant articles related to the descriptors selected, although only 8 articles [2,3,4,5,6,8,29,33] met all the inclusion criteria (Figure 1). The number of articles and reasons for exclusion were as follows: Twenty-five of them were associated with alternative diets (diets-ketosis, high in fat, low in CHO, supplements, and antioxidants), 10 studies with training and performance, 9 referred to pathologies, 5 to reviews, 29 failed to incorporate some of the inclusion criteria (24 referred to other sports and 5 analyzed the nutritional knowledge of athletes). Moreover, 9 studies were excluded because they focused on other outcomes.

Among the 8 studies selected, 6 were ultra-trail races held in Europe (1 in Spain, 1 in France, 2 in Holland, 1 in Switzerland, 1 in Scotland) [3,5,6,8,29,33] and 3 were in marathon trail races [2,4,29] held in Spain (Table 2).

The distances traveled in SOUT events were between 44 km covered by the “Tour des Dents du Midi” race in Switzerland [33] and 272 km covered by the World Championship held in Albi (France) [3]. However, 3 of the 9 SOUT events involved over 100 km [5,6,7,8,9,10,29], while the Glenmore 24 Trail Race event in Scotland was computed by the maximum distance traveled in 24 h, the range of which was 122–208 km [8]. One of the 6 events involved a trail marathon [29]. The cumulative elevation gain of the ultra-trail tests of more than 100 km ranged between 342 m in the Glenmore 24 Trail Race in Scotland [8] and 4448 m in the Ultra Mallorca Serra de Tramuntana in Spain [29]. Aditionally, that of ultra-trail races of less than 100 km ranged between 4448 m in the Transvulcania, Spain [29] and 2521 m of in the 67 km of the Ultra Mallorca Serra de Tramuntana, Spain [29]. The temperature at which the ultra-trail events were carried out was between 2.2 °C over 120 km in the Sixty of Texel [6] and 35.1 °C in the 272 km of the World Championship held in France [3].

As for the environmental conditions of the mountain marathon, this took place within a temperature range of 11.0–21.6 °C in the Ultra Mallorca Serra de Tramuntana, Spain, with an ascent of 1424 m [29]. It should also be taken into consideration that 1 study [29] covered the events in 2 categories: ultra-trail and trail marathon.

417 SOUT runners in total were included in this systematic review. Specifically, participants comprised 365 male and 52 female runners (Table 2), while participants were only men in 3 of the studies [2,4,5], and the remaining 5 studies included athletes of both genders [3,6,8,29,33]. Furthermore, the total number of runners included in the different ultra-trail events was 314 (271 men and 40 women) aged between 35.2 ± 8.4 years in the study by Martínez et al. [29] and 47 ± 6 years in that by Wardenaar et al., 2018 (22). The latter was a study carried out in the Ultra Mallorca Serra de Tramuntana (Spain) where maximum participation was recorded with 109 athletes (98 of whom were men) (35.7 ± 7.9 years) [29], while the one with the least number of participants was that described by Waardenaar et al. [5]. The total number of runners included in the marathon trail studies was 143 (91 men and 12 women) with an age of 36.6 ± 8 years [29], while the lowest number of participants was 20 [2,4], aged LOW:37.8 ± 9.4; MED: 37.2 ± 5.4; HIGH: 38.0 ± 6.8.

The CHO amount ingested during SOUT events ranged between 22.1 g/h in a 120 km race [5] and a maximum of 126 g/h ingested by a runner in a 160 km race [8] (Table 3). In the study by Wardenaar et al. [5], the highest CHO intake was 90.6 ± 38.2 g/h, corresponding to the 75–90 km section, while in the marathon trail the CHO intake was between 205.2 ± 81.2 g (the equivalent of (33.4 ± 13.5 g/h) [29]) and 120 g/h [2,4]. In terms of the type of CHO consumed, this is represented by solid, semi-solid and liquid foods [5], solid foods being fruit [3,5,9,29], gummies [29], pasta [29] and energy bars [5,29], as shown in Table 3. For its part, the liquid intake comprised chocolate milk [5], cola drinks [5,29] and commercial energy drinks in three studies [5,29,33]. One study described the type of CHO, but did not specify the food consumed by athletes [8]. Similarly, all studies allowed their athletes to consume the usual intake [5,6,8,29,33].

On the other hand, only three studies [2,3,4] provided nutritional instructions to their athletes for either the previous weeks and/or for ingestion during the race (Table 3). Likewise, one of these studies [3] allowed runners to modify the programmed intake during the race. It should be noted that among the studies on nutritional strategy, a recommendation of 90 g/h of CHO was not necessarily suggested, although 30–50 g/h was used as a target [3]. Despite not having provided athletes with a nutritional strategy, the percentage number in the study that followed recommendations was 22% (21.1% men and 12.5% women) [6] and 69% (100% male) in a further two studies [2,4]. Other relevant points noted in studies referred to the “ad libitum intake”, insofar as athletes were unaware of what was recommended for exercises of this duration and/or of the requirement [6], and so underestimated CHO consumption. In two studies [2,4] athletes had to ingest the amount of CHO determined by the group they were in. In addition, in these articles, an inclusion criterion was that athletes should carry out nutritional training assigned by their professional.

On the other hand, the fluid amount ingested during SOUT events ranged between 274 mL/h in a 120 km race [3] and a maximum of 765 mL/h ingested by a runner in a 160 km race [5]. Two of the studies [2,4] that analyzed a marathon reported an “ad libitum intake” by participants and the last an intake of 459 ± 256 mL/h [29]. Four studies [3,5,29,33] reported intake being maintained close to fluid recommendations of 0.4 to 0.8 L/h [28].

Two research articles [2,4] from the eight studies analyzed in this systematic review described internal exercise load, EIMD markers and post-exercise recovery outcomes (Table 4). In terms of exercise internal load and the evaluation of EIMD markers [4] and post-exercise recovery [2], the groups (HIGH or EXP), that consumed the largest amount of CHO per hour evidenced a lower exercise internal load level and EIMD markers (CK, LDK and GOT) after the race. Likewise, the HIGH or EXP groups evidenced better post-exercise recovery as observed by jumps and high intensity running capacity.

A common problem among endurance athletes is GI complications [26]. GI distress can be divided into upper: reflux, heartburn, belching, bloating, stomach cramps, vomiting, and nausea, and lower GI complaints may be: intestinal cramp, flatulence, urge to defecate, side ache, abdominal pain, loose stool, diarrhea and intestinal bleeding. The means used to measure GI symptoms was through surveys [5,6] with a nine-point scale (“no problem at all” to “the worst it has ever been”) or were consulted for complications at checkpoints throughout the race and at the end of the race [2,3,4,8].

Six [2,3,4,5,6,8] of the nine studies analyzed in this systematic review described GI problems (Table 5). Wardenaar et al. [6] and Costa et al. [8] showed an incidence of GI distress of 82% and 65% respectively, with two of them categorizing GI symptoms into upper and lower GI problems [5,6] The most recurrent upper GI symptoms include: nausea, heartburn, reflux, inflammation and stomach pain [6], and flatulence and diarrhea in the case of lower symptoms [5,6]. Another factor in GI symptoms could be high fiber consumption [6], and apart from nutritional factors, others such as warm environmental conditions, duration and/or intensity of exercise are also described [6]. Although some studies confirm the relationship between intake while competing [5] and GI symptoms, another study did not find this link [8]. In this sense, the alternatives proposed that favor CHO intake were via intestinal adaptation training [2,4,8].

The relative contribution of each macronutrient during an ultra-endurance race depends on previous energetic status, exercise intensity and duration and the nutrition strategy pursued prior to and during exercise [13], based on individual and personalized needs [17,26]. In particular, CHO is an important source of energy, although stores in the human body are limited, with muscle and hepatic glycogen being the most important storage areas [19]. For this reason, athletes must continuously ensure intake of this nutrient [12,50], whereby an individualized nutritional strategy may be developed that aims to deliver CHO to the working muscle at a rate that is dependent on absolute exercise intensity as well as the duration of the event [13]. Thus, international consensus establishes the recommendation to be 90 g/h when the activity exceeds 3 h, as well as the appropriate ratio of the type of CHO (glucose/fructose in 2:1 ratio) and previous gut training [8,28,49]. Importantly, ingesting different types of CHO means that absorption is carried out by several intestinal apical transporters (SGLT-1 for glucose and GLUT-5 for fructose) and thus, the ability to achieve maximum absorption and oxidation improves [26]. Conversely, it has been observed that the CHO oxidation rate can reach maximum values of 1.6 g/min when 100 g/h of glucose and fructose combination were ingested during cycling, thus reducing endogenous CHO oxidation [51]. Moreover, consuming glucose in combination with fructose resulted in 55% greater exogenous CHO oxidation rates, compared to ingesting glucose alone [52]. As such, high consumption of CHO with an appropriate composition during long-term exercises allows optimal blood glucose levels to be maintained, glycogen stores preserved, glycolysis enhanced, the efficiency of blood lactate production improved, and better sports performance attained [13,26,53,54].

On the other hand, the difficulties that the studies included in this work expose is the fact of not corresponding to updated recommendations during SOUT races linked to appetite suppression [8,10,33], lack of food education [8], difficulty in eating while running [33], factors that motivate food selection include taste [55], convenience, nutrition knowledge and beliefs, GI discomfort [5,8], flavors and tastes preferred by the athlete and the logistics associated with the race such as the great distance among the supply stations or the lack of variety of food in them [8]. Some authors, recommend the use of some nutritional strategies supervised by a professional dietitian or nutritionist or bringing their own food in order to increase food intake [37], although the 90 g/h goal is not always achieved. Accordingly, only two studies used a strategy that met the recommendation of CHO intake (90 g/h) and especially when (120 g/h) at least in one of the experimental groups [2,4]. In particular, a single runner who ingested 108 g/h during the 120 km race was able to adhere to recommendations without professional supervision [6]. However, it may be considered that in some cases the intake used for the strategy was lower than the recommendations (30–50 g/h) [3]. Studies that did not pursue strategies sought to ensure that athletes consumed the same as in training [5,6,8,29,33], except for two studies in which athletes had to pursue an intestinal strategy for at least two weeks [2,4].

Regarding the type of CHO food source, Jeukendrup [53,54] pointed out the importance of varying textures in order to avoid flavor fatigue. Along these lines, glucose transporter SGLT-1 is upregulated by molecular mechanism (mediated by enteric neurons) in response not only to glucose or galactose, but also to fructose, sucralose and others [42,56]. This regulation depends only on the bioavailability of these substrates, but not on their metabolism [42]. As the presence of some added sugars, sweeteners and/or flavors in foods can also upregulate glucose transport via SGLT-1 upregulation, flavor and food composition also have important implications through physiological mechanisms [42]. Moreover, one study highlighted the preference of athletes for solid foods as a source of CHO [29]. Thus, they gave a greater feeling of satiety, although some athletes preferred a sports drink, arguing that from a practical standpoint they can hydrate and ingest CHO at the same time without their having to load up with snacks or other solid foods [10]. Similarly, in terms of the consistency of food, Pfeiffer et al. [57] concluded that the average and maximum exogenous CHO oxidation rate was similar when athletes consumed a mixture of glucose + fructose in solid form (energy bar) or in a liquid state (sports drink). In other words, the gastric emptying rates were not substantially modified by the consistency of CHO intake, which would ensure an optimal consumption of CHO by offering variety in terms of food texture [3].

Overall, CHO intake by SOUT runners was below the updated recommendations regardless of whether a dietitian-nutritionist helped them during these events, although runners showed a preference for solid foods [5,29], also highlighting the consumption of gels and sports fluids [5,29,33]. For example, fructose consumption was represented by fresh or dehydrated fruits [5,29,33] without it being able to determine if the gels also contained fructose, since such information was not available in the relevant studies. Future studies may consider this type of content.

Linked to the concept of hydration, a suitable state during the race encourages the absorption of CHO [42] and prevents GI complications associated with the state of dehydration [26] caused by reduced blood flow to the digestive system. On the other hand, water is part of glycogen storage [58]. In four studies [3,5,8,33], faster runners reported higher fluid intake and recorded better performance, also matching an increase in CHO intake [5]. Hydration is an appropriate way to incorporate CHO [59], allowing an optimal state of hydration to be maintained [37] in order to avoid GI complications [16,20], which in turn entails disadvantages in the feeding plan and fluids designed by runners[6]. One strategy to ensure high CHO intake, avoiding GI complications associated with dehydration states, is through sports drinks or gels, ensuring suitable amounts of fluids in each of the latter [2,4].

On the other hand, one of the main drawbacks of ultra-endurance sports is the difficulty in reaching CHO intake recommendations [12]. Despite the benefits of high CHO/hour intake to maximize muscle glycogen reserves [12], delaying fatigue and improving post-exercise recovery [2,4], SOUT runners have shown an intake of 20–40 g CHO/h when such intake is “ad libitum” [37]. Only in two studies [2,4] did athletes consume the recommended quantity of (90 g/h) and the athletes achieved higher intake (120 g/h) due to the research protocol itself.

The SOUT events involve metabolic [60] and physical [61] requirements. For this reason, a nutritional strategy must be ensured that allows recovery times to be shortened between sessions and competitions [2]. Moreover, these races have a prominent component of eccentric strength [45], whereby the runners are subjected to high levels of EIMD and fatigue [15].

Ultra-endurance physical exertion causes physical and psychological stress [11,81], which could lead to changes in the intestinal tract and consequently affect athletic performance [37]. GI problems are common, especially in endurance athletes, and often impair performance or subsequent recovery [82]. The main GI symptoms associated with running might also be explained by [34]: dietary factors, alterations in intestinal or colonic water, electrolyte fluxes, disturbances in GI motility or underlying irritable bowel syndrome [83]. Moreover, these alterations and those related to mechanism may be different in the case of the upper (stomach) and lower (intestine) regions [42], with major differences between them. One study into long-distance triathletes who competed in extreme conditions demonstrated a prevalence of up to 93% in any GI symptoms [84]. Similarly, a survey of 707 participants in the 13th Annual Trail’s End Marathon in Seaside, Oregon (USA), showed a high incidence of GI problems, predominantly of the lower tract [83]. Between 30% and 90% of participants experienced some type of these GI problems, making it one of the main causes of the deficit in the intake of CHO while competing or training [5,6,8,83]. In addition, GI symptoms are usually more frequent when they are related to permanent states of dehydration, associated pathologies or high levels of stress or anxiety [85] and are exacerbated by hot climate and elevated humidity conditions [27]. Moreover, beverages with high osmolality (>500 mOsm/L) seemed to be associated with an increased incidence of symptoms [34].

The main pathophysiological mechanism by which this GI occurs in athletes, especially in the gut, is the intestinal ischemia motivated by the release of norepinephrine that is stimulated by long-term exercise and the high intensity that induces a secondary vasoconstriction. This in turn produces an epithelial lesion, changes in GI permeability and disruption of the epithelial barrier function, increasing serum gastrin related to exercise and exacerbating low GI problems such as diarrhea—resulting in osmotic shifts [86]. Gastric emptying is regulated in the stomach by different mechanisms such as neural control, hormone control and the vagal motor circuit [87]. Exercise impact on all these mechanisms has not yet become well established to the author’s knowledge, although how acute exercise impacts some gastric “braking” hormones like glucagon-like peptide-1 (GLP), leptin and cholecystokinin (CCK) is documented in scientific literature, affecting gastric emptying when exercise intensity is increased [88,89].

If exercise continues or exercise intensity increases and dehydration worsens, the splanchnic blood flow decreases even more, and, together with continuous mechanical impact and increased sympathetic activation, GI symptoms are aggravated accordingly [8,90]. However, this process could be reduced through gut training, as argued by Costa et al. [8] and Jeukendrup, [42], since exposure to high CHO intake can increase gastric emptying and stomach comfort, thus reducing GI symptoms and optimizing the monosaccharide transport rate. This in turn is mediated by increased SGLT1 protein expression and, therefore, improves the oxidation rate. [91], being one of the most interesting types of gut training proposed by Jeukendrup [42].

Regarding the appearance of symptoms and running speed, the study by Costa et al. [38] observed that faster runners experienced 2.5 times more GI problems than slower ones during the race, which made it difficult for them to continue with the established nutritional intake plan [6]. Conversely, Costa et al. [8] concluded that faster runners were less affected by GI symptoms. This difference in profiles may be due to the fact that the faster ones had made a nutritional plan that consisted of a greater intake during high volumes of training prior to the competition—hence, the adaptation of the stomach and gut in order to facilitate the absorption of nutrients in stressful situations [8]. Specifically, the relative intensity of each athlete may be one of the factors that highlight the differences among studies in terms of GI symptoms. As such, despite the major prevalence of mild or severe symptoms, the etiology of these GI complaints in endurance athletes is still not completely understood [34].

On the other hand, Wardenaar et al. associated a greater total nutrient intake with the lower incidence of symptomatology (except for fiber), although they were unable to determine whether the lesser symptomatology was due to higher intake or whether a better choice in the type of food enabled a higher intake [6]. Possibly, this study associates high intake with lower GI symptoms, given that athletes who practice sport daily with a suitably usual intake assume that high CHO intake is part of such daily practice. Moreover, a greater intake of fluids and foods helps to maintain better hydration status and, therefore, limits GI problems [41]. Symptoms are considered more likely with high fiber intake, fats and highly concentrated CHO solutions (i.e., hypertonic drinks) [92], while hypertonic solutions encourage net water secretion into the intestinal lumen, resulting in a temporary net loss of water from the body [93]. Despite this, high intake may not be associated with greater symptoms as long as suitable intestinal training has been undertaken, which makes it possible to increase the ability to digest and absorb CHO via the optimum regulation of SGLT1, GLUT5 and GLUT2 transporters [42].

Only two studies [2,4] detail the type of CHO and ratio. In them, only three athletes manifested GI complications, and these were participants with prior intestinal training that allowed high intake of CHO (120 g/h) to be attained without GI complications. One study [8] found no link between CHO intake and intestinal symptomatology. On the other hand, in another study [5] there were few complications, or the high intake of CHO was associated with a low frequency of symptoms [6].

The information available in the studies does not allow whether or not the type of CHO intake had an influence on GI symptomatology to be determined. It should be considered that in some studies some participants deemed it optimal to consume CHO in foods such as sandwiches, while in others consumption focused on gels, those being the ones where there was also intestinal training. This intestinal strategy does not occur in other studies, and so it is not possible to determine whether CHO intake in liquid or solid form is an advantage in avoiding complications. Only two studies [3,5] allowed the source of CHO used by participants to be analyzed with detailed GI symptomatology. In one of them [3], the symptoms were not transcendent or impacted on the performance of the runners. Those who evidenced the most symptomatology were the ones who consumed solid foods—they do not specify which ones—followed by those who consumed sports drinks. It should be mentioned that there was a third category: soft foods (all foods that did not require chewing). Solid foods have greater involvement in the digestive system than liquids and consequently lead to a delay in gastric emptying [57], which will imply a reduction in the oxidation efficiency of CHO [57]. The other study [5] reported fewer symptoms during the race than afterward. The athlete who reported the most complications—urgency to defecate—coincided with an intake of chocolate milk and other solids and fluids other than gels, energy bars or sports drinks. It should be mentioned that another participant who experienced discomfort in the upper intestinal tract—flatulence—consumed cola. This case could be explained as referring to carbonated beverages. The lack of information does not allow a conclusion to be drawn as to in which case an intake of CHO from a solid, liquid or soft source corresponds to a major or minor GI complication, or which area of the intestinal tract would be most affected.

The appearance of GI symptoms is a limitation in SOUT events [5,6,8,30,32,38,94], being more frequent in runners with pre-existing symptoms, in slower athletes [34] and in situations where the state of dehydration state has intensified, such as in hot and humid environments [45]. There is a disparity among results related to speed and GI symptoms, possibly due to the different objectives and methodologies used in different studies. Overall, it seems that moderate exercise has little effect on GI tract motility, but when exercise is more severe, there may be some inhibiting effects, especially in terms of gastric emptying [34]. In some of them, the faster runners did not evidence any GI symptoms since they undertook intestinal training [2,4], while in other studies, the faster runners reported GI complications when trying to consume more CHO given that they failed to undertake bowel training [38]. Moreover, as Pfeiffer et al. [20] suggested, a correlation between individual predisposition and GI distress should be taken into account by understanding intra individual variability of these mechanisms, in order to prepare personalized and periodized nutrition for each athlete [17].

The amount of CHO is as important as the time when CHO is ingested, since a high amount of CHO, usually through a hyperosmolar solution in a short period of time, would involve GI complications [26]. Inadequate fluid intake makes it difficult to ensure gastric emptying [32], and in this case, it is advised that a 2:1 glucose–fructose ratio be maintained to attain high amounts of CHO ingestion [2].