Gastrointestinal Complaints During Exercise: Prevalence, Etiology, and Nutritional Recommendations

Clearly, there is large variation in the reported prevalence in the literature and this seems to be attributable to, among other factors, the method of investigation (and the way in which gastrointestinal symptoms are defined and recorded). However, in addition, the reported prevalence of these symptoms varies in different studies depending on the study population, sex, age, and training status of the athletes, as well as the mode and intensity of the exercise studied, and the environmental conditions. There is also no consistent definition of what constitutes a severe or non-severe symptom. It has been suggested that a severe symptom is any symptom that impacts performance and/or health. So, for example, very mild nausea may not affect performance, but a higher degree of nausea is likely to have negative consequences. Flatulence is unlikely to have effects on performance, but vomiting, independent of the severity score provided by the athlete, is likely to have a negative impact on performance. This is an approach discussed by the authors in a paper in 2000 [2] and it is suggested that future studies in this area use a similar categorization of symptoms and scoring approach.

When analyzing the reported symptoms, it becomes immediately obvious that they are highly individual and there are no clear patterns with regard to the type of activity and the types of symptoms observed. There is a fairly large, yet well defined number of different gastrointestinal symptoms that can occur during exercise. Generally, the symptoms can be classified as either upper or lower gastrointestinal tract. Typically, lower gastrointestinal tract problems are more severe in nature, but all symptoms have the potential to impair performance. Symptoms are often mild and may not affect performance, but they can also be very serious and can not only affect performance, but also be health threatening.

Gastrointestinal symptoms are common in many sports but particularly in endurance events. One early review stated that in exhausting endurance events, 30–50 % of participants may experience one or more gastrointestinal symptoms [1]. A study in long-distance triathletes who competed in extreme conditions demonstrated a prevalence of up to 93 % for any gastrointestinal symptom [2]. Among elite endurance athletes, the prevalence of exercise-induced gastrointestinal symptoms was reported to be 70 % [4], and in an internet-based observational study of 1,281 athletes, 45 % reported at least one gastrointestinal symptom [5]. Gastrointestinal distress is a pervasive problem, especially in ultra-endurance events [6, 7, 8, 9]. Nausea, vomiting, abdominal cramping, and diarrhea have been reported in 37–89 % of runners participating in races 67–161 km long [6, 7, 8, 9], and fecal blood loss indicating gastrointestinal hemorrhage was reported in 85 % of participants in a 161 km ultra-marathon [6]. A recent study investigated gastrointestinal problems in a group of ultra-marathon runners, and observed that 9 of 15 runners experienced gastrointestinal distress, including nausea (89 %), abdominal cramps (44 %), diarrhea (44 %), and vomiting (22 %) [9].

The prevalence of symptoms varies considerably depending on the event, the environmental conditions, and the level of the athlete. Pfeiffer et al. [10] reported severe gastrointestinal distress ranging from 4 % in marathon running and cycling up to 32 % in Ironman races. Large differences were observed not only between the different events, but also between individuals within the same event. It was demonstrated that there was a strong correlation between gastrointestinal complaints and having a history of gastrointestinal symptoms, indicating that some people are more prone to developing gastrointestinal symptoms [10, 11], and suggesting that there is a large genetic component to these problems.

Gastrointestinal symptoms are not only inconvenient, they can also affect performance and, in extreme cases, have longer-term health implications. In one study, 43 % of triathletes reported serious gastrointestinal problems, and 7 % abandoned the race because of gastrointestinal problems [2]. In two 161-km ultra-marathons, nausea and/or vomiting were the main reasons for dropping out among non-finishers and were the second most common problem impacting race performance among finishers [7]. Clearly, gastrointestinal problems can have major effects on performance.

Among the reported deleterious manifestations of strenuous exercise are mucosal erosions and ischemic colitis, both observed after long-distance running [12, 13, 14]. For example, marathon runners and long-distance triathletes occasionally have blood loss in the feces in the hours following a marathon. Schaub et al. [15] observed epithelial surface changes known to occur during ischemia on colonoscopic inspection of one such triathlete following a marathon, and suggested that ischemia of the lower gastrointestinal tract induced the problems. Blood loss as a result of ischemic colitis is not uncommon in athletes, and can be profound in extreme cases. Proximal, distal, or pancolitis, and even small bowel, infarctions have been reported in athletes and in some cases have required surgery [14, 16]. Despite the high prevalence of mild or severe symptoms, the etiology of these gastrointestinal complaints in endurance athletes is still incompletely understood.

While it is recognized that the etiology of exercise-induced gastrointestinal distress is multifactorial, gastrointestinal ischemia is often acknowledged as the main pathophysiological mechanism for the emergence of the symptoms [5, 17, 18]. The other factors are mechanical and nutritional in nature.

Although much is still unknown about the etiology of gastrointestinal symptoms, a number of nutritional manipulations have been suggested to reduce the number or the severity of symptoms. For athletes who compete in endurance events, an interesting observation is that when a beverage is consumed that contains multiple transportable carbohydrates such as glucose and fructose, gastrointestinal symptoms seem to be reduced compared with the consumption of the same (large) amount of a single carbohydrate (glucose). This has been a consistent finding in a number of studies from the authors’ laboratory [42, 43, 44, 45]. Rowlands et al. [46] recently reported fewer gastrointestinal problems with multiple transportable carbohydrates (maltodextrin : fructose) in mountain bikers, and concluded that the ingestion of multiple transportable compares with a single carbohydrate-enhanced mountain bike race and high-intensity laboratory cycling performance. These results were found with inconsistent, but not irreconcilable, effects of gut discomfort as a possible mediating mechanism.

More recently, van Wijck et al. [19] argued that, if perfusion of the gut is one of the main causes of the problems, then manipulation of blood flow to the gut by upregulating nitric oxide production could be a way to reduce symptoms. There are a number of options to upregulate intestinal nitric oxide availability that may be of value for athletes experiencing abdominal distress associated with splanchnic hypoperfusion, including nitric oxide synthase-dependent (glutamine–arginine–citrulline) and nitric oxide synthase-independent (nitrate–nitrite) supplementation. Research is in its infancy, but of the ingredients that have been studied, dietary nitrate probably has the most potential to influence splanchnic perfusion [19]. However, current recognized vegetable sources of dietary nitrate are also sometimes associated with gastrointestinal symptoms.

For many years, it has been suggested that the gut can be ‘conditioned’ or ‘trained’ in order to reduce gastrointestinal distress, yet research in this area is still in its infancy [47, 48]. It has been shown that athletes who are not accustomed to fluid and food ingestion during exercise had a twofold risk of developing gastrointestinal symptoms than athletes who were accustomed to taking fluid and food during exercise [5]. In a study by Cox et al. [49], the adaptability of the gut was nicely demonstrated. In that study, 16 endurance-trained cyclists or triathletes were pair matched and randomly allocated to either a high carbohydrate group (high group; n = 8) or an energy-matched low carbohydrate group (low group; n = 8) for 28 days. It became apparent after 28 days that the high carbohydrate group had higher exogenous carbohydrate oxidation rates during exercise [49]. The higher rates were attributed to improved absorption, which is generally associated with improved tolerance of fluids and foods during exercise, and thus the training would have reduced the chances of gastrointestinal distress [48].

However, it also appears that training with a nutrition strategy can improve tolerance and stomach comfort, independent of changes in adaptations in gut function. Lambert et al. [50] assessed tolerance to fluid ingestion with repeated sessions of drinking while running. During five runs, subjects drank a volume of the solution every 10 min equal to their sweat production over 10 min. Although repeated sessions of drinking at a rate matching sweat rate improved stomach comfort, the gastric emptying rate did not change under such conditions.

More research is needed to understand the adaptations that occur in the intestinal tract, as well as the time course of such changes. This is essential to develop guidance for athletes on how often certain nutritional strategies need to be practised and the best way to conduct such training sessions.

Large numbers of athletes have reported using analgesics to relieve existing or anticipated pain [51]. The use of non-selective non-steroidal anti-inflammatory drugs (NSAIDs) has been associated with a three- to fivefold increased risk of upper gastrointestinal complications, mucosal bleeding, or perforation compared with no medication [52]. In a study at the Chicago marathon [53], it was found that ibuprofen (but not aspirin) ingestion during prolonged exercise may have increased gastrointestinal permeability and led to gastrointestinal symptoms. van Wijck et al. [54] recently demonstrated that ibuprofen aggravates exercise-induced small intestinal injury and induces gut barrier dysfunction, and concluded that the consumption of NSAIDs by athletes is not harmless and should be discouraged in those who experience persistent or recurring gastrointestinal symptoms.