Exercise practitioners are thankfully becoming more aware of gastrointestinal problems in athletes, and various strategies have been suggested. Simone do Carmo investigates the therapeutic usefulness of the low-FODMAP diet.

Despite months of training and careful preparation, a bout of gastrointestinal distress (GI) on race day can wreak havoc on an athlete’s performance and their subsequent recovery. The aetiology of GI distress is not fully understood due to multiple causative factors and a wide range of reported symptoms that are highly individual and vary in severity. This makes the condition challenging both to diagnose and treat.

Reported symptoms are generally categorised as either upper GI symptoms (e.g. heartburn, bloating and nausea) or lower GI symptoms (e.g. cramps, diarrhoea and intestinal bleeding). Although the latter symptoms are normally more severe, all GI symptoms have the potential to debilitate an athlete’s performance. The prevalence of GI distress is highly variable, ranging from 30 to 90 per cent, which can partly be explained by the different methodologies, study populations and environmental conditions in research studies. There also seems to be a large genetic predisposition towards developing GI distress, because a strong correlation between GI complaints and a family history of GI symptoms has been found in endurance athletes (1).

Potential causes of GI symptoms in athletes

A review by de Oliveira, Burini and Jeukendrup (2) provides an articulate outline of the aetiology of GI distress and describes the causes as physiological, mechanical or nutritional in nature. GI ischemia is thought to be the main physiological mechanism, caused by shunting of the blood to the active muscles. Changes in gut function and compromised gut permeability can follow, mainly due to a reduction in splanchnic blood flow, increasing the risk of endotoxemia where gram-negative bacteria and/or their toxins enter the blood circulation. This heightens the immune system to cause a systemic inflammation and can even lead to life-threatening endotoxic shock in extreme cases (3).

Mechanical causes are described as being related to a change in posture (2). This is observed in cycling due to increased abdominal pressure where upper GI symptoms are more prevalent. Mechanical trauma is also commonly seen in running, where lower GI symptoms are more predominant because of the bouncing repetitive impact that can irritate and damage the intestinal epithelium. This damage, combined with ischemia, is believed to be the cause of intestinal bleeding; a common symptom experienced by long-distance runners that can have profound effects, with some cases requiring surgery. Augmented respiration and drinking throughout a race, both commonly overlooked as causative factors, may also result in mild to moderate GI distress due to an increase of swallowed air.

As for the nutritional causative factors discussed by de Oliveira, Burini and Jeukendrup (2), dehydration and the consumption of high-fibre, high-fat, high-protein, fructose, milk and highly-concentrated carbohydrate drinks are all associated with a greater risk of developing GI distress. The association between nutritional practices and the development of GI distress symptoms was observed in 55 male triathletes who participated in an half-Ironman race (4). Fifty-two per cent reported an urge to defecate while 42 per cent complained of flatulence. Other symptoms included vomiting, abdominal pain, nausea, diarrhoea and intestinal cramps, which predominantly occurred during the running leg of the race. All of the triathletes who had eaten half an hour prior to the start of the race vomited during the swim leg of the race. High intakes of fat and protein were associated with vomiting, or the urge to do so. Of those who vomited, 93 per cent of triathletes had consumed a hypertonic drink and 40 per cent of those had severe complaints, compared to only 11 per cent who consumed a hypo- or isotonic drink. All triathletes who complained of intestinal cramps had consumed high-fibre foods before the race. Consuming too much food before any race or important training session is likely to cause a delay in gastric emptying and induce a mixture of GI symptoms like those mentioned in the study.

Compromised gut permeability, that results from the reduction in splanchnic blood flow, may also cause food-dependent, exercise-induced anaphylaxis, an IgE-mediated hypersensitivity (3). This has been associated with many foods, such as cereals, seafood, peanut, lentils, pistachio nuts, eggs, milk and vegetables. It occurs only after the ingestion of a specific food, followed by intense exercise within a few hours of ingestion. Additionally, the trigger of allergic symptoms is known to depend on the volume of food consumed (3).

Several nutritional strategies have been proposed to reduce GI distress. Examples of these include the use of functional foods (e.g. a concentrated beetroot product) and supplements (e.g. arginine and citrulline) that stimulate the production of nitric oxide, causing vasodilation and in turn improving splanchnic perfusion (5). Additionally, the use of multiple transporter carbohydrates (glucose and fructose) in sports drinks have been shown to increase carbohydrate oxidation efficiency (6). Since gastric emptying is slowed down as the concentration of carbohydrates increases in replacement fluid, current recommendations for athletes are to consume sports drinks below 8 per cent of carbohydrate concentrations (preferably a combination of glucose and fructose) in order to prevent dehydration during exercise, while maintaining adequate carbohydrate oxidation rates and plasma glucose levels (3).

The low-FODMAP diet

While these nutritional strategies have been successful for some athletes, symptoms of GI distress still persist in others, thus creating a need to develop innovative nutritional strategies such the low-FODMAP diet. In some athletes, GI distress may be triggered by certain food components that are poorly absorbed and rapidly fermented by colonic bacteria, causing GI distress symptoms such as diarrhoea, flatulence and abdominal cramps, that may impair their performance and subsequent recovery (7). These food components are known as FODMAPs; an acronym for Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols (see Figure 1 for an example of a high and low FODMAP list).

The low-FODMAP diet was initially developed for patients suffering from Irritable Bowel Syndrome (IBS) and has also been applied to other conditions, such as coeliac disease, fructose malabsorption and lactose intolerance. A review supports the efficacy of the diet and reports that up to 86 per cent of patients with IBS alleviate their GI symptoms while following it because of a reduction in osmotic load and gas produced by fermentation in the gut microbiota (8).


Figure 1: Example list of foods high in FODMAPS (left) and low in FODMAPS (right) (9).

The authors emphasise that long-term adherence to the diet is not recommended due to the high risk of inadequate nutritional intake and the potential negative effects of the altered gut microbiota. For example, restricting high-FODMAP foods generally leads to a reduction in fibre intake that can change the composition and activity of the microbiota. This may create adverse effects since the microbiota derives its nutrients from fibre. The fermentation of fibre results in the production of key short-chain fatty acids (SCFAs), such as butyrate, propionate and acetate, which are rich energy sources for the cells lining our colon and contribute to our colonic health. They also regulate carbohydrate and fat metabolism, and have anti-inflammatory properties, reducing the risk of type II diabetes, heart disease and other conditions.

The low-FODMAP diet is known to lower the relative abundance of bacteria that produce these key short-chain fatty acids and long-term adherence to this diet without any professional guidance could therefore put our health at risk of imbalance (10). This is where it is important to experiment with the individual patient, restricting certain high-FODMAP foods that are most aggravating rather than adopting a stringent, overall approach. The same applies to athletes, where emerging evidence shows they are reducing the consumption of these foods with the aim of improving their GI symptoms, allowing them to focus on their performance rather than their unpredictable gut.

A study surveyed 910 athletes to assess if they were avoiding certain foods in order to minimise GI distress symptoms: 55 per cent stated they were actively avoiding at least one high-FODMAP food. 93 per cent of the athletes reporting that that particular food elicited GI distress symptoms, and 86.5 per cent reporting an improvement in these symptoms through avoidance of the food (7). Lactose (86.5 per cent) was the highest FODMAP food reported to trigger GI distress symptoms, with an associated improvement in those who avoided it. Others included galacto-oligosaccharides (23.9 per cent), fructose (23.0 per cent), fructans (6.2 per cent) and polyols (5.4 per cent).

A low-FODMAP case study

A short-term, low-FODMAP dietary intervention was used in a case study to try and attenuate an athlete’s GI symptoms specific to running (11). The athlete’s habitual diet was compared to a six-day, low-FODMAP diet while keeping his training similar in both conditions. His GI symptoms were recorded after each training session and at the end of each day, along with his DALDA (Daily Analysis of Life Demands for Athletes) scores. The DALDA is a validated questionnaire that aims to quantify an athlete’s response to stress associated with daily life and the demands of training, thus providing a snapshot of an athlete’s psychological wellbeing. The athlete’s psychological wellbeing was assessed because there is a link between psychological stressors and increased GI symptoms. This is particularly relevant since stress and anxiety may be heightened during competitive events and could aggravate GI symptoms.

The low-FODMAP diet was successful in resolving the athlete’s running-specific GI symptoms compared to his habitual diet and he also showed a marked improvement in his psychological wellbeing. Although more randomised controlled trials are needed to examine the general effectiveness of a low-FODMAP diet in managing GI distress symptoms in athletes, this does not negate the n=1 effectiveness of a short-term, low-FODMAP diet as each individual athlete’s nutritional situation is unique and should be treated as such. Individualised approaches are moreover crucial as not all FODMAPs may be culprits and unnecessarily avoiding certain foods without adequate substitutions may risk inadequate energy and nutritional intake and compromise the gut microbiota (12). Therefore, until further research is conducted, a prudent approach is recommended by Lis et al (11); namely, to follow a low-FODMAP diet for two to three days prior to a race or important training sessions in order to alleviate the symptoms of GI distress.

Another contributing factor to GI distress that is unrelated to nutrition, but which warrants consideration, is that GI distress seems to increase with the use of non-selective non-steroidal anti-inflammatory drugs (NSAIDs), which athletes commonly take to alleviate pain. The use of NSAIDs has been associated with a three- to five-fold increased risk of developing upper GI symptoms, as well as intestinal bleeding (2).

Our gut is trainable

The battle against GI distress is a challenging one, but the upside is that the gut is a highly dynamic and adaptable organ, a fact that is often overlooked. This provides the opportunity for athletes to experiment with different nutritional strategies during training in order to determine what works best for them and then implement it on race day (13). Doing so can also boost an athlete’s gut function and improve their tolerance to certain foods.

The trainability of the gut was observed in a set of 16 trained cyclists or triathletes, who were pair-matched according to their maximal aerobic capacity, training status and training history, and then randomly allocated to consume either a high-carbohydrate or a low-carbohydrate diet that were each energy-matched during a 28-day training intervention (13,14). The athletes followed the respective diet for 28 days. The authors concluded that manipulation of carbohydrate availability in trained athletes leads to differences in metabolic adaptations to exercise, with the high-carbohydrate group revealing a greater increase in maximal citrate synthase activity and higher carbohydrate exogenous oxidation rates during submaximal exercise. These changes were attributed to enhanced intestinal absorption, which in turn has been associated with improved tolerance, reducing the likelihood of developing GI distress symptoms (2).

Like any athlete’s individual training programme, it is important to find the best way to nourish an athlete, considering that their gut is under repetitive stress and may be more prone to certain triggers. Eliminating certain high-FODMAP food triggers from an athlete’s diet with a prudent, individualised and focused approach may be an effective nutritional strategy to diminish GI distress symptoms, resulting in a calmer gut and a happier athlete. 


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