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Gastrointestinal (GI) Conditions in Endurance Athletes – Dietary Factors to Consider


How common is it?

Athletes who participate in endurance events frequently experience GI symptoms. It has been reported that over 40% of endurance athletes experience one or more GI complaints during or immediately after exercise. (Rehrer et al 1992b, van Nieuwenhoven et al 2004)









Photo: ©p-chambier_OT-Karellis

It is suggested that:

  • Lower GI complaints are more common than upper
  • Women experience more symptoms than men
  • Younger athletes experience more symptoms than older athletes
  • Exercising at higher intensity influences more symptoms than at lower
  • Running influences more symptoms than cycling, swimming or rowing


Study One: 160km Ultra Trail Race (Glace et al 2002)

During a 160km trail race runners eating strategies were documented.

  • 25,280kj was consumed – equivalent to daily intakes reported by cyclists during the Tour de France
  • 18L of fluid, 12g of sodium
  • GI symptoms were experienced by half the runners in the study. It was found that these symptoms were unrelated to food or fluid intake.
  • Upper GI symptoms occurred more often than lower and tended to occur after 88km
  • Nausea and vomiting were the most common symptoms  and these persisted from 88-133km
  • One runner pulled out of the race due to nausea and vomiting
  • Runners with GI complaints tended to complete fewer training miles 83kms vs 103km/week and do shorter training runs 76km vs 93km

Photo: © Timothée Nalet – www.peigné

What influenced their symptoms? It was thought that stress-mediated metabolic changes or changes in GI physiology (neuroendocrine changes, stress, non-steroidal intake, endotoxemia) were significant factors.

Runners in this event were better able to tolerate solid foods, concentrated beverages and fibre than athletes working at higher intensities over a shorter duration.

Runners who compete in these events tend to work at rates of about 45-50% maximal aerobic capacity.

Interesting Fact: Their total estimated energy expenditure was 56,680kJ – to put that in perspective that is approximately 8 times the basal metabolic rate (energy burned to keep you living) of a healthy sedentary adult. I don’t wish to say how many burgers that is equivalent to as that is not optimal endurance nutrition.

Study Two: London Pacers Distance Running Club (Sullivan and Wong 1992)

109 distance runners were surveyed about bowel function related to running

  • 12% had fecal incontinence while running
  • 62% had stopped to have a bowel movement while training
  • 43% had nervous diarrhoea before competition
  • 12% stopped during competition to have a bowel movement
  • 47% experienced diarrhoea after racing or after hard training session
  • 16% had seen blood in their stool after racing or after hard training session

Why do you need to use your bowels during exercise?

The short answer: It may be due to your sympathetic nervous system.

Long answer: When you exercise your sympathetic nervous system reduces blood flow to the gut and also relaxes your gut tone, this may increase the passage of what is currently in your colon (large intestine) into your back passage (rectum).

Why do you need to use your bowels after exercise?

This may be due to a decrease in colonic (large intestine) motility during exercise (this means there will be less resistance to colonic flow), combined with an increase in propulsive activity after exercise ? toilet.

Study Three: Over 600 well-trained endurance type athletes (runners, cyclists, triathletes) (Peters et al 1999)

Participants completed a mail questionnaire, covering the preceding 12 months of their training

  • Runners experienced more lower (71%) than upper symptoms (36%) during exercise
  • Cyclists experienced both upper (67%) and lower (64%) symptoms
  • Triathletes during cycling experienced both upper (52%) and lower (45%) symptoms, and during running more lower (79%) than upper (54%) symptoms

suggest GI symptoms in Endurance Athletes is Common!

It tends to be more common in females than males, it is dependent on the type of sport you are doing and the duration on whether this will more likely influence lower versus upper GI symptoms.

Types of GI symptoms

Endurance athletes experience a wide range of symptoms. Common symptoms reported are:

  • Abdominal pain
  • Urge to use the bowels
  • Diarrhoea
  • Reflux (more common in running)
  • Nausea
  • Runners trots (includes abdominal
    cramping, diarrhoea, fecal urgency, incontinence)
  • Vomiting
  • Side ache (physical tugging on
  • Intestinal cramps
  • Belching
  • Flatulence/wind

Reflux is more common in running and can be due to delayed gastric emptying. It is increased by pre-exercise food intake and may be related to inadequate lower esophageal sphincter pressure.

Belching is perhaps caused by swallowing air during running. This can then lead onto reflux due to the air that has now become trapped in the stomach influences relaxation of the lower esophageal sphincter ? reflux. Reflux is when the stomach contents including acid move back up into the oesophagus.

Why do endurance athletes get GI symptoms during exercise?

There’s an easy answer for that right? Unfortunately not. There has been limited research
into the cause of GI symptoms associated with exercise. Mostly there are speculations. So let’s look at some of these.

Gastrointestinal Blood Flow

  • When we exercise there is a reduction of blood flow to the gut. This is so the muscles
    receive more blood for the work they need to carry out, unfortunately the poor GI
    tract losses out. At maximal intensity exercise, blood flow to the gut can be reduced by 20% of normal resting value, and this occurs in both trained and untrained athletes (Jeukendrup 2000). The reduced GI blood flow that occurs with exercise, disturbs gastrointestinal motility, intestinal absorption and GI physiology? GI complaints.

Hormones, Training, Mechanical Vibration, Exercise Intensity and Exercise Duration

  • There are changes in gastroenteropancreatic (say that 10x quickly) hormones (including vasoactive intestinal peptide, peptide histidine methionine) which are thought to influence
    intestinal motility and/or blood flow (Brouns 1991, Sullivan 1991)
  • Training status – GI symptoms occur more frequently in untrained athletes. Training attenuates the reduction of gastrointestinal blood flow possibly due to adaptations that
    occur in the sympathetic nervous system with training. It is thought that this improves gut barrier function. Also an increase blood flow raises the rate of absorption of substances by reducing their concentration in the interstitial space, helping prevent against gut upset.
  • One author suggests that years of training influences a physical conditioning to manage
    the effects of exercise on the gut. Or quite possibly the more experienced/knowledge the athlete has the better self-evaluation and feedback mechanism they have e.g. they stop or are able to reduce problems before they occur. Or… is it possible that the athletes who are more likely to experience GI symptoms quit before they are ahead e.g. before they become experienced athletes and there is actually no effect of training status on GI symptoms..
  • Mechanical Vibration otherwise known as jarring of the gut or “caecal slap syndrome”. The
    pounding effect of running influences gut symptoms. Acceleration and deceleration is more than doubled in running than cycling. The rubbing and distension of the intestinal wall influences vasoactive intestinal peptide and prostaglandins to be released causing intestinal secretion which can then lead on to diarrhoea.
  • Exercise Intensity – the higher intensity that you train at the more likely you are to
    experience symptoms.
  • Exercise Duration – the urge to use your bowels or to urinate during exercise increases
    with increasing duration. If the athlete does not stop to go this will influence abdominal distress and cramping (just like it would occur if you did this at rest).

Psoas Muscle, Volvulus, Esophageal Sphincter, Abdominal Pressure

  • A more rare mechanical type of stress is where the psoas muscle (muscle that helps flex the hip, rotate the hip joint and flex the spine) may hypertrophy causing pressure
    on ascending colon when flexing hip (Dawson 1985)
  • Another more rare influence on gut symptoms – twist of the cecum by low body fat has been reported in distance runners (Pruet et al 1985)
  • Reduction in lower esophageal sphincter pressure
  • Cyclists increase their abdominal pressure when they lean forward on the bike which influences the upper part of the GI tract.

Non-steroidal anti-inflammatory drugs (NSAIDs), Environment, Dehydration, Individual Predisposition

  • NSAIDs have been associated with colonic bleeding, gastritis, ulcers, diarrhoea
  • Environment – When you exercise in hot conditions you tend to lose more total body water, experience a decrease in plasma volume and there can be a significant reduction in blood flow to the gut? GI complaints
  • Dehydration – a study reported that 3.5-4% of body weight loss was associated with an increased incidence of GI symptoms during exercise
  • A significant factor influencing gut complaints during exercise is if the athlete has a prior
    history of GI distress at rest (Peters et al 1999, Pfieffer et al 2011)

Gastric Emptying Rate

  • Gastric emptying rate and intestinal fluid absorption are not significantly affected
    during exercise at or <70% of VO2 max
  • At higher exercise intensity at 75-80% of VO2 max this may delay gastric emptying and decrease intestinal fluid absorption influence gut complaints
  • The average gastric emptying rate varies widely between individuals and it may be that the slowed emptying rate relative to ones norm is associated with gut complaints

Carbohydrate Beverage Concentration and High Carbohydrate Intake

  • Studies indicate that gastric emptying and intestinal fluid absorption are reduced when
    carbohydrate concentration in test solutions increase from 6% or higher increasing risk of gut complaints
  • A study by Shi et al 2004 found an increase in beverage carbohydrate electrolyte solution (6% vs 8%) and osmolality provoked stomach upset and side ache during intermittent, high intensity exercise (76%). The exercise involved running, hopping, jumping, sprinting and shuttle runs. To note though is that GI discomfort was relatively slight. For example on a 100 point scale for GI symptoms it appears to reach 12 points for the 8% solution
    and 9 points for the 6% solution.
  • A study by Rehrer et al 1992b found triathletes participating in half-traithlon who had vomited or had urge to vomit were more likely to have consumed a hypertonic beverage than those without symptoms.
  • It is thought that hypertonic solutions (more concentrated) cause GI upset via water
    retention to the intestine.
  • Regarding a high carbohydrate intake studies report that large amounts of carbohydrate
    consumed may be incompletely absorbed leaving residual carbohydrate in the intestine and this can influence GI complaints. Studies have shown residual carbohydrate in the intestine can influence gut complaints at rest.


  • A series of studies has shown that high rates of carbohydrate (1.4g/min) in the form of
    gels composed of glucose and fructose is well tolerated in majority of athletes
    in a 16km race. The duration was approximately 70mins (Pfieffer et al 2009). They did find higher score towards nausea for athletes when they consumed 90g carbohydrate/hr vs 60g carbohydrate/hr. It must be considered that this study was relatively short in duration and was carried out in mild environmental conditions.
  • A survey carried out in over 200 endurance athletes including triathletes, marathon runners and cyclists found that the average concentration of carbohydrate solution consumed across events was 10.5% (Pfieffer et al
    2011).  This is quite a bit larger than the general recommendation for carbohydrate composition of sports drink being 6%. Furthermore the individual concentration of carbohydrate solution ranged widely between 3.5-27%. The lowest average carbohydrate concentration consumed during the Hawaii Ironman was 8.8%. What is interesting is the study found no clear relationship between carbohydrate intake rates and GI distress. They did find that nausea and flatulence tended to occur more with high carbohydrate intake rates in the Hawaii Ironman and the half Ironman.
  • A study by van Nieuwenhoven et al 2004 found flatulence increased in athletes consuming carbohydrate sports drink compared to water. Studies indicate that carbohydrate intake can be a risk factor or nausea and flatulence during exercise however these symptoms appear to be quite minor and are less likely to influence performance compared with other symptoms like diarrhoea and cramping.
  • Therefore athletes need to strike a balance! Higher carbohydrate intake rates have been
    correlated with faster finishing times (Jeukendrup 2011, Pfieffer et al 2011). The athlete should practice with their intake needs and consume carbohydrate as tolerated. A Sports
    Dietitian can be very valuable in helping the athlete work out their race plan and nutrition needs.

 High Carbohydrate Intake and Multiple Transportable Carbohydrates

  • I mentioned a study (Pfieffer et al 2009) that looked at consuming high intakes of carbohydrate beyond the traditional recommendation of 60g carbohydrate / hr. Traditionally it was thought that it would be of no benefit to further increase beyond the amount of 60g carbohydrate / hr as its uptake or oxidation could increase no further. If it
    was increased beyond this amount it would increase the risk of gastrointestinal upset.


  • Jeukendrup and colleagues have found that although glucose absorption appears to become saturated at around 60g/hr other carbohydrates can be utilised simultaneously such as fructose (up to a rate of 30g/hr). Fructose uses a different transporter to glucose and/or the same and therefore when these two different carbohydrates are ingested by the athlete a higher amount of carbohydrate can be absorbed, taken up and oxidised. It has been shown that using multiple transportable carbohydrates can lead to up to 75 % higher oxidation rates than carbohydrates that only use the Sodium Glucose Transporter (SGLT1) (Jeukendrup 2011).
  • A 5hr cycling trial found that ingesting both glucose and fructose simultaneously at high rates compared to ingesting glucose alone in the same amount influenced a reduction in rating of perceived exertion and that cycling cadence was better able to be maintained. The other effect was that participants reported less stomach fullness with the glucose and fructose trial.
  • Studies have shown the higher carbohydrate intake rate to improve endurance performance (Currell et al 2008, Triplett et al 2010).
  • A study by Jeukendrup and Mosely (2010) found that the use of a combination of transportable carbohydrates e.g. glucose and fructose in solution can help to maintain gastric emptying and improve the delivery of fluid. A better tolerance of these drinks that use multiple transportable carbohydrates versus glucose alone has been shown when carbohydrate is ingested at high rates, greater >1g/min.

Still it seems like a lot doesn’t it.

Is it possible to train the gut?

  • Anecdotal evidence suggests the gut can be trained. Individuals who regularly consume
    carbohydrate or have a high carbohydrate daily intake may have an increased capacity to absorb it (Murray 2006, Jeukendrup 2011).
  • A study by Ferraris 2001 has shown that intestinal carbohydrate transporters can be up-regulated by exposing an animal to a high carbohydrate diet. There is limited evidence in humans although a study by Cox et al 2010 looked at whether altering daily carbohydrate intake affects substrate oxidation and exogenous oxidation. A higher oxidation rate
    after high carbohydrate diet (6.5g/kg BW/d; 1.5g/kg BW provided mainly as a carbohydrate supplement during training) for 28d compared with a control diet (5g/kg BW/d) was demonstrated.

Interesting Fact: The volume of an empty stomach is 50-100mL but can increase quickly to roughly 1L without an increase in intragastric pressure.  The gut can be trained to accommodate increasing volumes of food and drink and this is demonstrated in the following world records being set:

  • 2004 a Japanese person consumed 53.5 hot dogs in 12 minutes
  • An American female consumed 5kg of cheesecake in 9 minutes (I think I could beat that
  • Gastric emptying may be enhanced with training – learning effect (Gisolfi 2000)
  • At least 2 studies have reported gastric emptying rate and orocecal transit time to be enhanced in endurance athletes (Carrio et al 1989, Harris et al 1991)
  • Hyperphagia of SI in animal models in response to increased EI ? translate to humans? (Gisolfi 2000)
  • Athletes become more accustomed to training with fluid in their stomach.
  • It is possible that this adaptive response occurs due to the large energy intake required
    during the day by the athlete and to the repeated experience of ingesting food and drink during exercise

Carbohydrate Type FODMAPs

  • FODMAPs is a registered trademark of Dr Sue Shepherd and Professor Peter Gibson. Dr Sue Shepherd developed the low FODMAP diet and it is recognised as one of the most successful dietary therapies for Irritable Bowel Syndrome.
  • Another factor that may influence GI symptoms during exercise is the carbohydrate type and other food intolerances.
  • In some individuals gastrointestinal symptoms can be influenced by molecules in food in
    particular by poorly absorbed short chain rapidly fermentable carbohydrates referred to as FODMAPs™. I am not going to go over what FODMAPs in this article, to learn more about FODMAPs you can see the following ask me to write about it in another article.

Dietary Fibre, Fat and Protein and Timing of Meal

  • A study by Rehrer et al 1992b looking at diet during triathlon competition and the occurrence of GI symptoms found:
  • Those who consumed food high in dietary fibre, fat or protein before competition had more GI symptoms (esp. related to vomiting and reflux)
  • Timing of last meal – those who consumed (s) meal shortly before exercise e.g. 30mins prior to swim  vomited more

Summary and Practical Suggestions

  • There is an individual predisposition to GI problems and each individual varies in their
    tolerance during exercise therefore it is important that sports nutrition
    advice is tailored/specific to the athlete. See a Sports Dietitian
  • The better trained and acclimitised you are potentially less symptoms. For example,
    thermal tolerance, GI blood flow and GI physiology all improve with training
    and acclimation to the heat.
  • Increase training load and intensity gradually and be smart. Temporary reduction in training intensity to allow for gradual GI adaptation.
  • Stay hydrated minimise body weight loss. Ingesting sufficient water and nutrients during
    prolonged exercise can help maintain GI blood flow and reduce the risk of gut
  • Practice drinking during training – capacity to comfortably ingest water and nutrients
    can improve.
  • Avoid over-nutrition prior and during exercise e.g. need to also be careful not to go
    over budget regarding energy intake. Have a smart plan based on energy needs.
    And remember saturation of transporters and making good use of the variety
  • Eat ~2hrs prior to training or competition
  • Suggest low fibre, limit fat and protein in pre-competition meal
  • Have a loo stop prior to competition or training
  • Do not use anti-inflammatory drugs to prevent exercise induced pain during exercise
  • Avoid gulping fluids during training and some may find it beneficial to avoid agents that
    relax lower esophageal sphincter pressure  possibly caffeine, mint, chocolate, alcohol
  • Consumption of food attenuates the reduction of blood flow to gut induced by moderate to
    severe exercise therefore consuming food during exercise may help
  • Runners trots may require anti-motility medication prior to exercise. See your sports
    physician or medical specialist regarding this
  • If an athlete has fructose malabsorption, lactose intolerance or requires a low FODMAP diet or a low chemical diet e.g. Elimination Diet I suggest seeing a dietitian
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Stephanie Gaskell is an Accredited Practising Dietitian, an Accredited Sports Dietitian, and a competitive Ultra Trail Marathon Runner. She is available for Phone and Skype Consultations, and has published the FODMAP™ & Gluten Status Sports Supplement Guide.

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