Gut Feeling

What actually causes those gut problems during exercise?

Ask most long-course triathletes (and many in shorter distances), and they’ll tell you horror stories of gut problems ruining a training session or a race. For some, it’s a constant battle, and just finishing a race without vomiting or diarrhoea is an achievement. There are endless home remedies that seem to work perfectly for some athletes but not for others, and sports doctors, dietitians and coaches have often been left with no explanation and no solution.

Gut problems during exercise include stomach pain and bloating, nausea, reflux and vomiting; lower abdominal pain or bloating, cramping, urge to defecate and diarrhoea. In the past, scientific attempts to characterise these issues have consisted mainly of observational studies – researchers attending a race, surveying competitors and taking basic measurements. This research helped identify the factors that increase the risk of gut problems but does little to explain what’s actually going on in the body, and what can be done to prevent these issues. Because of this, both athletes and health professionals have been stuck using best-guesses and trial-and-error to try and eliminate the problem.

Recently, lab-based research has started investigating the underlying causes of gut problems. This has led to the realisation that gut issues in athletes are not one-size-fits-all, and culminated in a new term, “Exercise-induced Gastrointestinal Syndrome”, which describes the numerous ways that athletes can experience gut problems. Back in June, a new review article was published, outlining all the known causes of Exercise-induced Gastrointestinal Syndrome for the first time. The lead author was Dr Ricardo Costa, lecturer and researcher at Monash University, a former ITU/ETU professional and world junior duathlon champion.

The main causes of gut problems during exercise 

Testing: Staff member, Amy Giannotti, conducting one of the battery of tests used at Monash University to diagnose the cause of exercise-induced gastrointestinal syndrome. Breath testing assesses whether the carbohydrate an athlete consumes during exercise is being absorbed.

“If you look at the gastrointestinal symptoms that athletes experience during exercise,” says Costa, “there seem to be two main processes going on in the body to cause most of them. Firstly, the activity of the nervous system changes, affecting the way the gut functions. Secondly, during exercise more blood flows to the skin and working muscles. This reduces blood flow to the gut, which can also cause problems.”

There are two components to the body’s autonomic (or unconscious) nervous system, the Sympathetic and Parasympathetic Nervous Systems. The Sympathetic Nervous System is activated in response to various forms of stress and prepares the body for a “fight or flight” response. One action of the Sympathetic Nervous System is to reduce gut function – this includes movement of food and fluids through the stomach and intestines, digestion of food, and absorption of nutrients from the gut. Exercise itself activates this system to some degree, but increasing exercise intensities (racing compared to training), increasing exercise duration (especially over 4-5 hours), dehydration or overhydration, and a predisposition or established gut disease or disorder, results in an increased response and greater reduction in gut function.

The consequences of reduced gut function can follow. When food and fluids don’t leave the stomach fast enough, athletes can experience belching, stomach bloating, nausea, reflux or vomiting. Incomplete digestion or absorption of nutrients can cause the undigested material to enter the large intestine. Water enters the gut to dilute the undigested food, and bacteria in the gut feast on it, producing gas as a side-effect. The result – lower abdominal bloating, pain, excessive flatulence, urge to defecate or diarrhoea. And recently it’s been realised that nutrient malabsorption can cause nausea, vomiting and stomach bloating, through a phenomenon known as the ileal brake mechanism (the presence of the malabsorbed nutrients in the end of the small intestine prompt feedback signals to the stomach to slow or stop emptying).

Lack of blood flow is the other main cause of gut problems during exercise. Reduced blood supply restricts oxygen and nutrients to the muscles lining the gut, and the cells responsible for absorbing nutrients from food. As a result, the cells of the gut lining can become damaged, and the junctions between them start to break down. As well as malabsorption, the lining of the gut can also become inflamed or bleed. This can result in blood in an athlete’s stools during or after a race.

The reduction in blood flow to the gut is made worse when there is either a greater demand for blood supply to the muscles (higher exercise intensities like a race), the skin (hot, humid conditions) or both (caused by reduced blood volume from dehydration). The longer blood flow to the gut is reduced, the worse the damage becomes.  “This is most likely going happen in long-course racing,” says Costa, “because it usually takes at least two to three hours of continuous exercise to start to see significant gut damage occurring.”      

When you’ve just gotta go…

Perhaps the most frustrating problem for athletes is the sudden need to find a toilet. Urge to defecate ruins races for many athletes, who try all sorts of things, even complete avoidance of food and fluids for several hours before and during exercise, to ensure there’s nothing in their gut that needs to be emptied. As Dr Costa explains, there is a specific phenomenon in the body that can cause this urge during exercise: “The gastric-colonic reflex is a phenomenon that occurs whenever we eat. As food enters the stomach, it signals to the end of the large intestine to move its contents through, prompting you to go to the toilet. This seems to be worse with solid foods compared to fluids or gels. It’s also worse if you’re malabsorbing nutrients, because of the excess water, gas and undigested material in the large intestine.”

The damage you can’t see or feel

The cells lining the gut and the junctions between them play a crucial role in preventing anything unwanted from entering the bloodstream. This includes gut bacteria, either present in food or those living along the gut, and the substances they produce known as endotoxins. If the gut lining becomes damaged, gaps open up allowing endotoxins to enter the blood (endotoxaemia). “It’s literally a form of blood poisoning,” says Costa, “although in most cases the athlete’s liver and immune system do a good job of responding and we’re never aware it’s happened. But there are examples of endurance athletes requiring medical attention due to this systemic infection.”

In addition, it is thought that repetitive and severe damage to the gut lining during exercise may increase the risk of athletes developing ongoing gut problems later in life. These range from fairly mild Irritable Bowel Syndrome symptoms, through to significant inflammatory diseases of the gut.

Preventing gut problems during exercise

Preventing gut problems can be difficult because there are so many contributing factors and the relationship between them is so complex. “It’s not as simple as saying ‘you’ve got diarrhoea so this is the cause, and you’ve got nausea, so it’s that,’” says Costa, “the same factors during exercise can cause different symptoms in different people, and each symptom has multiple potential causes. Only by assessing all the potential reasons can you really drill down and figure out what’s going on.”

On the back of their research, Monash has now set up a first-of-its-kind clinic, where athletes come to diagnose their specific issue. “Athletes have been really excited,” says Costa. “Many of them have spent hundreds of hours of trial-and-error trying to understand their problem. After a single three-hour lab visit we have a very good indication of what’s causing their issue and can provide suggestions on how to address it more systematically.”

If you don’t have access to such facilities, there are some simple strategies that may assist. Ensuring you’re neither dehydrated or overhydrated is a good place to start. Some athletes with large sweat losses struggle to drink enough to prevent significant dehydration. Costa recommends “training the gut” – deliberately drinking substantial quantities of fluid (more than you would consume in a race) during shorter, higher intensity training sessions. “It will be a struggle at first, but your stomach will slowly get used to it, and you’ll start to cope better with drinking more during exercise. It’s not about deliberately overhydrating because that’s a problem too, but some athletes cannot physically tolerate the amount of fluid required to maintain normal hydration status.”

Some people have an impaired ability to absorb specific highly fermentable carbohydrates known as FODMAPs. This is often worse during exercise, so if you are FODMAP sensitive at rest, then avoiding them before and during exercise will probably help. Some athletes who aren’t FODMAP sensitive can become so during exercise, so avoiding these may still be beneficial (an Accredited Sports Dietitian can help with this). This is another area where gut training can benefit. Fructose is the main FODMAP in sports nutrition products (mainly sports drinks and gels), and Costa’s team have shown that just 10 days of running (one hour a day) while consuming large amounts of glucose and fructose can significantly improve tolerance.

Metabolic Cart: The same equipment used for VO2max testing also measures carbohydrate and fat oxidation rates, or the amount of each fuel source being used. This allows the team at Monash to determine how much of the consumed carbohydrate is actually being used to fuel the body at race pace.”

Many endurance athletes have shifted towards low carbohydrate, high-fat diets in recent times. One of the motivators is a large reduction (in some cases complete elimination) in the need to consume food during a race, meaning many gut issues become irrelevant. However, new research at Monash suggests that going without food during long training sessions or a race may not be wise. Rhiannon Snipe, an Accredited Sports Dietitian who is completing a PhD under the supervision of Dr Costa, studied factors that either increase or reduce the risk of gut damage during exercise. “We found that feeding even small amounts of carbohydrate or protein frequently during exercise almost completely prevented gut damage. However, the protein was poorly tolerated by participants, whereas the carbohydrate was fine,” says Snipe. “Even if they don’t feel symptoms, there is a risk of intestinal injury, local inflammation, and endotoxaemia in the short term or significant gut problems long term if athletes train or race for long durations with no carbs at all. Our recommendation is to consume at least some carbs during all sessions over two hours, but this can be as little as 15g every twenty minutes.”

The Monash team also reviewed and investigated the effect of dietary supplements (such as antioxidants, glutamine, L-arginine, L-citrulline, and bovine colostrum) and probiotics on gut disturbances and symptoms during exercise. “We haven’t seen any consistent and substantial benefit from these products, so their use prior to or during exercise is not justified. We actually found a negative effect with probiotic supplementation – greater endotoxaemia and systemic inflammation – with lactobacillus supplementation for seven days before two hours of steady state running in the heat,” says Costa. Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) like ibuprofen and aspirin are also known gut irritants and cause damage to the gut lining, and Costa recommends avoiding these before and during exercise wherever possible.

Finally, it should be noted that there are some athletes for whom there is no good solution. “We’ve had some athletes where the intensity or duration required to be competitive causes them problems, and the strategies we use to prevent them aren’t sufficient,” says Costa. “Some people have an unusually large Sympathetic Nervous System response to higher exercise intensities or long durations, and there’s nothing they can do about it, except lowering their exercise intensity.”


Exercise-induced Gastrointestinal Syndrome appears to be caused by two main processes – increased Sympathetic Nervous System activity and restricted blood flow to the gut. These cause either a reduction in gut function or physical damage to its lining. This can result in both the typical symptoms athletes experience during exercise, and potential local and systemic infections or long-term gut-problems. Higher intensity and longer duration exercise, especially in hot weather, dehydration, over-hydration and consuming foods and fluids above tolerance levels tend to increase the risk of problems occurring. Maintaining optimal hydration status, training the gut with at least as much (if not slightly more) food and fluid than is required on race day, and consuming at least small, frequent amounts of during races may assist athletes to prevent the debilitating gut disturbances, with or without symptoms, they experience.

More info

For more information on the Nutrition and Exercise Dietetic Research Team at Monash University, visit:

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Alan McCubbin

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