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The perfect race is unfolding. You’re right on pace through mile 13 and feeling strong. Suddenly, there’s a funny sensation in your stomach. The line of port-a-potties along the course is calling out your name, but is the pit stop worth losing out on valuable time?

If this situation sounds familiar, you aren’t alone. Runners of all types suffer from what is known as “runner’s diarrhea.” This unfortunate condition can rear its ugly head anytime, especially when you least expect it.

You Know it When You See it

Some things don’t need explanation. The “runner's trots” is a great example of something most runners can sympathize with, though it has no single definition. Several things differentiate runner’s diarrhea from the regular form that make it particularly annoying.

Runner’s diarrhea has been observed in endurance athletes for decades.

In contrast to normal diarrhea, runner’s diarrhea is exercise-induced, meaning it occurs in response to physical activity.

It’s characterized by frequent loose bowel movements that can occur during or immediately after a run–sometimes even in anticipation of a run.

Other than being uncomfortable and inconvenient, normal runner’s diarrhea isn’t pathological. Regular forms of diarrhea may indicate larger problems such as irritable bowel disease (IBD) or other GI disorders. Also unlike normal diarrhea, runner’s diarrhea doesn’t usually cause dehydration or electrolyte imbalance.DeOliveira2017 The only pathological symptom may be the appearance of blood in the feces. Progressing to this level might warrant a trip to your doctor.

Experiencing runner’s diarrhea can be unpleasant, embarrassing, and detrimental to training and competition.

Why are Runners More Susceptible?

Runner’s diarrhea is more common in distance runners than any other athletic population. In exhaustive endurance events, it’s estimated that 30% - 50% of runners may experience one or more GI-related symptoms.DeOliveira2014

Need more proof you’re not alone? 83% of marathon runners interviewed after finishing a marathon indicated they experienced GI symptoms sometime during or after running.Riddoch1988 Most commonly, the “urge to have a bowel movement” and diarrhea were reported by 53% and 38% of the respondents. These runners also reported encountering issues more during hard runs than easy runs.

Long-haulers might be more at risk. 93% of triathletes experienced GI symptoms during a race, and 7% of them dropped out due to symptom severity.Jeukendrup2000 GI distress seems to be particularly present in ultra-endurance events, with 43% of all finishers in a 67km run reporting GI distress; 15% reported diarrhea.Rehrer1992

Runners have almost twice the risk of suffering GI problems than participants in cycling, swimming, and other athletes.DeOliveira2014 Have runners been hexed? Not really. The higher prevalence of GI issues in runners can be easily explained.

The act of running leads to considerable mechanical trauma and jostling.

Compared to other sports, long distance runners also need to keep a steady intake of food and drink to maintain performance during competition. Both of these factors prime an upset stomach.

The Intestines and Exercise

The “rest and digest” state is where we occupy the most time. It is controlled by part of our nervous system called the parasympathetic nervous system (PNS). Dialing up the rest and digest state prioritizes blood flow to our digestive system. This allows us to properly process food that we have consumed, store and conserve energy to meet future physical stressors.

But what happens during the stress of physical activity? It’s the opposite of rest and digest. While we aren’t exactly running from a pack of wolves (hopefully), exercise mimics the “fight or flight” response, increasing sympathetic nervous system (SNS) activity to prepare our body for battle. The idea is to shift resource allocation between the systems of the body to prioritize the systems that will help us to survive the threat and shut off those that won’t.

During exercise, blood flow is shuttled rapidly to areas where we need it–working muscles, heart, liver, brain. This delivers vital oxygen and nutrients to these systems, allowing them to deal with the huge increase in output that occurs during exercise. The process is controlled by the SNS, which constricts blood vessels leading to digestive organs. The consequence? Intestinal regions receive very little blood flow and oxygen.

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During maximal exercise, GI blood flow can be reduced up to 80%.Costa2017 Forget digesting food, that can wait–the body screams at you, “run for your life!” Reduced blood flow to the GI tract during exercise leads to ischemia (lack of blood flow and oxygen), which can result in intestinal damage and function impairment, especially if it goes on a while. Dehydration may further reduce blood flow to your poor bowel. We lose blood volume during exercise through sweating, leading to thicker blood that’s even less likely to travel anywhere unnecessary.

This is an issue because gut function is still somewhat important to an athlete during exercise. It delivers water and nutrients to vital tissues, to help you push on while exercise saps water and fuel from your body. Actually, the intestines can accommodate mild-to-moderate intensity exercise without too much of a change in their function. Highly compromised digestion and absorption, and the ensuing runner’s diarrhea, may manifest only during severe exercise. Exercise at 70% VO2 max and above leads to the greatest overall disturbance in several GI functions:DeOliveira2014 the danger zone for runner’s diarrhea.

What Causes “The Trots?”

Exercise changes the physiology of the GI tract, which can contribute to runner’s diarrhea and other symptoms. It’s necessary to know what might be causing runner’s diarrhea before looking for remedies.

Most gastrointestinal symptoms (GI) are intensity-dependent–the harder you run, the more severe they get. This might be why runner’s diarrhea is prevalent during hard workouts and races.

The factor usually blamed for runner’s diarrhea is reduced intestinal blood flow. As we discussed earlier, at 70% VO2 max, delivery of blood to the GI tract is reduced up to 80%, leading to hypoxia (lack of oxygen) and ATP depletion.DeOliveira2014 The digestive tissues get starved of energy. Without adequate oxygen and blood flow, layers of the intestine begin breaking down, protein complexes known as “tight junctions” crumble, and intestinal permeability increases. Intestinal lining is a protection from various molecules and toxins.

“Leaky gut” allows bacterial endotoxin and other inflammatory factors to enter the intestinal lining. Heavy exercise exacerbates this. Runners who ran at 80% of their VO2 max displayed increased gut permeability compared to running at only 40% and 60% max capacity.Pals1997 An inflamed gut is primed for intestinal disaster. When blood flow resumes (after exercise stops), the "re-perfusion" process can even lead to further damage, like oxidative stress and inflammation, which probably contributes to the “post run runs.”

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Mechanical Factors

Running hard on the joints– and the intestines.

The repetitive vertical oscillation and vibration during running causes your internal organs to jostle around your abdomen like kids in a bounce house.

The harder you run, the more they bounce. This trauma can contribute to flatulence, an urgent need to use the bathroom, and diarrhea. It may also be one reason bloody stool is often reported in runners experiencing diarrhea.

When the foot contacts the ground while running, this generates massive braking forces that are transmitted throughout the body. These forces, for lack of a better term, emulsify bowel contents, loosening the stool. Braking forces when landing during running are twice those experienced by cyclists, likely another reason runners are more affected.

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Nutritional Causes

What you eat has an impact on both performance and your chances of GI issues during exercise. Nutrition before and during exercise can contribute to runner’s diarrhea, and certain dietary hacks may reduce symptoms. Since the digestive process lasts 24 hours - 72 hours, pre-run and race-day nutrition are equally important.

The carbohydrate composition and concentration of pre-and-mid-race beverages is known to play a major role in GI issues. Beverages with high carbohydrate concentration (above 6% - 8%) are known to delay gastric emptying, lead to fluid shifts in the intestine, and result in runner’s diarrhea.RehrerTwo1992

Many athletes think more is better when it comes to hydration and nutrition. Our body can only absorb 1g - 1.2g per minute of carbohydrate during exercise, so anything ingested above this level will lead to residual un-absorbed carbohydrates in the intestine. Combining a hypertonic beverage with exercise in the heat can exacerbate the intestinal damage because of dehydration.

A group of poorly absorbed carbohydrates known as FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols) may lead to excess intestinal fluid and fermentation, leading to gas and irritable bowel syndrome (IBS) symptoms. While few studies have evaluated the effect of FODMAPs on GI symptoms during exercise, restricting these foods around exercise may be precautionary to reduce diarrhea and other GI issues. In a study where athletes eliminated at least one FODMAP group from their diet (mainly lactose), 82.6% reported improvement in at least one GI symptom.Lis2016 As with all nutritional factors, tolerance may be highly individual.

Any foods that delay gastric emptying–fiber, fats, and protein–are candidates to induce GI symptoms. Other than sitting in the gut and leading to discomfort, these foods may also draw water into the intestinal lumen and lead to GI issues and diarrhea. A survey of a group of triathletes who experienced GI symptoms and cramps during a race found that all of them had consumed fat, fiber or protein in their pre-race meal.RehrerTwo1992 Best avoid the Raisin Bran, folks.

This image is of a large cheeseburger. The title is "nutritional factors". Under the image it says that foods high in protein and fat or poorly absorbed carbohydrates can delay gastric emptying, leading to gut discomfort and cramps.

Psychological Factors

Pre-race jitters? These may play a small but impactful role in runner’s diarrhea. In a survey of runners, GI symptoms were shown to be associated with having higher levels of stress and anxiety.Wilson2008

There may be an explanation for this. Psychological stress is known to induce small intestine permeability in humans,Vanuytsel2014 possibly related to the release of stress-response hormones. Stress may not cause ulcers, but it may make you poop.

Prevention Strategies

Plan A. Bring toilet paper. Plan B? Luckily, there are several steps to mitigate or completely prevent runner’s diarrhea by manipulating diet and training

Find the Right Beverage

Few athletes recognize the limit of how many carbohydrates the body can oxidize (utilize) during exercise, leading many to over-consume carbohydrate beverages and energy gels. The body’s inability to absorb all carbohydrates and fluid results in GI distress.

There are solutions to this. The right concoction of carbohydrates in your pre-or-mid-race beverage can prevent developing GI symptoms. The proper mix of multiple transportable carbohydrates reduces the presence of carbohydrates and fluid in the intestine during exercise. Glucose + fructose or maltodextrin + fructose are common blends found in hydration beverages due to their high absorptive capacity.

Ingesting a beverage of glucose + fructose or maltodextrin + fructose in a concentration of 6% - 8% allows for optimal absorption.Jeukendrup2017 Many beverages also contain sodium, since the uptake of glucose through GLUT5 carbohydrate transporters is sodium-dependent–they occur together.

Just Say No

Avoiding certain foods or food groups may help alleviate symptoms of GI distress and runner’s diarrhea.

Stay away from high-fiber foods, or those high in fat or protein, and certain artificial sweeteners for up to three hours before exercise.

In addition, a “low-residue” diet in the day(s) leading up to a big event will result in less intestinal content. Restricting consumption of FODMAP-containing foods before a race may alleviate symptoms, but this depends on tolerance and preference. In one preliminary study, runners reduced GI symptoms by following a low FODMAP diet for six days.Lis2018

Hydrate (With Good Ol’ H20)

Maintaining adequate hydration before and during exercise is crucial to prevent runner’s diarrhea.

Dehydration exacerbates symptoms of GI distress. Losing just 2.7% of body mass due to dehydration impairs water flux, increases GI symptoms, and even promotes an intestinal inflammatory response during exercise.Ryan1998

Avoid over hydrating. Hyponatremia (low sodium concentration in the blood) also is associated with GI symptoms.

This image is of a purple-colored neuron like you would see in the brain. The title of the image is "psychological factors" and under the title it says "the brain plays a role too." Under the image it says that high levels of stress and anxiety are associated with GI symptoms, which might be due to increased gut permeability and stress hormones.

Training the Gut

There is evidence the gut is capable of adapting to increased demands of exercise, just like muscles and the cardiovascular system. Many GI symptoms, such as runner’s diarrhea, may simply be a lack of “intestinal fortitude.” A gut not trained to handle both exercise and fluid intake won’t be happy when you ask too much of it.

Practice Makes Perfect

Gut training can be accomplished by regularly taking in fluids and carbohydrate-based drinks during exercise.

One key benefit here is acclimating to the feeling of fullness during exercise. Improved liquid tolerance will reduce many symptoms of GI distress during exercise and help performance. It only takes about three days of gut training for the stomach to get used to larger volumes.Jeukendrup2017

Residual fluid and carbohydrate volume in the stomach are the main cause of runner’s diarrhea. With training, the stomach can improve how quickly it can empty fluid, and the intestines can improve how well they absorb carbohydrate and water. Improved absorption means less sloshy water in the gut during exercise, and less tummy trouble.Jeukendrup2017

How do you train the gut? Practice drinking during exercise.

Runners who consumed a glucose and fructose-rich carbohydrate beverage during runs for two weeks improved GI symptoms during a three-hour run challenge.CostaTwo2017 This challenge involved stressing the gut by consuming 30g of carbohydrates every 20 minutes. The training gave the runners iron stomachs. Fewer GI issues, more glucose absorption and two weeks of training combined to improve performance by just over 5%.

Can improving fitness decrease the severity of runner’s diarrhea? Trained athletes have greater intestinal blood flow and better gut barrier function during exercise compared to untrained subjects.Bandyopadhyay2011 This suggests that as you improve your running fitness, “gut fitness” also improves. This may be why runner’s diarrhea occurs early in your training, during races, or when when you increase running volume or intensity. As the going gets tough, all of your resources (blood) must go to the muscles, sparing little for your intestines. Once you can better distribute blood flow, GI issues may ease up a bit.

One or two days per week, crush a tempo run or hard workout after consuming a simulated pre-race meal (with the right amount of time beforehand) and hydrate with your competition beverage.

The Importance of Training Diet

Research shows that your regular diet will influence how you respond to nutrition before and during exercise.

Targeted carbohydrate supplementation during training for athletes following a normal mixed diet has been shown to be helpful. Carbohydrates were provided before, during, and after training and this improved glucose absorption during exercise and lowered GI symptoms.CoxGA2010

On the other hand, restricting carbohydrates (ketogenic or low-carb diets) results in a lower capacity to absorb and empty carbohydrates during exercise. This is most likely due to a lower number and activity of carbohydrate transporters in athletes on these diets. They’re simply not needed as much. If a keto athlete tries carb-loading on race day, this could spell disaster. Some keto athletes use fat as fuel on race day; as they burn more of their own fat, they need to eat less to sustain race pace and therefore reduce possible GI issues.

If you are following a low-carbohydrate or ketogenic diet, periodized carbohydrate intake can improve your tolerance for certain carbohydrate-containing beverages and foods. This strategy will involve eating your normal diet but supplementing with carbohydrates for certain training sessions.

The alternative? Many ketogenic athletes forgo carbohydrates during competition, instead opting to fuel with fats like MCTs or exogenous ketones such as beta-hydroxybutyrate (BHB). If you’re going this route, make sure to practice with MCT or ketone ester ingestion before and during your training runs to get adapted.

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Practical Recommendations

When it comes to athletes and nutrition, there isn’t a one-size-fits-all approach. This is especially true when it comes to preventing issues like runner’s diarrhea. Find your tolerance. There are, however, plenty of evidence-based recommendations you can try out to prevent the dreaded runner’s diarrhea.

Prevent the mid race port-a-potty detour by keeping your stomach happy. Hydrate properly, and avoid upsetting foods close to exercise–this means those high in fiber, protein, and fat. Keep nutrition simple.

Don’t try any new food on race day. This is not a time to experiment with a new meal or beverage. Training is the time to test out different foods, gels, and drinks to find what works best for you.

For many runners, urinating and having a bowel movement prior to exercise can prevent a mid-run disaster. It may help to ingest a mild laxative the night before to move things along and ensure you're starting exercise with a clear colon.

And if nature calls? First of all, know where rest stops are located on your run or race. If you want to avoid the woods (or your neighbor’s backyard), having a backup plan is smart. Many runners opt to carry toilet paper on their runs. If you do have to duck into the bushes, TP is a better option than using leaves (or poison ivy).

Persistent, nagging, and inconvenient, runner’s diarrhea can ruin a race. Fortunately, nutritional strategies and gut training can help runners to avoid many symptoms of runner’s diarrhea and even prevent it from happening.

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