Become a Member

Get access to more than 30 brands, premium video, exclusive content, events, mapping, and more.

Already have an account? Sign In

Become a Member

Get access to more than 30 brands, premium video, exclusive content, events, mapping, and more.

Already have an account? Sign In



What Your Poop Might be Telling You

How to get to the root of GI problems while running and racing.

Heading out the door? Read this article on the new Outside+ app available now on iOS devices for members! Download the app.

Porta Potty pit stops are a necessary part of running. But only to a point. 

No matter what you are training for, having to take multiple bathroom breaks in the middle of a run isn’t just annoying, it can be indicative of a deeper issue. 

Whether it’s stomach cramping, gas, bloating, or diarrhea, having stomach issues is one of an endurance athlete’s worst nightmares. If it continues to happen, it can lead an athlete down a dizzying maze of frustration. 

Occasionally, navigating gastrointestinal problems during training or racing could be caused by an acute issue that can be remedied quickly. 

  • Meal and Snack Composition and Timing: The composition and timing of your pre-exercise meal or snack is one of the first places to look if wanting to examine causes of issues.Having too much fiber or fat, which are harder to break down and digest, can cause cramping, gas, bloating, and urgency. To stay on the safe side, limit fiber to <5 g pre exercise and less than 10g of fat. Timing of your meal or snack should allow for ample digestion, which can also vary depending on the size and composition of your meal or snack. A good rule of thumb is to give yourself at least one to three hours to digest your pre-workout fuel choices.  And don’t forget the hydration to top off your fluids (8-10 oz should be enough to get you started off on the right foot).
  • Fueling choices during training and racing: Figuring out a fueling plan that works for you can be an entire puzzle in itself (see our previous reporting here). Ensuring that you are taking in enough, not too much, and the right kinds of calories, fluids, and electrolytes every hour is a good starting point. From there, evaluate calorie sources to ensure that you aren’t getting too much fat or protein. Carbohydrate sources should also be varied, to ensure that you aren’t getting too much fructose or glucose that would overwhelm your digestive system
  • Too much caffeine: Caffeine is a gastrointestinal stimulant, meaning that it increases gut motility and can cause diarrhea, gas, and bloating. Everyone has an individual tolerance to caffeine intake, but the consensus for sports performance is 3 to 6 mg/kg of body weight pre-exercise (An example: for a 150-pound runner, this would be 205-409 mg, which is equivalent to about two-four cups of drip coffee). More than 6 mg/kg body weight may not offer additional benefits over the 3 mg/kg body weight.

RELATED: Should You Cut Caffeine Before A Race?

Next-Level Considerations

If you are continuing to have stomach issues and have exhausted other acute possibility options, it might be time to think outside of the box. 

  1. Look at your microbiome: Your gut microbiome is a complex system of tiny living organisms that impact your ability to have regular bowel movements, digest your food properly, absorb nutrients, influence inflammation, and orchestrate an immune response. 

Each individual has their own microbiome composition, which is impacted by genetics, environment, diet, and sport. This microbiome can change, and while there is still a lot to learn about it, we do know that if there are too few or too many bacteria in the gut, this can be problematic. Without enough bacteria, you don’t have enough of the workers to do the work and digest your food. With too many, you end up with excess production of gas, bloating, and gut pain. In certain cases of overgrowth, an athlete may have a particular condition called small intestinal bacterial overgrowth, or SIBO, which is defined as the presence of excessive number of bacteria in the small intestine, and this can be caused by motility issues, antibiotic overuse, stomach acid production, and structural issues of the GI system itself (1).

  • Possible Infections (Bacterial or Parasitic): While rare, bacterial and parasitic infections can be present in endurance athletes. Giardia, a parasite which can come from contaminated water or food sources, can show up with watery diarrhea, cramping, and bowel movement urgency, while clostridium difficile (C.Diff) can come from the disruption of the normal microbiota due to use of antibiotics, and exposure to the bacteria and causes watery diarrhea and stomach pain. These are just a few out of the many possibilities of infections. 
  • Intestinal Lining Hyperpermeability: Our intestines are designed to be semi-permeable to be able to absorb fluids and nutrients. When functioning properly, our intestinal cells are lined up tight together (otherwise known as tight junctions). Excessive amounts of jostling (like the exercise), inflammation, food sensitivities, antibiotics and dysbiosis of the microbiome can all cause these tight junctions to become loose or hyperpermeable, disrupting the mucosal lining and gatekeeping capabilities of the cells. This can lead to chronic stomach pain, diarrhea, inflammation and enteric nervous system disruption (2).
  • Gluten/Other Food Allergy Issues: When looking at possible food allergy issues, we have to be careful not to start removing anything and everything from our diet or try to do it right before an upcoming race. For athletes that suspect they have celiac disease or lactose intolerance, there are clear testing protocols. For figuring out other possible food reactions, an elimination diet is the gold standard, not those IgG food sensitivity tests you order online.   
  • Under-Fueling: Under-fueling is also a major contributor to gut issues. Chronic under-fueling signals to the body that it doesn’t have enough resources to run body functions properly. This can manifest in the digestive system as a slowed, dysfunctional GI system with symptoms ranging from constipation, bloating, excessive gas, and diarrhea. It is always a good idea to address overall energy intake for your level of activity to ensure you are getting in enough. 
  • Chronic Dehydration: An acute instance of dehydration might land you with stomach issues during your training or racing session, but chronic dehydration on a daily basis should not be overlooked as a cause of frequent issues.   Lack of fluids in the digestive system can lead to problems with acid reflux, constipation, and stomach pains.  A good general rule of thumb for daily hydration is to aim for half your body weight in ounces as a baseline and then add 16-20 ounces for every hour of exercise you do.

What to do if you suspect you could have issues?

This article is not meant to discourage you from seeing your doctor or a gastroenterologist. However, athletes often walk away with a diagnosis of irritable bowel syndrome and a low FODMAP diet handout, which can be frustrating if you feel like there is more going on. Plus, going on a restrictive diet while training for endurance events can be a slippery slope to under-fueling. If you do suspect that something is off, and feel like you aren’t getting the answers you need, there are other options. PCR stool testing can be offered by some dietitians or functional medicine professionals, and they can give some insight into some of the issues that were mentioned above. While there is a lot that we don’t know about the microbiome, there is a lot that we can glean from a stool test that can help develop a better plan of attack.

RELATED: Beating Gut Troubles in the Heat

What happens once you get PCR stool testing done?

Once you get a PCR stool test done, it will come back to you with pages of data to sift through. This is where a professional can be helpful to help you interpret the results and lay out a plan. Sometimes, this will involve you taking the test to the gastroenterologist to help you get antibiotic treatment, while others it will involve a targeted nutrition strategy along with a temporary supplement protocol. 

No matter what approach you decide to take, don’t settle for stomach issues as your new norm. Take matters into your own hands and get to the root of your problems for good. 

  1. Southward K, Rutherfurd-Markwick KJ, Ali A. The Effect of Acute Caffeine Ingestion on Endurance Performance: A Systematic Review and Meta-Analysis. Sports Med. 2018 Aug;48(8):1913-1928. doi: 10.1007/s40279-018-0939-8.
  1. Zhou Q and Verne GN. J Clin Invest. 2018;128(11):4764-4766.


Kylee’s mission is to separate facts from fads in the endurance nutrition space and works to provide easy nutrition solutions to help improve health and performance.  Kylee is the founder and owner of her sports nutrition business Flynutrition, which helps runners, triathletes, cyclists, and skiers to learn not only the ‘why’ but the ‘how’ behind fueling for performance. Her work has appeared in Trail Runner and Women’s Running Magazines.



Into the Route: Trail Running in the Alps

A quest to create a multi-stage tour designed specifically for runners