Five Reasons You Might be Experiencing "Runner's Stomach"

By Jen Rawson, RD

Ask any runner and they’ve probably got a tale to tell about an upset stomach or an unfortunate porta-potty stop that wrecked their race time. It’s estimated that 30-50% of athletes experience gastrointestinal (GI) issues during activity (1). Runner’s stomach occurs due to one of three reason - mechanical (the jarring up and down movement while running), physiological (reduced blood flow to the digestive tract and increased stress while running) or nutritional (the type of food you eat impacts digestive symptoms). While we can’t change the mechanical or physiological causes of GI distress, we can adjust nutrition.


Timing is everything when it comes to running. Eat too early and you feel hungry or lethargic on the run but eat too late and you have food still digesting which feels like a rock rolling around in your stomach.

Individuals vary widely on their digestive tract speed and sensitivity. Guidelines suggest eating 1-4 hours pre-run. A good place to start is giving yourself 1-hour post snacks and 2-hours post meals before running and then adjust per your own tolerance. Trial and error is the only way to figure out exactly what works best for your body.

High Fat or High Fibre

To limit runner’s stomach, you want to limit the amount of food being digested during the run. Nutrients that slow digestion, like fat and fibre, in your pre-run meal or snack can lead to increased cramping, gas, or the dreaded runner’s trots. For those with a slow digestive system, even eating high fat or fibre foods the day before a race can have an impact.

Examples of high fibre foods include legumes, bran muffins and cereals, berries, and nuts. High-fat foods include deep-fried foods, burgers, bacon, cheese, avocado and nuts/seeds.


Caffeine speeds up gastrointestinal motility. Some runners make use of the laxative-like-effect to get their bowels moving before a run whereas others find the effect unpredictable leading to a bathroom stop mid-run. Because individual tolerance to caffeine varies, it’s recommended to trial any use of caffeine or caffeine-containing products during training and not on race day. 

Beyond the morning cup of coffee, caffeine can also be found in a lot of runners’ supplements such as gels, gummies, etc. Caffeine is often an added ingredient because it “reduces the perception of fatigue and allow exercise to be sustained at optimal intensity for longer” (2). For those whose guts are sensitive to caffeine, it’s important to check any drinks, gels, or gummies that you consume during a run for added caffeine. 

Sports Supplements

Sports supplements are often necessary for endurance runners but they can also be the cause of stomach problems. The goal with sports supplements is to give the body rapidly absorbable carbohydrate to fuel the muscles.

Sport gels and gummies are a convenient highly concentrated source of sugar, however, they must be diluted to digest properly and not cause stomach upset. Many runners make the mistake of taking supplements without water or taking it with a sports drink such as Gatorade, which has added sugar. Highly concentrated sugar in the gut will create an osmotic pull of water into the bowel leading to cramping, pain, and a need to find a port-a-potty quickly.

There are also different types of sugars used in sports supplements including sucrose, fructose, dextrose, etc. Fructose (also found in honey) is a fermentable carbohydrate that can cause gas, pain, and diarrhea for people who are less tolerant of it. If you are experiencing these symptoms it might be worthwhile to check the sugar source of your supplement and experiment with a different type.


Often times it’s not food that causes GI distress but rather a lack of water. Dehydration leads to decreased blood flow to the digestive tract which can slow down the digestion of your pre-run meal and your sports supplements. Adequate hydration before, during, and after running is imperative to keep your stomach feeling good.

Stomach problems in runners are common but the causes can vary widely between individuals. Use your training to experiment with different foods, sports supplements, hydration and timing. Journalling what and when you ate and drank with how you feel is a good way to see intake patterns and create your best fueling strategy. And of course, always follow the golden rule - no new things on race day.


  1. Oliverira, E, Burini, R. & Jeukendrup, A. (2014). Gastrointestinal Complaints During Exercise: Prevalence, Etiology, and Nutritional Recommendations. Sports Med, 44(1); 79-85.

  2.  Thomas, D., Erdman, D. &, Burke, L. (2016). Nutrition and Athletic Performance Position of Dietitians of Canada, the Academy of Nutrition and Dietetics and the American College of Sports Medicine. J Acad Nutr Diet, 116(3); 501-528