Gut Training for Endurance Athletes: How to Fuel Without GI Distress
It is well known that endurance athletes need adequate carbohydrate (carb) intake to maintain muscle glycogen stores, to maintain high energy levels, and optimize performance. However, consuming the recommended 60 to 90 grams of carb per hour during endurance training or racing can be challenging and may result in gastrointestinal (GI) distress. In fact, GI symptoms are common among endurance athletes, which as many as 70% percent of athletes may experience GI issues during events, such as cycling, swimming, triathlons, running, mountain biking, and climbing. These symptoms can significantly impact an athlete’s training, racing and to be able to perform their best.
The exact mechanisms causing GI discomfort during exercise is not fully understood; but as intensity increases during exercise, blood is shunted away from the gut reducing blood GI flow, increasing intestinal permeability, known as “leaky gut”. This allows bacteria and toxins to enter the bloodstream and trigger inflammation, resulting in abdominal cramping, heartburn, gas, nausea, bloating, or diarrhea.
Other factors that increase the risk of GI discomfort include the following:
Dehydration impairs gastric emptying increasing risk of nausea, cramps and diarrhea during exercise.
Food intake before exercise:
Consuming high-fat, or high-fiber meals before exercise can slow digestion and delay gastric emptying.
Eating a large meal within 30 minutes before exercise may increase the risk of GI discomfort because there is not enough time for adequate digestion.
Fitness level:
Novice athletes may be more susceptible to GI distress since they have less time to adapt to the demands of prolonged exercise and repeated GI stress.
Underlying GI conditions such as gastroesophageal reflux disease (GERD), peptic ulcers, GI cancer, IBS, IBD, constipation, and diverticular disease.
Excessive use of protein powders and carbohydrate supplements (gels, chews)
Large amounts of protein powders, carbohydrate gels, chews and other concentrated fueling products may contribute to GI symptoms.
Consuming carbohydrate-containing fluids with a high carb concentration can increase osmolality of the fluid, potentially slowing gastric emptying and contributing to GI distress.
Consuming more than 60 grams of carb per hour, relying on a single source of carb from glucose or fructose, may increase the risk of GI symptoms. Combining different carb sources may reduce these symptoms.
Age and sex:
GI distress may be more common in female athletes, especially during hormonal fluctuation during menstrual cycles, peri-menopause and during menopause.
Younger athletes may also experience changes with GI symptoms during puberty as they undergo hormonal and physiological changes.
Nutrition Strategies to Avoiding GI Distress
Training your gut means consistent, purposeful, intake of carbs throughout endurance training over two weeks can lower GI symptoms. Like your muscles adapt to your training, your gut can adapt to the food and fluids during exercise to improve nutrient delivery.
What type and amount of carbs should you eat during exercise?
The amount and type of carbohydrate matters when training your gut. If you are new to endurance training, consuming 20 to 30 grams of carbohydrate per hour, typically does not lead to GI discomfort; but when training or racing over several hours, carb intake should increase to 60 to 90 grams of carb to prevent muscle glycogen breakdown.
Your body absorbs carbohydrates in the small intestine. It uses two transporters, SGLT1 to transport glucose and maltodextrin, and GLUT5 to transport fructose. You may experience GI discomfort when consuming more than 60 grams of carb from a single source of glucose/maltodextrin; therefore, to lessen GI issues, adding additional 30 grams of carb from fructose, plus an approximately 40 to 60 g of carb from glucose/maltodextrin, can improve gastric emptying and tolerance of carbs.
The ratio of 2:1 of glucose to fructose carb gel, food or chews, matters when increasing your overall carb intake during endurance training. Recruiting both transporters is key to reaching carb goals during exercise; therefore, it is important to test out different carb gels/chews during endurance training to know what will work on race day.
How to train your gut?
If you are new endurance athlete, it may take up to 8 to 10 weeks to be able to go from 20 to 30 grams of carb per hour, up to 60 to 90 grams of carb per hour. The loading needs to progressive and intentional. Depending on your GI symptoms, each week, add an additional 5 to 10 grams of carb every two weeks during endurance training or during high intensity workouts over an hour. If no GI symptoms occur, continue adding carbohydrates until you reach your carb goal per hour on race day.
Example:
Weeks 1-2: 30 grams of carb per hour (glucose + fructose)
Weeks 3-4: 40 grams of carb per hour
Weeks 5-6: 50 grams of carb per hour
Weeks 7-8: 60 grams of carb per hour
Weeks 9-10: 70 grams of carb per hour, test up to 90 grams per hour as tolerated
How does hydration impact carb absorption?
Dehydration during endurance events causes reduced plasma volume, which can negatively impact GI blood flow. This means, less oxygen and blood flow to the gut contributing to GI discomfort.
In addition to reduced blood flow, gastric emptying is slower during dehydration, causing fluid and carbs to remain the stomach longer. This can cause symptoms of feeling full, bloated or nausea. Overtime, if fluid intake is not adequate during endurance training or competitions, while consuming carbs from gels, chews, or food, the beverage becomes more hypertonic causing GI issues.
Lower osmolality drinks (more diluted) are easier on the stomach. When we compare the tonicity of a sports drink we compare it to human blood, which impacts how quickly you can absorb the fluid into your bloodstream. Sports drinks can be either hypotonic (lower concentration), isotonic (about the same concentration as human blood), or hypertonic (higher concentration).
Hypotonic sports drinks are best used for endurance events since they are absorbed quickly into the bloodstream to help maintain hydration. Since the solution has a lower concentration than blood, fluids cross the gut into blood vessels more easily, preventing GI issues and “sloshing” when training for longer events.
During training, fluid intake varies based on body weight, weather conditions, and sweat rate. You may need anywhere from 12 to 24 ounces (or more) of fluid per hour when training over an hour. To avoid dehydration and GI distress, focus on drinking 4 to 8 ounces every 10-15 minutes or every mile, to train your gut to tolerate the amount of fluid per hour. Having a hydration plan, and not just drinking to thirst, is key to preventing dehydration, being able to absorb the carbs consumed per hour, and avoiding GI symptoms.
Key tips to reducing GI discomfort:
Eat main meal 3-4 hours before competition
If you have less than 2 hours before competition, reduce amount of protein and fat, eating more easily digesting carbs, i.e. ½ PB sandwich + fruit
Cut back on high fiber foods and gassy vegetables (Brussel sprouts, broccoli, cauliflower) before competition or intense training sessions
Choose foods with less than 3 grams of dietary fiber per serving before
Avoid eating large salads the day before or on an intense workout or competition
Start the event well-hydrated
Consider eating low FODMAP foods during a carb load before competition
Use multi-source carbs from glucose/maltodextrin + fructose to maximize absorption
Practice race day fueling during long endurance training sessions
Remember gut training is not short-term, it is a progressive, repeated process over weeks to allow your gut to adapt to the amount of fluid and food you are eating. Over time your gut will be able to tolerate your goal fluid and carb intake so you have a successful race day!
Resources:
Mlinarica J and Mohorko N. Nutritional strategies for minimizing gastrointestinal symptoms during endurance exercise: systematic review of the literature. Journal of the International Society of Sports Nutrition. 2025, Vol.22, No.1. https://doi.org/10.1080/15502783.2025.2529910.
Naderi A, Gobbi N, Ali A, et al. Carbohydrates and Endurance Exercise: A Narrative Review of a Food First Approach. Nutrients. 2023. Mar 11;15(6):1367. 10.3390/nu15061367
Ribichini E. Scalese G, Cesarini A, et al. Exercise-Induced Gastrointestinal Symptoms in Endurance Sports: A Review of Pathophysiology, Symptoms, and Nutritional Management. Dietetics. 2023, 2(3), 289-307. https://doi.org/10.3390/dietetics2030021
Monash University. 2019. The Low FODMAP Diet. website: https://www.monashfodmap.com/
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