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Why running in the heat causes gut issues — and how to avoid them

By Courtney Berg

If you’ve ever running through a hot Saskatchewan summer, you might have noticed that the frequency of gut-related issues increases as the temperature rises. But why does this happen and how do you avoid it? As a Registered Dietitian I’ve come to understand the connection between heat and gut function. By recognizing how the body responds to exercising in the heat, you can minimize potential gut issues and adopt strategies to maintain performance during hot weather! Here’s how I talk about it with my clients.

How running in the heat impacts the body

  • Increased Sweat Rate: Physical activity generates heat and, when this is combined with hot conditions, the body temperature rises. In response, the body produces sweat as a cooling mechanism to lower body temperature allowing organs (including the gut) to operate properly.
  • Increased Heart Rate: Heart rate increases to maintain blood flow to both the skin for cooling and to the muscles for running. Heart rate can also increase due to reduced blood volume from sweat losses.
  • Dehydration & Electrolyte Imbalance: Excessive sweating can lead to dehydration if fluids are not adequately replaced. Additionally, sweat contains essential electrolytes like sodium. Dehydration and electrolyte imbalance can disrupt normal body functions (eg. gut function) and exercise performance (eg. muscle function).
  • Fatigue: Heat stress accelerates fatigue and can reduce physical performance. In more severe circumstances, an elevated core temperature can lead to heat stress or even heat-related illnesses such as heat exhaustion or heat stroke.
  • Impaired Digestion: Blood is diverted away from the digestive organs to the skin for cooling and to the muscles for the delivery of nutrients and oxygen. This redirection of blood flow from the core (ie. organs) to the periphery (ie. skin and muscles) can lead to impaired digestion and gut issues.

Why does running in the heat cause gut issues?

  • Blood Flow Redistribution: Blood is diverted from the gastrointestinal tract to the skin and muscles to aid in cooling and performance. This redirection in blood flow can negatively impact digestive processes. In more extreme scenarios, this reduced blood flow can decrease oxygen and nutrient delivery to the gut, which can potentially damage the gut lining.
  • Sweating and Dehydration: Sweating is the body’s primary cooling mechanism and sweat rate increases in the heat. Excessive sweating can lead to dehydration, which reduces blood flow to the gut, and electrolyte imbalances, which can present as stomach cramps or diarrhea.
  • Heat Stress and Intestinal Permeability: In more severe circumstances, heat stress can increase intestinal permeability. This can allow bacteria and toxins to enter the bloodstream, which can cause inflammation and gastrointestinal distress.

How to avoid gut issues when running in the heat

  • Stay Hydrated with Fluids: Drink fluids regularly before, during, and after running. Work with a Registered Dietitian to calculate your sweat rate and determine the appropriate hydration strategy for you! Some tips:
    • Before: Drink water with your pre-run meal.
    • During: Registered Dietitians recommend consuming between 400-800mL of fluid per hour when running >60 minutes.
    • After: Drink water with your post-run meal and consume fluid-rich foods such as fruits, vegetables, smoothies, or soups.
  • Replenish Electrolytes: Consume electrolyte-rich foods and supplements before, during, and after a run. Registered Dietitians recommend consuming at least 300 mg of sodium per hour for runs >60 minutes. Sodium can be found from a variety of sources during a run such as electrolyte-rich fluids, salt tablets, salt added to fluids, or salty intra-run snacks like pretzels. Sodium is an essential electrolyte for endurance athletes and it is not recommended to restrict sodium before, during, or after a long-run unless otherwise directed by a healthcare professional.
  • Optimize Pre-Run Nutrition: Avoid high-fat or high-fiber meals immediately before running or during a long run. High-fat and high-fiber foods can be difficult to digest and exacerbate gut issues. Instead, consume easily digestible carbohydrates and moderate amounts of protein and fat!
  • Consider Intra-Run Nutrition:
    • Easy-to-Digest Carbohydrates: If running for >60 minutes you will benefit from consuming easy-to-digest carbohydrates during the run to optimize performance. Choose carbohydrates that are low in fiber and fat to prevent digestive upset (eg. pretzels, dried fruits, honey, gels, or electrolyte beverages).
    • Train Your Gut: If you are running longer distances (eg. >60 minutes) it is important to train your gut to take in adequate carbohydrates to fuel performance. Gut-training means starting out with smaller amounts of carbohydrates during exercise and gradually increasing as your gut adapts (eg. 30-90g of carbohydrate per hour of running). Training your gut can improve carbohydrate absorption, glucose availability in your blood, and can substantially reduce gut discomfort.
    • Combine Carbohydrates Sources: By including both fructose and glucose as carbohydrate sources, you can increase total carbohydrate absorption to enhance available energy. Furthermore, using both glucose and fructose minimizes this risk of gastrointestinal stress that can happen when using a single type of carb. For example, a runner might combine carb sources by having a sports drink that contains glucose and fructose or by using natural sources like consuming different types of foods such as dried fruit and an energy bar.
  • Use Cooling Strategies: Adopt strategies to keep your body cooler, such as drinking cold fluids during a run, wearing lightweight or moisture-wicking clothing, or using cooling towels if they are available to you.
  • Acclimate to the Heat: Gradually increase exposure to running in hot conditions to help the body adapt. This can improve sweat response and reduce the risk of dehydration and heat-related issues!
  • Personalize Your Approach: Every runner is different and it is important to experiment with different foods, timing, and quantities to find what works best for you. Keep a food and performance journal to track what you eat before and during runs, how you feel, and your performance outcomes.

By understanding the body’s physiological response to the heat and taking proactive measures to support the body during hot weather, runners can minimize the risk of gut issues and optimize their running performance! Working with a Registered Dietitian to optimize your fluid, electrolyte, and carbohydrate strategy for long-runs may be important for more complex challenges related to gastrointestinal function.

Courtney Berg is a Registered Dietitian and the owner of Vitality Nutrition, a collective of Registered Dietitians supporting clients in-person in Saskatoon and Regina and virtually across Saskatchewan. Courtney and her team of nutritionists incorporate a unique and meaningful approach to food, fitness, and performance that empowers their clients to build life-long habits and create lasting results.

References

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