by Mimi Winsberg, M.D.

The term “gut feeling” originates from the notion that the enteric nervous system acts as a primitive brain and that our guts are closely tied to our brains. The neurotransmitters that regulate our brain function are also active in the gut. Given this physiological link, it’s no surprise that we often feel emotion in our gut.  

Many of us have gut reactions to racing. At least 50% of endurance athletes have experienced gastrointestinal (GI) distress during races. But, just as our muscles adapt to training, so can we train our guts (1). Here is an overview of common triggers for GI distress, and how to avoid the “runs” that are not on the course.

GI issues most commonly affect athletes while running (2). The aero position on the bike, however, can trigger upper GI issues, and swallowing water or air during an open water swim can cause bloating and gas. Upper GI distress manifests as heartburn, vomiting, bloating or just stomach pain, and lower GI distress manifests as cramping, gas and diarrhea.

Multiple factors will determine whether an athlete gets GI distress including:

• Exercise Intensity, Environment, and Fitness - At sub-maximal intensities (less than 70% of VO2 max) in trained athletes, blood flow to the gut is not impaired, but with more intense exercise or with particularly hot conditions, blood is diverted away from the gut to working muscles and skin resulting in significantly decreased blood flow to the gut and delayed stomach emptying (2). There is no calorie per hour intake limit that applies to all athletes without considering individual fitness, exercise intensity, and climate.  
  
  
• Diet - High protein and high fat diets, too much fiber, and highly concentrated carbohydrate beverages can predispose athletes to Gi issues (3). Supplements such as creatine, ginseng and amino acids can be problematic, as well as large doses of vitamin C, magnesium, iron, calcium and potassium. Caffeine stimulates large intestine contractions, and can lead to cramping and diarrhea.
  
  
• Timing of Meals - Timing meals too close to exercise can exacerbate GI issues.  
  
  
• Hydration Status - Dehydration contributes to GI issues during exercise (4). Sipping fluids frequently during longer sessions helps maintain gastric absorption, and reduces the risk of GI distress (5).
  
  
• Illness - Some athletes harbor underlying illness. Celiac disease, IBS, lactose intolerance, GERD, Crohn’s disease, ulcerative colitis, and gallbladder disease can all be culprits of repeated bouts of GI distress. A medical evaluation to rule out any of these causes can be helpful if the problem has been pernicious or severe.
  
  
• Emotional State - Stress, anxiety, and even the power of suggestion can contribute to GI distress. When the body’s fight or flight reaction kicks in, the rate of cortisol and other stress hormone production increases, resulting in changes in intestinal motility and the urge to eliminate.

Given the multitude of factors, how to approach the problem? Here are some ways to get clarity:

1. Log
   • Keep a food and symptom log. Note intensity of exercise when symptoms arise.
   • Experiment with foods in training, not races.
   • Slowly build exercise intensity to give the GI system a chance to adapt.
   • Figure out what works for you at race pace, in similar conditions that you will face on race day.
2. Timing  
   • The more intense the exercise, the more time needed for digestion.
   • Consume large meals 3-4 hours before exercise and small meals 1-2 hours before exercise.
   • Experiment with liquid nutrition during higher intensity sessions or races.
3. Hydration
   • Sip fluids frequently.
   • Carbohydrate drinks are most easily digested in a non-carbonated 6% solution. Higher percentage solutions are likely to induce GI distress (6).
   • Monitor your caffeine intake and limit doses to 100mg every 4 hours.
4. Nutrition
   • If certain food groups are problematic (such as dairy), then try eliminating foods and re-introducing them one at a time, noting accompanying symptoms.
   • Optimize nutrition in the days leading up to competition and avoid supplements in this window.
   • Gradually eliminate sources of fiber in the three days leading up to competition.
   • Eat smaller doses of calories in easy to digest simple carbohydrates the day before racing.
   • Try liquid meals closer to training and racing.
5. Emotional
   • Notice your anxiety and incorporate calming pre-race rituals into your routine.

If problems are severe or seem complicated, take the time to discuss your symptoms and patterns with your physician to get advice and rule out an underlying medical condition.

1) Broun F, Beckers E. Is the gut an athletic organ? Digestion, absorption and exercise. Sports Med. 15: 242-257, 1993.

2) Sullivan, SN. Exercise induced symptoms in triathletes. Phys Sports Med. 15: 105-108, 1987.

3) Peters et al. Gastrointestinal problems as a function of carbohydrate supplements and mode of exercise. Med Sci Sports Exerc. 25: 1211-1224, 1993.

4) Rehrer et al. Physiologic changes and gastrointestinal symptoms as a result of ultraendurancerunning. EurJ Appl Physiol. 64: 1-8, 1992.

5) Ploutz-Snyder L et al. Gastric gas and fluid emptying assessed by magnetic resonance imaging. EurJ Appl. Physiol. 79: 212-220, 1999.

6) Shi X, Horn MK, Osterberg KL et al. Gastrointestinal discomfort during intermittent high-intensity exercise: Effect of carbohydrate-electrolyte beverage. Int J Sports Nutr Exerc Metab. 14: 673- 683,2004.