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An Athlete Needs To Train Their Gut, Too

Especially important for endurance athletes

Athletes tend to do a good job of training their muscles, heart and lungs. But some of them (particularly endurance athletes and those in running sports) commonly fail to train their gut. As one marathoner reported, "I was so afraid of getting diarrhea during long training runs that I did not eat or drink anything beforehand. I really struggled after 14 miles..." A high school soccer player admitted, "I'm so afraid I'll throw up if I run with food in my stomach." He ate only a light lunch at 11:00 and then practiced on fumes at 3:30. No wonder he had a disappointing season.

An estimated 30-50% of endurance athletes (including up to 90% of distance runners) have experienced gastro-intestinal (GI) issues during and after hard exercise. They fear bloat, gas, nausea, stomach cramps/pain, side stitch, diarrhea, vomiting, and urge to defecate. These issues arise during long bouts of exercise because blood flow to the gut is reduced for an extended period of time. When combined with dehydration, elevated body temperature and high levels of stress hormones, normal intestinal function can abruptly end.

For an athlete with a finicky GI tract, restricting their diet before and during exercise will not solve the problem. They need to learn how to train their gut to accommodate performance enhancing carbs and water. That way, they can train better - hence compete better - without stressing about undesired pit stops.

Thankfully, the gut is trainable. Competitive eaters have proven this point. Google Nathans' Hot Dog Eating Competition and watch the video of a champ who stuffed 72 hotdogs into his stomach in 10 minutes. Clearly, he had to train his gut to be able to complete that task.

If competitive eating isn't the goal, learning to fuel wisely in order to perform optimally should be. While some "keto-athletes" choose to train their bodies to rely on fat for fuel (fat is less likely to cause GI distress), training the gut is a far easier alternative for most.

Here are some tips to help an athlete exercise with digestive peace

  • Drink enough fluids. Dehydration triggers intestinal problems. The goal is to drink enough to prevent 2% dehydration (sweat loss of 2 pounds per 100 pounds of body weight from pre- to post-exercise). For a "big guy" who sweats heavily, this can be a lot of fluid. For example, a 200-pound football player could easily lose 4 pounds (a half-gallon) of sweat in an hour of exercise. He needs to train his gut to handle fluid replacement during training. He could need as much as 12 to 16 ounces every 15 minutes during a two-hour practice.
  • Learn how to empty the stomach. Feeling "full" and "bloated" during exercise indicates fluids (and foods) have not emptied from the stomach. This commonly happens during really hard exercise, when reduced blood flow to the stomach delays stomach emptying. Hot weather and prolonged exercise in the heat can also reduce stomach emptying.  The answer is to dilute highly concentrated carbs (i.e., gels) and drink enough water during exercise (i.e. 16 oz. water per 100 calories gel) to help speed up gastric emptying.
  • Train the gut to handle fats as well as carbs. Athletes who plan to eat a peanut butter on a bagel before they compete should also routinely eat that snack before important training sessions. This helps train your gut to accommodate fat (sustained energy) as well as carbs (quick energy).
  • When planning what to eat during extended exercise, choose from a variety of carbs with a variety of sugars
    • This helps prevent the glucose transporters from getting saturated. Too much of one kind of sport food can easily create GI problems.
    • Once carbohydrate (such as sport drink, gel, banana, or gummi bears) empties from the stomach, it enters the smallBanana intestine and is broken down into one of three simple sugars (glucose, fructose, galactose). These sugars need "taxi cabs" to get transported out of the intestine and into the blood stream.
    • Too many gels or chomps without enough transporters can lead to diarrhea. By training with your race-day carbs, you can increase the number of transporters.
    • For athletes who typically eat a low-carb Paleo or keto-type diet and then on the day of, let's say, a marathon,   decide to fuel with carb-rich gels and sports drinks, their body won't have the capacity to optimally transport the sugar (carbs) out of their intestines and to their muscles. They could easily end up with diarrhea.
  • "Real foods" such as banana, raisins and cereal, have been shown to be as effective as commercial sport foods. The body processes "real food" every day and has developed a good supply of transporters to deal with the carbohydrate an athlete  commonly eats. By experimenting and learning what works best for the body, an athlete can fuel without anxiety about undesired pit stops.
  • For exercise that lasts for up to two hours, research suggests about 60 grams (240 calories) of carb per hour can empty from the small intestine and get into the blood stream. Hence, that's a good target. For longer, slower, events, the body can use 90 g (360 calories) carb per hour from multiple sources, as tolerated. Again, train your gut!

The bottom line:   

  • Train with relatively large volumes of fluid to get the stomach used to that volume.
  • Routinely eat carbohydrate-based foods before training sessions to increase the body's ability to absorb and use the carbs.
  • During training, practice race-day fueling. Mimic what might be eaten before the actual competitive event, and tweak it until the  right balance is found.
  • If concerned about diarrhea, in addition to preventing dehydration, limit fiber intake for a few days pre-event (fewer whole grains, fruits and veggies).
  • Reduce intake of onions, garlic, broccoli, apples, and sorbitol might help reduce GI issues during exercise.
  • Meet with a sports dietitian to help create a fueling plan that promotes intestinal peace and better performance.

Sports nutritionist Nancy Clark MS RD CSSD has a private practice in the Boston-area (Newton; 617-795-1875). She helps both fitness exercisers and competitive athletes create winning food plans. Her best-selling Sports Nutrition Guidebook, and food guides for marathoners, cyclists and soccer are available at nancyclarkrd.com. For online workshops:  www.NutritionSportsExerciseCEUs.com.


Jeukendrup, A. Training the Gut for Athletes. Sports Med 2017; 47 (Supple 1): S101-S110

Prado de Oliveira E., Burine, R, Jeukendrup A. Gastrointestinal Complaints During Exercise; Prevalence, Etiology and Nutritional Recommendations. Sports Med 2014 (Supple 1): S79-S85.