Tips for runners with intestinal problems

Have you ever experienced nausea or unpleasant stomach symptoms that impaired your training or performance? Though this may not be a popular topic of conversation, studies suggest that as many as 60% of distance runners experience intestinal problems related to exercise. Females are more likely to experience symptoms than males, especially during menstruation and younger athletes also appear to be more susceptible to gastrointestinal problems than older adults. Risk for gastrointestinal upset also increases with exercise intensity and during more prolonged bouts of activity.

Many runners suffer from ongoing intestinal issues related to running. The good news is that after making some changes in their training diet or fueling routine, many were able to significantly reduce their symptoms. This article focuses on the causes of gastrointestinal upset in runners, potential triggers and strategies that may reduce the occurrence of symptoms.

Why does running increase gastrointestinal upset?

Studies have shown that runners experience more intestinal issues than other athletes. The following are physiological factors that may contribute to this fact:

  • Jostling of the digestive tract stimulates intestinal activity
  • Rapid fluid shifts into the colon during activity may cause irritability and cramping
  • Diversion of blood from the gastrointestinal tract to muscles impairs digestion and absorption
  • Dehydration further reduces blood flow to the gastrointestinal tract, increasing risk for gastrointestinal upset
  • Changes in intestinal hormones during exercise can increase gastrointestinal issues
  • Increased stress and anxiety before events can contribute to intestinal distress

What factors increase risk for gastrointestinal upset?

Some runners report the ability to eat high-fat foods like pizza or burgers right before running, while others can't eat anything within several hours of exercise. Though intestinal response during running varies widely among individuals, certain factors can increase the likelihood of intestinal symptoms in susceptible individuals. These include:

Lactose intolerance - Some individuals lack the enzyme required to digest the carbohydrate in milk, which leads to stomach symptoms such as gas, bloating and diarrhea after consuming dairy products. Many runners may experience only mild or inconsistent symptoms at rest, and do not realize they are lactose intolerant. Others may be lactose intolerant, but unaware that certain foods contain lactose. Thus they do not attribute their gastrointestinal symptoms to this issue.

Dehydration - Studies show that intestinal symptoms increase when runners have lost more than 4% of their weight through sweat (about 6 pounds for a 150-pound runner). Many of these individuals believe that drinking more fluids contributes to diarrhea, and therefore they limit their fluid intake.

High-fiber foods - High-fiber foods are great for health, but can increase gastrointestinal symptoms during activity. Fiber speeds movement through the gastrointestinal tract, increasing risk of bowel movements or diarrhea. Bacteria also feast on fiber in the lower gastrointestinal tract, which can produce gas and abdominal cramping.

Cruciferous vegetables - Cruciferous vegetables like cabbage, broccoli, and cauliflower contain raffinose - a gas-producing compound that is also found in beans.

High-fat/protein foods before activity - The digestive process in the human body can take 4 to 72 hours. High-fat meals take longer to digest and remain in your gastrointestinal tract longer, increasing risk for upset.

Caffeine - Though individual response can vary, caffeine has been shown to have a laxative effect on the gastrointestinal tract, and high doses may lead to gastrointestinal problems during exercise.

Artificial sweeteners - Sugar alcohols such as mannitol and sorbitol, used as artificial sweeteners, can lead to stomach upset. These are commonly found in diet sodas and flavored waters, sugar free chewing gum and candy, and other "diet products". Sports drinks or gels containing fructose as the main sugar source can also cause gastrointestinal upset.

Sodium bicarbonate and citrate - Sodium bicarbonate and citrate are used as buffering agents to neutralize the effects of lactic acid during exercise. Athletes have reported gastrointestinal symptoms such as vomiting and diarrhea after using these substances.

Excessive carbohydrate intake during activity - A recent study reported that up to 90 grams of carbohydrate (about 360 calories) per hour, taken in the form of gels, was well tolerated during exercise. Sports drinks have been shown to be well absorbed at concentrations of 6-10% (about 50-70 calories per cup). When carbohydrate intake exceeds these levels, fluids are pulled into the gut to dilute the contents. This is can lead to loose stools and diarrhea during activity.

NSAIDS - Commonly used non-steroidal anti-inflammatory drugs, such as aspirin and ibuprofen, have been associated with gastrointestinal distress, especially when used in the high amounts that are common for some athletes.

Strategies for reducing gastrointestinal symptoms

To reduce gastrointestinal symptoms, try the following strategies:

Allow sufficient time for digestion - Allow at least three hours to digest a large meal and 1-2 hours for a small snack. Energy products such as gels and sports drinks are the easiest to digest and absorb within one hour of activity.

Maintain hydration - Drink sufficient amounts of fluid to maintain body weight during activity. For most athletes, this constitutes about 16-24 ounces per hour.

Reduce your fiber intake - If you are prone to gastrointestinal distress, try reducing your intake of high-fiber breads and cereals, beans, lentils, cruciferous vegetables, and fresh and dried fruit. If symptoms improve, try substituting lower fiber foods like pasta, rice and bagels and low-fiber fruits and veggies like zucchini, tomatoes, grapes and grapefruit.

If you suspect lactose intolerance - If you suspect lactose intolerance try avoiding dairy products for a week. This includes milk, yogurt, ice cream, soft cheeses, and products containing milk solids (creamy salad dressings, cream sauces, cream soups, etc.). If symptoms subside, try substituting soy, rice or almond milk for cow's milk, or dairy-free or lactose-reduced products.

Test your tolerance of caffeine, medications and buffering agents - Try limiting your intake of caffeine-containing beverages to one cup, and pay attention to the caffeine content of energy products. Test your tolerance of non-steroidal anti-inflammatory drugs and buffering agents before using them during an event.

Keep a diary - The best way to identify "triggers" for gastrointestinal symptoms is to keep a diary. Track your intake of food, beverages, medications, and timing of meals, workouts and symptoms. You should also document energy products used during activity, intensity of exercise, and other factors affecting your workouts. Level of stress and hormonal status for women can also be factors. Track this information for a week or two, and look for associations. Remember, it can take several days for food to move through the digestive systems, so symptoms can occur 1-3 days after a food is ingested.

The bottom line

Do not let intestinal issues keep you from your run. Gastrointestinal tolerance can improve with training and increased fitness, as muscles divert less blood from the stomach. The ability to consume fluids during activity can also improve with practice, reducing the risk of dehydration-related gastrointestinal symptoms. Experiment with training during different times of the day or varying your exercise intensity. If you are still unable to get relief, consult a sports nutritionist who can aid in the evaluation of your symptoms or consult a physician about the occasional use of anti-diarrhea medications.

Credits - would like to thank Heather Nakamura for the permission to reprint the article "Dealing with GI distress" by Heather Nakamura. Text © by Heather Nakamura. Heather Nakamura, MPE, MS, RD is a registered dietitian with Masters Degrees in both Exercise Physiology and Nutrition. She works as a Personal Wellness Coach, to help active individuals develop nutrition and training programs that focus on improving performance, maximizing energy, and achieving optimal body composition and health. For more information visit her website at

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