Runner's diarrhea, intestinal problems and abdominal cramps - causes and prevention

Gastrointestinal problems are very common in endurance athletes. Runners experience symptoms such as belching, nausea, heartburn, cramps, bloating, loose stools, and diarrhea. The problems are divided into upper and lower digestive tract symptoms.

How often does this happen?

Estimates show that 30-50% of endurance runners experience intestinal discomfort during a race. In studies of marathons, up to 80% of runners report at least one symptom. The most common are the urge to defecate and diarrhea. In triathlons or longer races, the problems are even more common. Problems occur more often with running than with cycling or swimming, and more in women than men.

Why problems arise

Physiological causes

During intense exercise, the body redirects blood to the muscles and blood flow through the digestive tract can decrease by up to 80%. Lower blood flow impairs the function of the intestinal wall and can lead to abdominal pain or diarrhea. In more severe cases, damage to the intestinal mucosa can occur. In addition, nervousness before a race affects intestinal motility through hormonal reactions and can contribute to loose stools.

Mechanical causes

Running creates repeated shocks and pressure on the abdominal cavity, which strains the digestive tract and promotes the development of lower intestinal problems. Runners sometimes experience minor bleeding into the digestive tract, which increases with the length of the race. Upper GI problems are more common in cycling, and swallowing air during increased ventilation and drinking from bottles can also contribute to discomfort.

Nutritional causes

A higher risk of problems is associated with foods high in fiber, fat, protein and fructose - especially in concentrated form. Dehydration worsens the problem. Drinks with high osmolality (for example, more than 12 g of carbohydrates per 100 ml together with electrolytes) increase the risk of diarrhea. Lactose can also cause discomfort in sensitive people.

How to reduce the risk

It is not possible to guarantee that intestinal problems will never occur, but their risk can be significantly reduced with proper preparation and digestion training.

1. Limit fiber before the race. Choose a low-fiber diet a day or two before the start. Skip whole grains, legumes, and higher-fiber vegetables. Opt for white rice, pasta, or white bread.
2. Avoid taking medications like ibuprofen or aspirin. Nonsteroidal anti-inflammatory drugs increase intestinal permeability and may worsen the risk of complications.
3. Limit milk with lactose. If you have even a mild intolerance, lactose can be a problem during exercise. Instead, reach for lactose-free alternatives.
4. Avoid products containing only fructose. Drinks or foods with pure fructose can cause cramps and diarrhea. A combination of glucose and fructose is better.
5. Make sure you stay properly hydrated. Lack of fluids increases the risk of intestinal problems, but too much water before the start is also not advisable. Drink gradually and continuously, not a large amount at once.
6. Don't use new foods and supplements on race day. Test everything you plan to use in training. Avoid products you're unfamiliar with.
7. Train your digestion Just like you train your muscles, you can train your digestive system. Regular carbohydrate intake during training improves their absorption and reduces the risk of problems in the race. Long-term carbohydrate restriction, on the other hand, worsens tolerance.
   

Summary

Intestinal problems during sports have combined causes - physiological, mechanical and nutritional. They most often occur during running, especially at higher intensity. The risk increases with dehydration, consumption of foods high in fiber, fat, lactose or concentrated fructose.
The key to prevention is simplicity: a light and tried-and-tested diet, plenty of fluids, limiting risky foods, and gradual digestion training.

Just like you train your body, train your digestive system - thanks to this, you can run at your full potential and without unnecessary stops.

Resources:

1. Jeukendrup, A. E. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine , 44(1), 25-33.
2. Burke, LM, & Maughan, RJ (Eds.). (2015). Sports Nutrition: More Than Just Calories - Triggers for Adaptation.
3. de Oliveira, EP, Burini, RC, & Jeukendrup, A. (2014). Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine , 44(1), 79-85.