Decreased gastric juices – investigate training tolerance for race tolerance

Gastrointestinal distress is a common problem in endurance sports. Upwards of 75% of endurance athletes will experience some variety of GI problems during training and racing. These issues tend to effect more people as the events get longer and longer (Ironman, ultramarathon, etc). In recent years, carbohydrates have been getting a lot of the blame. So, what’s really going on? Are carbs the ‘bad guy’ here?

Gut function & the gut – brain connection

We all know the basics of the GI system: mouth, esophagus, stomach, small intestine, large intestine, rectum, anus. The food and drink we consume, takes a long journey down all of these pipes. Along the way, your food is broken down and absorbed.

It’s a long way for food to travel: covering a massive surface area and providing plenty of opportunity for things to potentially go right or wrong.

The GI system is also home to the enteric nervous system. This large network of nerve cells is often referred to as the second brain. It houses between 100 and 600 million neurons that line the entire length of your gut’s walls from mouth to anus. Your gut is responsible for identifying, digesting and absorbing nutrients, plus weeding out any pathogens or toxins to eliminate on a second by second basis 24/7.

There’s a lot of messaging between the enteric and central nervous system. We often think that all the communication starts in the brain and moves in one direction – “downstream” going out to all body parts. But, actually, there is two way communication between the brain and the GI system. The gut sends more messages “upstream” to the central nervous system via the vagus nerves than downstream. The vagus nerves are the main nerves of your parasympathetic nervous system that run through the gut, chest and through the neck to the brain stem.

What is GI distress in endurance athletes?

GI distress is defined by various symptoms. The nine most common symptoms are:  nausea and vomiting; reflux, heartburn and regurgitation; fullness and bloating; intestinal cramps; side stitch; flatulence; defecation and diarrhea; constipation; bloody stool. And, an athlete could experience any combination of these at any given time.

The reality is that these issues have a negative impact on performance in training and racing.  And, with decreased performance, confidence in your body’s ability to perform also drops.

The argument for more carbs = improved performance and decreased GI distress

There are more and more studies coming out all the time showing that carbohydrate intake during training and racing improves performance. In a study of ultramarathon runners, researchers looked at inflammation and gut injury as potential causes for GI distress during a 60k event. Nearly 75% of athletes reported some level of GI distress during the race. Inflammatory markers did rise after the event, but this happened to ALL runners even if they hadn’t complained about GI distress. Gut specific injury and inflammation did not appear to be the mechanism causing GI problems.  The one thing that was associated with symptoms was the amount of nutrition taken during the event. The runners that took in the most nutrition, especially when that nutrition was carbohydrates, experienced the fewest and least severe symptoms. Researchers stated that the reason for this appears to be that when carbs were being ingested, this improved blood flow to the gut through various mechanisms and prevented many of the symptoms often associated with low blood flow states. (Van Venrooij, et al)

The intake of carbs improved gut blood flow, improving digestion.

Other studies have compared high protein intake with high carb diets on performance in cyclists.  The high protein diet group’s time trial (TT) performance decreased. The results suggested that a high carbohydrate diet supports better performance during cycling TT. (Schwabenbauer)

And, there are plenty of studies showing that too much carbs or the wrong carbs for an individual could wreak havoc on their gut. One study of the type of carbohydrate consumption showed that indeed the wrong type of carbs taken in will impede performance. This study made the general recommendation to focus on glucose or glucose plus fructose intake. (Oliviera)

Root cause vs trigger of symptoms – symptom by symptom approach

According to current science and research, there is not one known definitive cause of GI distress in endurance athletes. But there is a lot of research about the origins of the various nine symptoms listed above, individually. When working to find how to improve gut problems, each symptom must be looked at one by one. Then, do a lot of detective work along the way.

Here are just a few of the origins of those nine problems:

1. Nausea and vomiting:  reduced gut blood flow (especially during intense exercise), eating high fat foods before workouts, high dose of caffeine, psychological stress or anxiety.
2. Reflux, heartburn, and regurgitation: all of the above in #1, plus: consuming chocolate, citrus fruits, spicy foods, acidic foods, coffee, alcohol or carbonated fluids, reduced gut blood flow.
3. Fullness and bloating:  dehydration, excess food consumption too close to workout time, high fiber diet, drinking hypertonic fluids (sports drinks) during workouts, reduced gut blood flow.
4. Intestinal cramps: NSAID consumption, carbohydrate malabsorption, reduced gut blood flow.
5. Side stitch: movement causing up and down jostling or twisting of the abdomen (running), reduced gut blood flow, ingesting hypertonic fluid during exercise, posture.
6. Flatulence: aggressive carb intake during exercise, carbohydrate malabsorption.
7. Defecation and diarrhea: reduced gut blood flow, up and down movement,  GI infections, medications (antibiotics, metformin, selective serotonin update inhibitors, etc), supplements, caffeine.
8. Constipation: inadequate fluid intake, low carb or low fiber diet, overall low dietary energy intake, medications.
9. Blood in stool:  reduced blood flow, NSAIDs, cecal slap syndrome.

The symptoms vary quite a bit and an athlete can have any combination of these symptoms and origins. It’s fair to say that most athletes will have some type of gut problems during training or racing. Although there are many potential causes of these problems, reductions in gut blood flow are likely to contribute to most of these gut symptoms. Reduced gastric juices (hydrochloric acid and digestive enzymes) can be a root cause for some athletes and preexisting GI medical conditions (IBS, colitis, etc).

Coach’s note: Anyone experiencing persistent GI issues: I highly recommend Patrick Wilson’s book: The Athlete’s Gut and/or working with a highly experienced sports oriented Registered Dietitian to help you get to what is at the root of your issues and get on your way to better, happier training and racing.

One size does not fit all

The reality is that GI distress is a complex, multilayered issue. It’s different for each person and needs to be looked at symptom by symptom and food by food, fluid by fluid, medication by medication, underlying conditions, etc. We are all unique and this issue must be approached on an individual basis. To get to the root of your issues and get significant improvement, you must be your own detective and conduct your own “science experiments of one” over time.

Factors that influence the occurrence or severity of gut symptoms:

1. Exercise intensity and duration: increasing exercise intensity and duration can increase gut problems because blood flow is reduced to the GI system.
2. Age: Getting older is actually associated with a decrease in gut problems mostly because as we get older we simply can’t train as long, as often or as hard.
3. Training experience (train your gut!):  The more you train your body, the better it adapts to taking in food when training, thus improving blood flow to your gut and less likelihood of experiencing GI distress. And, there’s more time to work with the trial and error process to get it all figured out.
4. Sex: Women are more likely to experience gut problems and those who are younger in age and/or training age.
5. A history of gut symptoms: The simple truth is that if you’ve had gut problems in the past, you are more likely to have them in the future.
6. Sleep:  Because our GI systems run on a 24-hour rhythmic biological clock, consistently not getting that 7+ hours of sleep at night can lead to gut upset.

Please note that underlying health conditions, known or unknown, can contribute to GI distress during training and racing. And, several other medications have an impact on GI function that can lead to potential problems.

There are many different types of sports foods and regular foods to take in during training and racing. This is another opportunity to understand, through some trial and error and/or guidance from a registered dietitian, what works best for you.

Trigger or cause?

This is far from a black and white issue; more like many shades of gray for each individual athlete to investigate. While carbs may or may not be the underlying issue, it’s important to understand whether something is a trigger or a cause. Is it triggering your symptoms or is it the root cause of those symptoms? Although carbs take the heat for athletes’ gut problems, more often than not they are not the culprit or at least not the only culprit.References & Resources:

Prado Oliveira, et al. Carbohydrate-Dependent, Exercise-Induced Gastrointestinal Distress.  Nutrients. 6(10):4191-4199. Oct 2014.

Sankoff, Jeffrey, MD. More Carbs Correlates with Less GI Distress in Runners.  Triathlete Magazine, June 20, 2022.

Schwabenbauer, Kim.  Want free speed? Stabilize your gut.  Triathlete Magazine:  September 15, 2022.

Schwabenbauer, Kim. Does low-carb equal low testosterone in men?  Triathlete Magazine:  June 14, 2022.

Sims, Stacy. Ask Stacy: Why is gut health so important?  Triathlete Magazine: November 4, 2020.

Van Venrooij, N., et al.  The association between gastrointestinal injury, complaints, and food intake in 60-km ultramarathon runners. Applied Physiology, Nutrition, and Metabolism. 47:5. May 2022.

Wilson, Patrick. The Athlete’s Gut: the inside science of digestion, nutrition, and stomach distress.  Velo Press, 2020.