Endurance athletes face numerous gut problems during training and competition, and there are undoubtedly numerous potential explanations for each. Indeed, there are so many possible causes that I wrote a 300-plus page book on the topic. For those who want to cut to the chase, here are five of the leading ways endurance athletes trigger exercise-related gut woes.
1) They Gobble NSAIDs
It’s no surprise that lots of endurance athletes struggle with daily aches and overuse issues. In one 2003 study of 844 novice and intermediate-level runners participating in a 13-week training program, about three out of every 10 reported a running-related injury. A study published a decade later found a similar injury incidence among a cohort of 200 recreational runners who were followed for 12 weeks. If you follow runners for even longer periods of time, say a couple of years, upwards of two-thirds suffer from at least one overuse injury.
Surely these figures aren’t a revelation to any veteran athlete, but they do help explain why the rate of anti-inflammatory medication use is astronomically high among this population. Indeed, a recent survey of 806 runners recruited through Parkrun UK (an organization that facilitates weekly 5-kilometer runs across the United Kingdom) found that about 9 out of every 10 runners used a non-steroidal anti-inflammatory drug (NSAID) in the past year, and 80% had taken NSAIDs in the previous four weeks.
While popping a few over-the-counter NSAID pills may seem relatively harmless, these drugs can impinge on many of the body’s organ systems, and the gut is no exception. In that study of Parkrun UK runners, roughly one-third of those who had used NSAIDs said they experienced at least one adverse drug reaction that impacted the gut (nausea, discomfort, heartburn, GI bleeding, etc.). Likewise, a survey from the Bonn Marathon found that runners who used pain relievers like NSAIDs pre-race had a tenfold higher rate of intestinal cramps than non-users and were about twice as likely to withdraw from the race because of gut cramps.
2) Their Hydration Is Imbalanced
If you are training or competing for an hour or less, drinking solely based on your perception of thirst is usually the simplest and most effective hydration strategy. Once exercise extends beyond a couple of hours, though, hydrating to optimize performance can become a real conundrum.
I say this because both overhydrating and underhydrating in these situations can cause major issues. On the underhydration side, large fluid deficits can compromise your blood volume and ultimately the delivery of oxygen and fuel to your muscles, as well as to your gut. Some research has even documented that exercising in a dehydrated state slows emptying of foodstuffs from the stomach and aggravates the severity of gut issues like nausea and stomach cramps. In other words, if you become dehydrated because of a failure to drink enough fluid before and during exercise, your gut’s function may become compromised. And that could make it even harder to correct the problem, as any fluid you ingest will sit like a brick in your stomach.
On the other end of the spectrum, downing bottle after bottle of fluid can also cause major gut problems. On the most basic level, drinking fluid at a high rate heightens perceptions of fullness, bloating, and nausea, which are generally not conducive to performing at your peak. In one example investigation, drinking fluid at an average rate of 1.4 liters per hour (which was done to minimize dehydration levels) did not lead to better half-marathon performance in comparison to simply drinking to thirst (~0.4 liters per hour), despite the fact that drinking more reduced body weight losses (1.3% vs. 3.1%) and lowered heart rates by 4 beats per minute. Importantly, drinking well above thirst doubled the severity of gut discomfort, which likely overshadowed any cardiovascular or thermoregulatory benefits from drinking the additional fluid.
So, if drinking to thirst worked well in the previous study, then why not just recommend it across the board for all runners? Personally, I’m hesitant to say that drinking to thirst works best in every situation because as exercise duration gets longer, small mismatches between sweat rates and drinking rates can add up to a hefty total. If you’re not sure what I mean, it can be helpful to consider an example. Say a 150-pound runner who drinks based on his thirst ingests 0.5 liter per hour and sweats 1 liter per hour during a 5-hour trail marathon. Ultimately, this mismatch in drinking and sweating may not add up to much 2 hours into the race (i.e., a couple of pounds), but by race’s end, his net loss would be about 5.5 pounds of fluid, which has the potential to impact his performance.
Bottom line, there is no one-size-fits-all hydration strategy that will work for every athlete in every situation. While drinking to thirst is a good option for many endurance athletes, especially during relatively brief workouts, a regimented approach may offer some advantages for longer exercise bouts. On a final note, you should never drink above your sweat rate during exercise, as this can raise the risk of complications like hyponatremia. (Here’s an example of how to calculate sweat rates.)
3) They Ignore Pre-Race Nerves
Anecdotally, many athletes believe that excessive nerves are a source of their gut woes during competition. Although scientific studies confirming this are few and far between, one decades-old survey of London Paces Distance Club members revealed that 43% said they experienced “nervous diarrhea” before competition. It wasn’t until very recently that more systematic research attempted to quantify the relationships between psychological stress, anxiety, and gut complaints in athletes. It just so happens that I’ve published a few of the only existent studies on this topic.
The first one was published in the Journal of Sports Science in 2017. In short, I had 150 seasoned runners track their gut symptoms after every run for a month, and at month’s end, they completed questionnaires on life stress and anxiety. After crunching the numbers, I found that scores on the stress and anxiety questionnaires were positively correlated with the percentage of monthly runs that participants reported at least one substantial GI symptom. It’s important to note that the correlations in this study were not super strong, meaning that other factors were undoubtedly involved in the runners’ gut problems. Still, the correlations were as big as those from other studies looking at predictors of gut troubles in athletes. In reality, we should rarely expect any single factor to fully explain why a runner gets gut distress.
Based on these initial results, two of my colleagues and I carried out a follow-up study to see if this same phenomenon occurs during endurance races specifically. In the resulting paper, we reported that having higher levels of anxiety increased the odds of suffering from several GI issues (nausea, reflux, cramping) during races. Moreover, being anxious on the morning of a race seemed to be somewhat more predictive of gut problems than just having general life anxiety.
Before we get carried away, it’s important to remember that these findings are correlational. In other words, we did not prove with 100% certainty that stress and anxiety are causing these gut symptoms. It’s certainly possible that athletes who are stressed or anxious are engaging in other behaviors that provoke their gut. Similarly, these athletes’ anxieties may be the result of—rather than a cause of—their gut woes.
We are currently planning follow-up studies to see whether easing anxiety leads to fewer gut complaints in runners and other athletes, so hopefully we’ll have more concrete evidence in coming years. Even so, there are few, if any, possible downsides to managing anxiety through various interventions like slow deep breathing and meditation.
4) They Don’t Prepare for Hot Conditions
Acclimatizing to the heat is crucial to performing well as the summer months roll around each year. The physiological adaptations your body makes to repeated training in hot conditions—larger blood volume, enhanced sweating, less cardiovascular strain, et cetera—not only help you perform well but may just also lower your chances of being plagued by gut disturbances during exercise.
Runners who don’t acclimatize to sweltering conditions can experience upper gut symptoms like nausea, bloating, and burping at rates that are 2–4 times higher when they exercise in hot conditions as compared to when they exercise in temperate environments. Although there are several explanations for this observation, a drop in gut blood flow is perhaps the most important one. A study published over 50 years ago showed that exercising in super-hot conditions (110°F) diminished blood flow to the liver (which reflects gut blood flow generally) more so than exercising in moderate conditions (78°F). This is partly because a notable share of blood flow is diverted to the skin in hot conditions, leaving table scraps for the gut.
The details of any heat acclimatization plan will vary depending on the athlete’s experience, goals, and access to resources. For more context on how to implement heat acclimatization training, check out this PodiumRunner article.
5) They Fast or Under-Eat
You don’t need a scientific study to tell you that overeating—especially within 30–60 minutes of starting intense exercise—is a recipe for digestive difficulties. (If for some reason you doubt this, check out this clip from The Office.) What’s perhaps less intuitive is that undereating or outright fasting before exercise can also provoke certain gut symptoms, the most prominent of which is nausea.
Surprisingly, few studies have looked at how working out on an empty tank influences gut perceptions despite the growing popularity of fasted training. Even so, we do have a decent amount of circumstantial evidence from other arenas that can help inform us. Another situation where people often fast is before surgery, which is usually done to lower the risk of aspirating foodstuffs into the lungs under anesthesia. Traditionally, patients getting surgery are told to fast overnight, which typically equates to at least 8–12 hours sans eating. More recently, however, some of these guidelines have shifted, in part because it’s been observed that long fasts actually worsen perceptions of nausea before and after surgery.
Obviously, these studies aren’t completely applicable to exercise, but they do bolster anecdotes of heightened nausea that some people report with fasted workouts. Plus, one study has shown that doing high-intensity cycling for 20 minutes provokes more nausea if a person fasts overnight in comparison to exercising 60 minutes after eating.
Exactly why fasting may inflame nausea is still being explored, but one of the likeliest explanations is that it boosts the secretion of stress hormones like adrenaline into your bloodstream. These hormones can act on receptors in the brain to induce nausea. Coincidentally, a rise in these hormones in the blood may help explain why super-stressful life events and high-dose caffeine ingestion are also capable of exacerbating nausea in certain people.
Patrick Wilson, PhD, RD, is a registered dietitian, associate professor of exercise science, and author of The Athlete’s Gut: The Inside Science of Digestion, Nutrition, and Stomach Distress. He has published over 50 peer-reviewed scientific articles, many of which are on the causes of gut issues in endurance athletes.