5 Common Mistakes Runners Make That Mess with Their Gut, According to a Dietician

Runners often experience gastrointestinal issues during training and competition, and there are likely multiple potential explanations for each. Indeed, there are numerous possible causes, which is why I wrote a book on the topic. Here are five common mistakes runners make that trigger gut problems.

1. Taking Too Many Pain Relievers to Soothe Aching Muscles

It’s no surprise that many runners struggle with daily aches and overuse injuries. Approximately 50 percent of runners experience an injury once a year. The majority of these injuries involve the knee, shins, and ankles. Running takes a toll on the lower back, too.

Surely these figures aren’t a revelation to any veteran runner or running coach, but they do help explain why the rate of anti-inflammatory medication use is astronomically high among this population. A 2020 survey of 806 runners recruited through Parkrun UK (an organization that facilitates weekly 5-kilometer runs across the United Kingdom) found that approximately 9 out of every 10 runners used a non-steroidal anti-inflammatory drug (NSAID) in the past year, and 80 percent had taken NSAIDs in the previous four weeks.

NSAIDs Can Lead to Cramping

While taking 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. Hydrating Too Much or Not Enough

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.

Dehydration Impairs Gut Function

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.

Overhydration Often Causes Bloating

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 percent versus 3.1 percent) and lowered heart rates by four 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.

How Often Should You Hydrate?

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 their thirst ingests 0.5 liter per hour and sweats 1 liter per hour during a five-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 runners, especially during relatively brief runs, 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. (You can calculate your sweat rate here.)

3. Ignoring 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 percent 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 existing studies on this topic.

The Link Between Anxiety and Gut Problems

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 the end of the month, 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 during which 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 percent 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.

4. Not Preparing for Hot Conditions

Acclimatizing to the heat is crucial for performing well as the summer months approach each year. The physiological adaptations your body makes to repeated training in hot conditions—larger blood volume, enhanced sweating, and reduced cardiovascular strain, among others—not only help you perform well but may also lower your chances of being plagued by gut disturbances during exercise.

Runners who don’t acclimatize to sweltering conditions can experience upper gastrointestinal symptoms, such as nausea, bloating, and belching, at rates that are two to four times higher when exercising in hot conditions compared to when exercising 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 in 1965 showed that exercising in extremely hot conditions (110°F) reduced blood flow to the liver (the liver breaks down the blood coming from the stomach and intestines into usable nutrients for the body) significantly 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.

5. Fasting or Under-Eating

You don’t need a scientific study to tell you that overeating, especially within 30 and 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 when Michael eats too much fettuccine alfredo before a race.) 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.

(Ahead of your next race, try a breakfast of oat bran with cherries and almonds. This meal is full of carbohydrates and won’t upset your stomach. Follow easy recipe step by step by clicking the video above.)

Not Eating Enough Makes You Nauseous

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 to 12 hours of no eating. More recently, however, some of these guidelines have shifted, in part because it has been observed that people who fasted for a shorter length of time reported feeling less nauseous ahead of their surgery, and that fasting for fewer hours did not pose a risk to patients undergoing 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 found that people who did intense exercise after fasting reported feeling nauseous.

Exactly why fasting may cause 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, 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.

Want more of Outside’s Health stories? Sign up for the Bodywork newsletter.