As a friend and I dipped into the bushes for our fourth pre-race bathroom break one morning, he asked, “How do our bowels know we’re racing today?” The answer, a new study suggests, could be that our innards were bearing the brunt of what was going on in our minds.

The research, published in the Journal of Sports Sciences, found that runners’ gastrointestinal problems on training runs and in races were significantly correlated with the runners’ stress and anxiety levels. Put another way, even when other known causes of GI issues were accounted for, such as caffeine and workout intensity, runners who reported more stress and anxiety in daily life were more likely to have flatulence, stomach distress, mid-run pit stops, and so on while running. This study was the first of its scale (150 runners) and length (30 days) to gather data on a topic that some runners have ample anecdotal evidence on.

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For the study, 76 women and 74 men between the ages of 18 and 65 kept detailed journals for 30 days of their runs (duration, intensity, frequency), GI problems during their runs, and levels of perceived stress and anxiety. The subjects were dedicated runners who did at least 32 kilometres per week and had run at least one 16-kilometre run in the month before the study.

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The runners rated their level of GI distress for each run on a scale of 0 to 10, with 0 being “no discomfort,” 5 being “moderate discomfort,” and 10 being “severe discomfort.” They reported having at least one symptom of GI discomfort at a level of 3 or more on 45 percent of their runs.

One of the strongest links to GI issues was run intensity – the harder the effort, the more likely the runners reported at least one GI symptom. This finding is consistent with, well, pretty much all previous research and the experiences of everyone who has done a speed workout too soon after eating. The faster (and, eventually, farther) you run, the more that blood is diverted to working muscles from other, temporarily less-important body parts, such as those involved in digestion.

But here’s the striking result from this study: The runners’ reported level of stress and anxiety was the other top factor linked to on-the-run GI problems. The correlation between stress and GI problems while running was greater than commonly addressed culprits such as caffeine, medication, and carbohydrates consumed on the run. In fact, of all the contributing factors tracked, the strongest link was between daily stress and anxiety and upper GI distress (nausea, regurgitation/reflux, and stomach fullness).

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To be clear, the runners weren’t reporting feeling stressed about their running. They responded to questions on the Perceived Stress Scale (PSS) and the Beck Anxiety Inventory (BAI), two standard tools used to measure perceived stress and anxiety in daily life. The PSS cover subjects such as, “How often have you been upset because of something that happened unexpectedly?” and “How often have you felt that you were on top of things?” The BAI deals mostly in physical manifestations of anxiety, such as racing heart, difficulty breathing, and feeling flushed, but also includes psychological symptoms such as feeling afraid and being unable to relax. Both are based on people recalling their daily lives, not a specific activity such as work or, in this case, working out.

That’s not to suggest that stress about running stress might not be a factor in this study’s findings. “I would say that if an athlete experiences gut distress before, during, or after other stressful or anxiety-provoking events (an interview, public speaking, a date, etc.), then it makes it more likely that her GI symptoms with running are stress- and anxiety-related,” Patrick Wilson, Ph.D., an assistant professor at Old Dominion University and the study author, wrote in an email.

How do you know whether it’s your boss or your existential angst contributing to your stomach problems? Wilson says that GI symptoms can occur for many reasons, so you’ll need to rule out other causes by a process of elimination. “One way to narrow down the list of causes is to document when, and under what circumstances, the symptoms occur. It would also be useful to document the specific symptoms occurring. If an athlete is experiencing flatulence after eating milk or yogurt, for example, then that would indicate a problem with lactose intolerance,” says Wilson, who is also a registered dietician.

If you do decide that daily life is causing your running GI issues, then try experimenting with practices to reduce your overall stress levels. A couple short sessions of yoga, deep breathing, or meditation per week can make a significant difference for many people. If your symptoms are more anxiety-related, you might benefit from seeing a psychologist or other counselor who can teach you cognitive behavioural therapy, which helps you challenge automatic thoughts that can lead to undesirable behaviours. Having more enjoyable runs thanks to experiencing less stress sounds like one of the better gifts you can give yourself.

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