The Science Behind How Stress Affects Your Gut Function

Plus, ways to avoid stomach upset when running and stressed.

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One of the most powerful ways that exercise affects your digestive tract is by constricting gut blood flow. Both intense and prolonged exercise result in a profound shift in blood flow away from your gut, toward your skeletal muscles and—especially in hot and humid environments—toward your skin. As it turns out, psychological stress can also redirect blood flow away from your gut. (Although if it’s only mild in nature, stress has little impact on gut blood flow.)

Over the past few decades, advancements in ultrasound technology have allowed scientists to noninvasively probe how gut blood flow changes with a variety of stressors, including those that are psychological in nature. However, you might be surprised to learn that these shifts in gut blood flow were documented almost two hundred years ago. In 1822, a Canadian fur trader named Alexis St. Martin was accidentally shot with a musket, resulting in a hand-sized wound that, once healed, still left an opening large enough that a person could see directly inside St. Martin’s stomach. William Beaumont—the physician stationed at the site of the accident—initially treated St. Martin but didn’t think he would survive. St. Martin did manage to pull through, and Beaumont saw an extraordinary opportunity to study what had long been out of the reach of scientists: the inner workings of human digestion.

Of course, Beaumont’s dreams of studying the mysteries of digestion and becoming a world-renowned gastric physiologist hinged on St. Martin’s willingness to serve as more or less a human guinea pig. Fortunately for Beaumont, St. Martin was unable to pay his hospital bills and agreed to work and live in Beaumont’s home as an indentured laborer. As a part of this Odd Couple–like arrangement, St. Martin allowed Beaumont to conduct more than two hundred gastral experiments on him over roughly a decade. Among other things, Beaumont inserted and extracted small muslin bags filled with various foods into and from St. Martin’s stomach and even stuck his tongue inside the hole in St. Martin’s side. (How else would you figure out how the inner coating of the stomach tastes?) Beaumont’s mini-studies—the results of which were published in his book Experiments and Observations on the Gastric Juice and the Physiology of Digestion—were pioneering in that few others had successfully probed the process of digestion in humans. Beaumont’s book paints St. Martin as somewhat of a cantankerous figure, although if you keep in mind that he was subjected to hundreds of mildly unpleasant experiments, you can hardly blame St. Martin for his irritability. This may also help explain why St. Martin was prone to overindulging in alcoholic beverages on occasion. In one entry, Beaumont writes:

St. Martin has been in the woods all day picking whortleberries... Stomach full of berries and chymifying ailment, frothing and foaming like fermenting beer or cider; appears to have been drinking liquor too freely.

Among the discoveries that arose from his experimentation on St. Martin, one of Beaumont’s most interesting revelations was that a person’s psychological state influences how much blood flows to their stomach. Specifically, Beaumont observed that “fear, anger, or whatever depresses or disturbs the nervous system—the villous coat becomes red and dry, at other times, pale and moist.” These changes in appearance from red to pale (and vice versa) were almost certainly the result of alterations in blood flow to St. Martin’s visceral organs. In St. Martin, Beaumont had, quite literally, a gastric window into the man’s state of mind.

Over the next 150 years, other cases of patients with permanent openings into their digestive tracts corroborated Beaumont’s observation that mood state impacts gut blood flow. One such patient, a man known as Tom Little, had an artificial opening into his stomach surgically created after his esophagus was permanently damaged from swallowing scalding hot clam chowder at the age of 9. In 1941, a pair of physicians, Stewart Wolf and Harold Wolff, convinced the then 56-year-old Little to serve as a subject in a series of tests focusing on psychosomatic changes and gastric function. Stewart Wolf later recalled the following about their experimentations on Tom Little:

The exposed gastric mucosa of Tom made it possible to observe the vascularity of the stomach... In Tom and in other fistulous subjects... we found that fright, depression, and attitudes of being overwhelmed were associated with pallor of the mucosa.

When you pair the information gleaned from historical cases like Alexis St. Martin and Tom Little with the revelations from contemporary studies, it’s impossible to deny that an individual’s psychological state influences blood flow to the gut.

Aside from its effects on gut blood flow, psychological stress can also influence smooth muscle activity in the gut, otherwise known as motility. In particular, stress can tone down stomach motility, and one of the first scientists to document these alterations was Walter Bradford Cannon, a leading American physiologist of the early 20th century. During some of his experiments, Cannon noticed that young male cats were restive when restrained while older female cats remained mellow, and that these differences in temperament corresponded to differences in stomach activity. He further observed that “by covering the cat’s mouth and nose… until a slight distress of breathing is produced, the stomach contractions can be stopped at will.” In an experiment conducted years later, Cannon went so far as to place a barking dog near a restrained cat and then proceeded to take blood samples from the cat to see what effect the “excited blood” had on a strip of intestinal muscle. (It caused a relaxation, or a reduction in motility.) As it relates to athletes, attenuated stomach motility could exacerbate upper gut problems like nausea, fullness, bloating, and reflux, especially when they’re trying to consume sizeable quantities of food or fluid in and around the time of competition.

In contrast to the dampening of stomach activity, stress can result in a livelier colon. Early experiments undertaken by physician Thomas Almy at New York Hospital in the 1940s and 1950s documented that simply discussing emotional topics could abruptly throw the colon into spasm. In a 1949 article, Almy describes measuring colon motility in patients as the researchers led them through discussions of unsettling life events. Almy used a latex balloon placed in the colon to measure pressure changes, which gave him an indication of muscle activity. In a constipated German housewife, for instance, colon contractions intensified when, as the researchers put it, she “expressed resentment over her husband’s ability to have regular bowel movements.” (Of all the things you could take umbrage at your spouse for, focusing on his propensity to defecate seems peculiar.) In another case, the colon of a 26-year-old man went into a frenzy after he revealed that a woman he had been seeing “humiliated and scorned him for his inability to satisfy her sexually.”

In another article, Almy and his colleagues describe going to even greater lengths to manipulate colon function in an unlucky medical student referred to simply as L. L. After positioning a proctoscope in L. L.’s lower colon, the researchers carried out an elaborate deception to make him believe he had a potentially cancerous lesion. The researchers told L. L. that they had to take a biopsy (even though they never took one), and during this time the motility of L. L.’s colon became progressively more intense. After 20 minutes of dupery, the researchers finally fessed up and told L. L. the procedure was fake. In the associated paper, Almy and his coauthors state that L. L. accepted their reassurances that nothing was truly wrong and that L. L. held no “resentment for the . . .anguish he had been through.” Despite Almy’s proclamation, something tells me L. L. wasn’t completely bitter-free about this skullduggery.

The effects of stress on colon motility observed in Almy’s eccentric experiments have largely been confirmed in more contemporary studies. To put it plainly, psychological stress can make the colon go wild. It’s somewhat puzzling, then, that the symptoms arising from a more active colon vary drastically. In many cases, cramping and urges to go numero dos are stimulated. In others, constipation and bloating are more prevalent. Thinking about it logically, one would assume that frequent and strong contractions of smooth muscle would translate to a heightened urge to poo, but it really depends on whether those contractions are coordinated and propulsive or of a spastic variety. (Experiencing spastic, uncoordinated smooth muscle contractions in your colon could mean you’re headed for constipationville.) That said, anecdotes about having to defecate before stressful competition seem to be reported with more regularity than constipation.

Interestingly, humans have the power to consciously override nerve-induced urges to poop in many situations, but animals that are of a less-inhibited nature are much less capable. As it turns out, rats are particularly inclined to poo when stressed, and, anecdotally, exposing a rat to a new environment (such as a maze or a living space) is a good way to get said rat to empty its bowels. If you’re not big on anecdotal evidence, you’re in luck, as an experiment carried out in the 1930s found strong correlations between rats’ emotional responses (measured via their willingness to eat in a strange enclosure) and their proclivities for defecating. Much like rats in a cage, the psychological stress and anxiety experienced by athletes before endurance races is undoubtedly one reason lines for Porta-Johns are often as long as those for the latest and greatest ride at Disney World. Unlike rats, though, humans can usually keep it in long enough to avoid making a public mess.

There are a few physiological explanations for why reduced stomach motility and amplified colon activity accompany acute psychological stress. One of the most consistent scientific findings is that a hormone called corticotropin-releasing factor (CRF) contributes to the ebbs and flows of gut activity. Your body responds to stress by secreting CRF from your hypothalamus; CRF then binds with receptors in your brain that modify the activity of your autonomic nervous system, the primarily unconscious part of your peripheral nervous system that helps control gut function. Across a variety of studies, directly administering CRF to animals dampens gastric motility and acid secretion while it also spurs colon motility. The role of CRF in provoking gut symptoms with psychological stress is presented in the figure below.

Illustration on how acute stress affects the gut
Moderate-to-severe acute stress triggers the release of CRF from the hypothalamus, which modifies gut activity through the fibers of the autonomic nervous system. Illustration: Andrew J. Nilsen

During times of extreme psychological stress, these motility changes and their associated symptoms become commonplace. Few situations are as stressful as what soldiers experience during combat; in contrast to the run-of-the-mill pre-competition jitters experienced by many athletes, absolute fear is the emotion soldiers often feel before and during combat. In some cases, this fear is associated with spontaneous defecation; in an article in a long tome titled Encyclopedia of Violence, Peace, and Conflict, the authors report that one-quarter of World War II veterans admitted to defecating in their pants during combat. In these situations, CRF secretion almost certainly contributes to a colon and rectum that are—to put it in layman’s terms—massively overstimulated.

Urgent impulses to poo and the nervous shits are common manifestations of acute severe stress. With respect to the other end of the gut, nausea represents an equally troublesome stress-invoked symptom. The secretion of CRF plays a role in triggering said nausea, but there’s another hormonal response that may be as equally influential: the release of catecholamines. High-intensity exercise, altitude, and—most directly—injecting catecholamines all increase blood catecholamine levels and can provoke nausea. Acute psychological stress is also a source of catecholamine secretion. Many studies have demonstrated psychological stress’s effect on catecholamine secretion, but I’ll focus on one that used what’s called the Trier Social Stress Test, a task that’s become the gold standard for examining psychosocial stress in the lab. Though variations exist, the Trier Social Stress Test typically subjects participants to a mock presentation in front of a panel (often presented as a job interview), followed by a challenging arithmetic task (e.g., counting backward from a large number by increments of 13). In the example study I’m focusing on, participants had plasma levels of the stress hormone norepinephrine (also called noradrenaline) measured before and after the Trier Social Stress Test, and as expected, norepinephrine levels shot up, with levels ending up about twice as high as they were beforehand.

The Trier Social Stress Test is a reliable method of inducing stress in a laboratory, but it’s sometimes criticized because it doesn’t reflect the diversity of tense and trying situations that people encounter in everyday life. With this in mind, researchers have assessed the catecholamine response to stress in more naturalistic situations—academic exams, a visit to the dentist’s office, even skydiving. Specific to sports, a slew of investigations have evaluated stress responses to real-life competition. Take for example a study that contrasted the stress responses of elite German tennis players under practice and tournament conditions; urine epinephrine (a.k.a. adrenaline) levels were about twice as high two hours before a tournament than in practice.

Despite the fact that elevations in catecholamines are nearly ubiquitous in stressful situations, not all athletes experience nausea before competition. Although the stress response to athletic competition isn’t usually potent enough to induce nausea and vomiting in most athletes, there will always be exceptions. And of course, the bigger the stage, the more likely an athlete will experience stress severe enough to trigger nauseousness. In Without Limits, the definitive running biopic about legendary runner Steve Prefontaine, there’s a notable scene in which Prefontaine pukes under the bleachers before the 3-mile race at the 1974 Hayward Field Restoration Meet, which was renamed the Prefontaine Classic after his tragic death. In the scene, Prefontaine laments that he doesn’t think he can hang with the likes of Frank Shorter and Gerry Lindgren anymore, even as the crowd chants his name. The scene perfectly sums up what countless athletes have felt in the moments leading up to their biggest competitions.

If you do happen to perpetually suffer from stress-induced nausea, you could consider implementing the following strategies to dial down the magnitude of catecholamine release in the hours leading up to competition:

  • Shun caffeine and other stimulants.
  • Avoid dehydration and hypoglycemia by consuming adequate amounts of fluid and carbohydrate beforehand.
  • Consult with a sports psychologist or other qualified mental health professional, as they can help you implement a variety of strategies—from breathing exercises to meditation to cognitive behavioral therapy—that could blunt catecholamine release prior to competition.

I want to be clear that these strategies are based primarily on indirect evidence. Even so, there are probably minimal risks in trying them. On a final note, one other gut function change that can stem from psychological stress is heightened gut permeability. While it’s transient and relatively harmless in many situations, gut leakiness may, in some cases, contribute to heat illness during and after prolonged exercise. Psychological stress increases gut leakiness via the actions of stress hormones like CRF as well as through its actions on various components of the immune system. Much like with stress-induced nausea, it’s conceivable that interventions such as deep breathing and meditation could blunt the stress response to competition and subsequent onset of gut leakiness, though I’m not aware of research that’s documented those effects specifically.

Adapted from The Athlete’s Gut by Patrick Wilson with permission of VeloPress.

The Athlete’s Gut