The Causes of Runner's Diarrhea and What You Can Do About It

Few things are more humbling (read: humiliating) than your plain-sailing run being suddenly interrupted by a gurgling noise in your stomach and a feeling that you have to poop, now. And this urgent need to drop a watery number two mid-jog is pretty common: In fact, 62 percent of distance runners have stopped their training to have a bowel movement, according to a small survey published in the Journal of Clinical Gastroenterology.

So, what causes runner's diarrhea, and is there anything you can do to prevent it? Here, Greg S. Cohen, M.D., a gastroenterologist and a clinical associate professor of medicine at Northwestern University, breaks down everything you need to know, including the factors that may up your odds of experiencing runner's diarrhea and steps you can take to reduce your risk.

What Is Runner's Diarrhea?

In case the name didn't clue you in, runner's diarrhea is an acute form of diarrhea (read: loose, watery bowel movements) that develops during or immediately after a run, according to the Mayo Clinic. Along with those liquidy stools, some athletes may experience abdominal cramps, nausea, vomiting, or rectal bleeding after racing or particularly tough runs, according to the Journal of Clinical Gastroenterology survey.

The Potential Causes of Runner's Diarrhea

Currently, runner's diarrhea is largely an understudied topic, and there isn't much research into the causes, says Dr. Cohen. That said, experts do have a few theories. For one, the physical jostling of your organs as you pound the pavement or treadmill belt could lead you to need to hit the restroom ASAP, he says.

Your autonomic nerves, which control the speed at which the muscles in your GI tract contract, may also play a role, says Dr. Cohen. "We know in other circumstances that stress and anxiety can trigger autonomic nerves to speed up the GI tract, creating cramps, urgency, and diarrhea," he explains. "It's purely speculative, but presumably, vigorous exercise like running could have similar effects on autonomic nerves and result in similar kinds of outcomes." In other words, as you dash down the sidewalk, your GI tract's muscles may contract more vigorously, potentially leading you to need to drop a watery deuce.

Changes in blood flow during your run can also impact your GI tract and contribute to runner's diarrhea. During intense exercise, blood flow to contracting muscles increases in order to supply them with more oxygen, which is needed to produce energy (aka ATP), research shows. In turn, blood flow to your GI tract is reduced, says Dr. Cohen. In extreme cases, some folks may develop ischemic colitis — inflammation of the colon caused by a lack of blood flow, which may lead to bloody diarrhea. "Usually once a year or so, I'll see a few patients around leading up to the Chicago Marathon who, once they get into their real distance runs, they'll present with ischemic colitis," he adds. "...So it stands to reason that maybe some of the milder runner's diarrhea is related to less extreme blood flow shifts that could trigger irritation in the GI tract but on a more minor level."

The final potential player? Your diet, specifically if it includes foods that are high in FODMAPs, says Dr. Cohen. FODMAPs include fermentable oligosaccharides, disaccharides, monosaccharides, and polyols — short-chain carbohydrates that are poorly absorbed during digestion and are highly fermentable, according to the American College of Gastroenterology (ACG). When these carbohydrates are fermented (turned into gas and chemicals) in the colon, they can cause bloating, cramping, and changes in bowel habits and may exacerbate GI symptoms in folks with sensitive GI tracts, according to the ACG. "A low-FODMAP diet is one of the most well-researched diets that has a [positive] impact on irritable bowel syndrome [IBS] symptoms," adds Dr. Cohen. "So avoidance of foods that are high in FODMAPs is speculated to be helpful for runner's diarrhea."

Factors That May Increase Your Chances of Having Runner's Diarrhea

While it's possible for anyone to experience runner's diarrhea, folks who suffer from IBS — a chronic condition that can lead to constipation, diarrhea, or both — may be more likely, says Dr. Cohen. "I view runner's diarrhea as a micro-version of IBS, so if your someone who has IBS, you're for sure going to be more prone to runner's diarrhea," he adds.

The intensity of the run, based on your personal fitness level, could also come into play. For example, if you usually avoid cardio at all costs and then suddenly decide to go for a mile-long, fast-paced run, you may be more likely to experience runner's diarrhea than, say, a highly trainedathlete completing the same workout, says Dr. Cohen.

How to Treat Runner's Diarrhea

Diarrhea, in general, usually clears up without any treatment, but you'll want to stay on top of your fluid intake and consume electrolytes (think: sodium, potassium, magnesium, chloride, and calcium) to prevent dehydration, according to the National Library of Medicine. Reminder: Electrolytes are required to maintain fluid balance, muscle contractions, and neural activity. Sodium, in particular, is lost through sweat, and when you're running low on the nutrient, you may also experience muscle cramping and dizziness, according to the American Council on Exercise.

You should also steer clear of dairy products for 24 to 48 hours, which can make diarrhea worse. And if your symptoms are a bit more severe (think: you're also experiencing some cramping or stomach pain), you can take over-the-counter anti-diarrhea medications to get them under control, according to the ACG. That said, you'll want to chat with your doctor if your runner's diarrhea is bloody, as this can be a sign of ischemic colitis, says Dr. Cohen. "This is more of a serious issue and could potentially require some IV fluids to aggressively rehydrate you," he explains.

How to Prevent Runner's Diarrhea

If you're worried about having to make a pit stop mid-run, consider nixing high FODMAP foods, such as wheat, milk, and legumes, from your diet in the day or two leading up to your run, suggests Dr. Cohen. Ensuring you're properly hydrated and consuming electrolytes can also help keep runner's diarrhea at bay, as it may "mitigate against blood flow changes that deprive the GI tract of blood when you run." If you do chat with your health-care provider about your symptoms, you may be prescribed anti-spasmodic medications to take before running to suppress cramping, he adds.

You can also try dialing back your workout's intensity, acclimating to it, then slowly increasing the length or intensity of your sessions to prevent runner's diarrhea, but it's not a guarantee, says Dr. Cohen. "On average, I do think that if someone is [taking those steps] they're going to be better," he says. "But there's a lot that we don't understand about IBS — and I'm calling runner's diarrhea a very specialized subcategory of IBS. Sometimes people go through a period of days or weeks or months where their symptoms are flaring up with no particular reason or logic behind them." And it's possible those same incomprehensible flare-ups of runner's diarrhea can also take place, he adds.

While feeling like you're going to poop your pants a mile into your run may hurt your pride, there is good news: "Runner's diarrhea is not really a dangerous set of symptoms by itself," says Dr. Cohen. "Just having some runner's diarrhea doesn't mean you're at risk of a serious medical problem."