Let’s be real, marathons are gross affairs. And while reaching the finishing line (and slapping that 26.2 bumper sticker on your car, your Instagram, and all over your disinterested friends’ and coworkers’ lives) is a crowning achievement for many athletes, they are often stained and smeared with less-triumphant moments.

(By the way, I’m talking about shitting yourself.)

While most people can tell you that exercise and tummy aches go hand in hand, the science surrounding the phenomenon was as flimsy and scattered as a sprinter’s intestines. But a team of Australian scientists compiled the best research on the subject to present a definitive review on exactly how strenuous activity damages your gut and, thankfully, how you can fight back.

In doing so, they evaluated dozens of scientific experiments, including one 1997 study that showed that along with permeability of the small intestine, rectal temperature does in fact increase, along with exercise intensity. The review was published Wednesday in Alimentary Pharmacology & Therapeutics.

Some things are still unknown, like how strenuous exercise affects people with irritable bowel syndrome who supposedly benefit from taking a light jog every now and then. But it turns out that serious workouts, especially running for a long time, do a great deal of damage to your insides.

Bad news for the gym bros out there — this happens to everyone, regardless of fitness level. The threshold for intestinal and digestive harm is based more on how much of an effort you’re making, measured by calculating how efficiently you’re replacing the oxygen in your system, which could be different for different people. In particular, most symptoms appeared after about two hours of exercise at about 60 percent efficiency.

chart graph pain marathon ultramarathon gastrointestinal distress poopy
The symptoms felt by runners in a nonstop 24-hour ultramarathon.

The scientists evaluated claims related to exercise and gastrointestinal permeability and injury, endotoxemia (the proliferation of bacteria in the bloodstream), gastrointestinal motility (see aforementioned pants-shitter), and finally how nutrients aren’t absorbed as well during exercise. In short, all of that stuff gets screwed up.

Once they hit that 2 hour, 60 percent threshold, studies show that runners tend to have a high concentration of a protein called Intestinal Fatty Acid-Binding Protein in their system. The protein is prevalent in parts of the body that aren’t getting enough blood delivered to them. When the exercise occurred in hot weather, the protein’s concentration was even higher.

The studies surveyed for this new literature review concluded that runners generally faced more gastrointestinal symptoms than people doing other forms of exercise. This could in part be because running involves constantly flexing those washboard abs of yours — squeezing all your guts — to hold yourself upright.

But this gives runners, especially marathon and ultramarathon runners, a whole big pile of shitty symptoms to deal with, including vomiting, bloating, pain, gas, heartburn, flatulence, and an undying urge to shit, often resulting in diarrhea or blood loss. In fact, that last bloody mess is one of the top reasons why people withdraw from marathons and other long races.

Also, when your body is at its highest temperatures and most dehydrated, your intestine becomes more permeable, allowing harmful bacteria normally contained in the gut to flow into your bloodstream.

graph chart table gastrointestinal distress marathon run exercise poopy :)
Don't worry, it's not just you.

Fortunately, these symptoms are generally short-term and reversible. Once you stop exercising and take some time to recover, you’ll be back to eating and pooping like a champ. But during your exercise, you may find that it’s hard to go. Scientific research suggests low or moderate activity for a shorter period of time might help jog your system along, but anything more strenuous or longer-lasting will have you stopped up.

To help things move along, you should drink lots of water. That’s the number one way to stave off distress, both gastrointestinal and psychological, from crossing the finish line with a big ol’ brown streak going down your leg. Back when I was an NCAA athlete, I had a teammate who didn’t want to drink any water during a competition because she didn’t want to go to the bathroom. To no one’s surprise, she lost right away. Marathon runners who stayed hydrated to a reasonable extent suffered fewer intestinal injuries and had a lower intestinal permeability, meaning fewer bacteria got into their blood.

Also, research suggests that eating small amounts of carbs throughout the workout could help prevent injury and flush out your system. On the other hand, eating protein during exercise just made everything way worse. Just about any sort of sports-based extreme diet also tended to exacerbate problems just because of how abnormal they were.

So while there are some ways to alleviate the pain during extreme exercise, you’re probably going to have some sort of tummy troubles. Just try to keep your chin up and remember that it’s all temporary. And if you find yourself in a dire situation, do your best to hold it in and remind yourself that this too shall pass.

Abstract: Background: “Exercise-induced gastrointestinal syndrome” refers to disturbances of gastrointestinal integrity and function that are common features of strenuous exercise. Aim: To systematically review the literature to establish the impact of acute exercise on markers of gastrointestinal integrity and function in healthy populations and those with chronic gastrointestinal conditions. Methods: Search literature using five databases (PubMed, EBSCO, Web of Science, SPORTSdiscus, and Ovid Medline) to review publications that focused on the impact of acute exercise on markers of gastrointestinal injury, permeability, endotoxaemia, motility and malabsorption in healthy populations and populations with gastrointestinal diseases/disorders. Results: As exercise intensity and duration increases, there is considerable evidence for increases in indices of intestinal injury, permeability and endotoxaemia together with impairment of gastric emptying, slowing of small intestinal transit and malabsorption. The addition of heat stress and running mode appears to exacerbate these markers of gastrointestinal disturbance. Exercise stress of ≥2 hours at 60% VO2max appears to be the threshold, whereby significant gastrointestinal perturbations manifest, irrespective of fitness status. Gastrointestinal symptoms, referable to upper and lower-gastrointestinal tract, are common and a limiting factor in prolonged strenuous exercise. While there is evidence for health benefits of moderate exercise in patients with inflammatory bowel disease or functional gastrointestinal disorders, the safety of more strenuous exercise has not been established. Conclusions: Strenuous exercise has a major reversible impact on gastrointestinal integrity and function of healthy populations. The safety and health implications of prolonged strenuous exercise in patients with chronic gastrointestinal diseases/disorders, while hypothetically worrying, has not been elucidated and requires further investigation.

Photos via Monash University