There are a bunch of different theories for why runners get GI problems.
There are a bunch of different theories for why runners get GI problems. (Photo: Scott Markewitz/Gallery Stock)
Sweat Science

Can Probiotics Help Marathoners with GI Distress?

New research suggests that a month of probiotic supplementation reduces GI symptoms in runners

There are a bunch of different theories for why runners get GI problems.

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Let’s skip the euphemisms and get straight to the point. There are six main gastrointestinal symptoms that plague marathon runners: bloating, nausea, urge to vomit, flatulence, urge to defecate, and stomach cramps. None of them helps you run faster. But it’s not entirely clear what causes them, or how they can be prevented.

As anyone who’s ever listened to the conversation (or other sounds) on a Sunday long run knows, these symptoms are surprisingly common. A study of participants in the Liverpool and Dublin marathons published last year found that 43 percent suffered from GI symptoms classified as moderate (at least 4 on a scale of 1 to 7) or worse in the week prior to the marathon, and 27 percent reported moderate or worse symptoms during the race itself. But that study didn’t find any connection between what the runners ate or drank and how likely they were to suffer symptoms.

Now the same research team, led by Graeme Close of Liverpool John Moores University, has a new study published in the European Journal of Applied Physiology in which they propose a potential countermeasure. They assigned 24 runners to take a daily pill—either a probiotic or a placebo—for 28 days, then run a full marathon on a standard 400-meter track. That’s pretty cruel, but it allowed the researchers to accurately monitor pace and in-race perception of GI symptoms in real time. The results were cautiously encouraging.

There are a bunch of different theories for why runners get GI problems. During prolonged exercise, blood flow is redirected away from your gut to your muscles (to supply oxygen) and skin (to dissipate heat). This reduced blood supply disrupts the intestinal barrier, allowing toxins to leak into your bloodstream and triggering an immune response. Trying to consume a lot of carbohydrates to stay fueled during a marathon or other long bout of exercise may also lead to malabsorption. Probiotics, some scientists believe, may help with both those problems, helping to maintain the intestinal barrier and boosting carbohydrate absorption.

The probiotic used in this study was a multistrain capsule from ProVen Probiotics containing 25 billion colony-forming units of four different strains: Lactobacillus acidophilus CUL60, Lactobacillus acidophilus CUL21, Bifidobacterium bifidum CUL20, and Bifidobacterium animalis subsp. lactis CUL 34. (And yes, you guessed correctly: the company that makes ProVen funded the study.) The placebo group got the same capsules, but with cornstarch inside. During the marathon itself, every 20 minutes the runners took a gel with 22 grams of carbohydrate along with 200 mL of water. That’s a lot of carbohydrate, but is consistent with the upper range of sports nutrition recommendations.

At the start of the experiment, the two groups reported equivalent levels of GI symptoms, and they stayed pretty much the same for the first two weeks of supplementation. But in the second two weeks, differences began to emerge. In the placebo group, there were no statistically significant changes in the frequency or severity of symptoms. In the probiotic group, on the other hand, there was a significant improvement. For example, the probiotic subjects had an average of three days with moderate or worse GI symptoms during the first two weeks, but just one day during the next two weeks.

During the race itself, a similar pattern emerged. There’s lots of individual variability, so it’s worth looking at the raw data. Here’s each subject’s average GI symptom score, on a scale of 0 to 10 (higher is worse), which was measured every 15 minutes and then averaged for the first, second, and third parts of the race. Black dots are probiotic subjects, white dots are placebo subjects.

(European Journal of Applied Phys)

There’s plenty of scatter, but looking at the averages, you see that there’s not much difference during the first two thirds of the race, when most people are feeling pretty comfortable. But in the final third, when the going gets tough, a difference of a few points emerges. It’s not a miraculous cure, but the average probiotic subject is feeling a couple of points better than the average control subject.

And that, it appears, is enough to make a difference in performance, too. Here’s the lap-by-lap average splits for the two groups:

(European Journal of Applied Phys)

Again, the two groups are initially pretty much indistinguishable. But sometime after the halfway mark, some daylight begins to open up and the placebo group has a more pronounced slowdown. If you again break the race into thirds, you find that it’s only in the final third of the race that the placebo group slows significantly more than the probiotic group relative to their initial pace. Causality is hard to establish, but it’s tempting to suspect that the slowdown in the control group is connected to their greater GI discomfort—and indeed, there’s a statistically significant correlation between each runner’s GI score and how much he or she slowed down in the final third.

There are a few caveats to point out. The authors note that “reductions in running speed during the final stages of a marathon can elicit negative emotions”—a very understated way of expressing how crappy it feels to hit the wall. It’s possible, then, that the miserable runners whose races had already gone off the rails for one reason or another were more likely to exaggerate their level of gut discomfort, creating a false association between GI symptoms and late-race slowdown. This doesn’t seem particularly likely to me, but it’s nice to see they were thinking hard about possible sources of error.

The researchers also took blood samples before and after the race and measured a whole series of markers related to gut function and immune response, and overall didn’t find much in the way of smoking guns to illustrate why runners get GI problems or what probiotics do to help. That means the mechanics remain unclear. It also means we don’t really have any meaningful answers about which probiotic strains might be best, and why. We just know that this particular combination seems helpful—which is convenient for the company that makes it, which (as noted above) funded the study.

To be clear, I think this is a solid study. (One of the methodological details I liked from the paper: “The randomization code was held by a third party, unlocked for analyses upon sample analysis completion.” I enjoy picturing a deep subterranean vault protected by lasers where the secrets of who got the real probiotic were kept.) And, taken with previous research, it bolsters the case that probiotics are worth considering if you’re an endurance athlete.

But you only get answers to the questions you ask. Do other probiotic supplement strains do the same thing? Can you get the same benefits from kefir and kimchi? We don’t know (and the subjects in this study were actually asked to refrain from eating probiotic foods like yogurt for the duration of the study). Until we understand more about the mechanisms of which probiotics actually work, that keeps it in the maybe column for me. But if you’re one of those data points with a GI symptom score of 6 or 7 out of 10 during the final miles of your marathons, you might weigh the evidence a little differently.

My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.

Lead Photo: Scott Markewitz/Gallery Stock

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