GI Issues in Focus: Research on Gastrointestinal Problems During Running and What You Should Do About It

“Joggers trots.”  “The runs.”  Whatever you call it, gastrointestinal problems are no stranger to many runners.

Though they might not be pleasant to talk about, anyone who has experienced them knows how much trouble they can cause, both in workouts and in races.  This week, we’ll be taking a scientific look at intestinal problems of both the upper and lower gastrointestinal (GI) tract.

Upper GI tract issues include heartburn, nausea, and abdominal cramps, while diarrhea and urgent bowel movements are characterized as  lower GI tract issues.  We’ll look to the research to find out if these kinds of problems can be predicted or avoided.

Prevalence of GI problems

To establish the prevalence of GI problems during running, we will turn to a 1988 survey study by Riddoch and Trinick at Queen’s University in Ireland.  They questioned 471 of the roughly 1,800 finishers of the Belfast City Marathon on various gastrointestinal problems they encountered in training.

In all, 83% of the respondents reported experiencing GI problems at least occasionally during training, with diarrhea and urgent bowel movements being the most common.

These issues, along with upper GI tract disturbances like nausea and heartburn, were also more common in women than men.

The runners felt that their gastrointestinal problems were more frequent and more severe during longer and harder runs.

Finally, younger runners seemed more susceptible to GI issues than older runners.

Looking for answers on why runners have GI issues

Though Riddoch and Trinick’s surveys revealed some useful information on diarrhea, urgent bowel movements, and other GI problems in runners, they conceded that there was little more than speculation on the causes of these issues.

Scientific papers that followed have taken a more cause-oriented approach to GI problems in athletes; one of particular interest to us is a 1990 study by N.J. Rehrer and colleagues at the University of Limburg in The Netherlands.

In their study, 16 male runners underwent an intensive series of tests designed to monitor how their stomach processed fluids and nutrients during exercise.  The subjects consumed a “liquid meal” that was high in carbohydrates and low in fats and protein before embarking on a two hour treadmill run at an easy pace.

During the course of this run, the researchers measured how fast the liquid meal was emptied from the runners’ stomachs—a parameter known as the gastric emptying rate.

Two variants of the experiment were conducted; one where the subjects were well-hydrated, and one where they were intentionally dehydrated during the run by increasing the temperature and humidity of the laboratory.

Rehrer et al. found that when the athletes were dehydrated and running in the heat, their stomachs emptied significantly slower and a large proportion of the subjects complained of GI troubles.

Conclusion

While tangling apart cause and effect is difficult given the design of the experiment, Rehrer et al. nevertheless concluded that:

• Slower gastric emptying, coupled with dehydration and heat exposure, are plausible culprits for gastrointestinal disturbances in runners.

• A contributing factor is also bloodflow to the intestines during running.  As described by H. P. F. Peters and other researchers in a 2001 review article, the body diverts blood that would otherwise be used by the intestines to the muscles and to the skin.

• This could also explain the worsening of GI issues in the heat—when it’s hot, your body needs to divert more blood to the skin to prevent overheating.

Unfortunately, because Rehrer et al. didn’t isolate the effects of heat and dehydration, it is difficult to confirm this.  It may also be that water is prioritized for sweating and cooling off instead of aiding digestion, further confounding the issue!

Peters et al. also point out that the mechanical bouncing associated with running is strongly suspected to play a role in the relatively high incidence of GI problems when comparing running to lower impact sports like cycling or swimming.  Unfortunately, there’s been no research as of yet into this factor.

Suggested GI prevention methods

• When it comes to prevention, the evidence to date suggests that you monitor what you are eating and ensure that you are well hydrated.  A 2011 review study by Erick Prado de Oliveira and Roberto Carlo Burini in Brazil cites studies, which demonstrate an increased incidence of GI problems during running when the drinks and meals that preceded the exercise were high in fiber, fat, protein, or concentrated sugars.

• Meat, fibrous foods, and concentrated sports drinks or gels should all be limited or avoided before and during runs if you have a history of gastrointestinal problems.

• While concentrated sports drinks and gels are efficient from a fuel intake standpoint, they are what physiologists call hypertonic—they have a greater concentration of sugar and electrolytes than the ambient fluids in your body, effectively “pulling” water from your body into your stomach fluid.

• Drinks with more than a 10% concentration of carbohydrates are associated with cramps, diarrhea, and nausea, and Prado de Oliveira and Burini recommend that fluids be at or below 5% carbohydrate concentration to avoid GI problems.  For reference, “standard” Gatorade is around 6% carbs.  Watering it down 1:1 or 1:2 with water can help if you have a sensitive stomach.

Summing it up

In sum, gastrointestinal problems like nausea, diarrhea, bowel movement urgency, and cramping affect a large percentage of runners, both recreational and competitive.

• Long and hard sessions of running exacerbate the problems, probably due to a combination of the greater dehydration, blood demand, body heat buildup, and mechanical oscillation that occur during long, high-intensity runs.

• Meals that are high in fiber, fat, and concentrated carbohydrates will also cause problems, and you should be especially careful in the summer—hotter temperatures outdoors will essentially recreate Rehrer et al.’s lab experiment, causing dehydration and higher body temperatures, which are both linked to higher rates of GI problems.

• Women in particular appear to be more susceptible to GI issues while running, though the reasons behind this are not clear.  Regardless, staying hydrated, eating bland, carbohydrate-based meals that won’t upset your stomach before you run, and switching to less-concentrated sports drinks (perhaps simply by watering them down) will go a long ways towards preventing gastrointestinal distress when you run.

You may have to experiment for a while to find out the perfect formula for yourself, but once you do, it will be one less thing to slow you down.