I'm preparing for my first marathon. Whenever I do a long training run and add in fuel, I really need to find a restroom. While these urges are infrequent, they are very serious, and I don’t want it to happen midrace! What should I be eating or not eating to avoid a problem situation like this?

You’re not alone. Many long distance runners have run into this frustration. “Stomach issues” including intolerance issues, runners’ trots, and nausea can strike even the most seasoned runner.

This is because, during long runs and races at hard effort, your system prioritizes blood flow toward your muscles and shunts blood away from your GI tract. Because the blood flow to your digestive tract is reduced, it can be difficult to handle fluids and fuel during hard and long efforts.

But you needn’t hang up your racing flats just yet. Consider some of the reasons distress can occur and ways to “train” your gut to avoid the porta-potty on race day.

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GI distress can hit at different times during a run and for different reasons, so take note of when it happens to you. If runners’ trots are occurring only in the later part of the run, or immediately after you take a gel or energy bar, then the distress may be related to the building concentration of the fuel in your stomach. The stomach effectively digests fuel when it’s diluted (i.e. why it’s often easier to tolerate a sports drink compared to an entire gel), and because of the high concentration of carbohydrate in the gels/blocks/beans/whatever, you must follow the high-octane fuel with water. Otherwise, you may fall victim to dumping—the fuel goes right through you, forcing you to sprint to the nearest porta-potty.

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If runners’ trots derail your run every three miles, beginning at mile three, you may be reacting to something you ate in the past 24 to 48 hours. If this is the case, consider if there may be something in your diet that you eat frequently but might not be tolerating. It’s different for every runner, but some common culprits include caffeine, alcohol (the night before), fiber, high fructose corn syrup, dairy, and so on. Start a food journal and see if there is something that you frequently eat that might be causing distress. And take a look at the ingredient panels of your favorite foods to see if there might be a certain ingredient that is widespread in your diet. If so, try taking out the potential culprit for a few days prior to the long run and see if the distress resolves.

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Another culprit of GI distress might be what you consumed the morning before or night before the run. Take a moment to look back at the occasions when you’ve found yourself making a pit stop: What did you eat the night before, and what time was the meal consumed? Aim for your big prerace (or pre-long-run) meal at lunch the day before. In addition to completing your carbo-load at the prerace pasta dinner (and there’s no need to skip this meal), try to consume your “main meal” earlier to ensure plenty of time to enjoy the meal (without prerace jitters) and time to digest. At dinner the night before, opt for something small in volume, rich in carbohydrates, yet bland (i.e. non-greasy, non-spicy, no roughage!).

4. Try small amounts of fuel at a time.
You may find you tolerate small amounts of fuel, such as one chew every so often, rather than one gel every few miles. Try to take in small amounts of fuel yet while still meeting your carb needs by fueling early and often. And if you consistently fuel early (and often) during training, your gut will become accustomed to this “load” and timing of fuel and won’t need to be overwhelmed by a huge amount of high-octane fuel at once.

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Have you noticed it the GI distress occurs with one brand/type of fuel but not with others? Many runners find that their GI distress goes away when they switch fuel sources. There are many gels, beans, chews, blocks, bars, and drinks on the market. I suggest trying out different forms and sources, keeping in mind that the first one you try out should be the brand / type offered on race day so that there are no GI surprises!