If you’ve ever tried noshing on gels, chews, waffles, or something similar to maintain your energy levels while training for or participating in a race, you’ve probably experienced an unwelcome side effect: pooping.

But what’s causing it? The trigger isn’t necessarily caffeine (a well-known laxative), considering a lot of these products don’t even contain the added stimulant. The answer is actually less about the ingredients—usually a mix of different kinds of carbs and sugars plus sodium and electrolytes—and more about the highly-concentrated doses of those ingredients that you're consuming.

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“Your gastrointestinal system, and more specifically your small intestine, optimally absorbs nutrients, such as carbohydrates, within a certain concentration range,” says certified strength and conditioning specialist Carwyn Sharp, Ph.D., chief science officer for the National Strength and Conditioning Association. “Gels, chews, and waffles have strong amounts of carbs, sugars, and electrolytes to fuel your body, but if you haven’t consumed sufficient amounts of water, your small intestine may not be able to handle them efficiently.”

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That causes a pretty severe chain of events. First, to dilute this mixture of carby stuff, water moves out of your blood and into the intestine to help it process things, which can lead to—dun dun dun—dehydration. Once it hits that point, your body has to take blood away from the GI tract to help your working muscles keep moving, explains Sharp. That leaves less blood available to absorb the nutrients you just consumed, and so they move on to the large intestine. That’s where bacteria sits and feeds on sugars…resulting in gas, bloating, and diarrhea. Add that to the repetitive bouncing motion of running, and voila, you have the recipe for some seriously messy pants.

All that said, you don’t have to give up your energy godsends. Really what you need to do to prevent potty problems (the number-two kind at least) is drink plenty of water to dilute the high concentrations, says Sharp. And a tip that may seem like a no-brainer but way too many people skip: Read the directions. Most packets will tell you how much to consume in relation to your duration of exercise. (They should also remind you to take them with water.)

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And one more word to the wise: Experiment with these guys a few times during trial runs. Some people have more sensitive stomachs than others—not something you want to find out on race day. (Take it from me.)

The article Why Your Energy Gels Are Giving You Diarrhea originally appeared on Women’s Health.

Marissa Gainsburg Marissa Gainsburg is the Features Director at Women's Health, where she oversees the magazine's news-meets-trends Warm Up section and Love & Life section.

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