Why Do Running Gels Upset Your Stomach? Causes and What to Use Instead

You're ten miles into a long run. You take your second gel of the morning, wash it down with a few sips of water, and within ten minutes your stomach is sending very clear signals that it disagrees with your fueling strategy.

If this sounds familiar, you're not alone and you're not weak. GI distress during running is one of the most common performance problems endurance athletes face, and gels are frequently in the conversation. But the reason gels cause stomach problems is more nuanced than "gels are bad" and understanding the actual mechanism helps you fix it, or find a better alternative or addition to your nutrition strategy.

Here's what's actually happening when running gels upset your stomach, and what to do about it.

First: Running Itself Is Hard on Your Gut

Before blaming the gel entirely, it's worth understanding that running creates GI stress independent of what you eat.

The repetitive vertical impact of running increases intestinal motility (i.e. sloshing in that tummy of yours), essentially speeding up how fast things move through your digestive system. Simultaneously, exercise diverts blood flow away from your GI tract toward your working muscles. Less blood to the gut means slower, less efficient digestion. Put those two things together and you have a digestive system that's already under stress before a single gel enters the picture.

This is why runners experience more GI issues than cyclists or hikers at equivalent effort levels. A cyclist can often eat foods that would destroy a runner mid-effort, simply because the mechanical impact isn't there.

Why Gels Specifically Cause Stomach Problems

On top of the running-induced GI stress, gels add several specific variables that can tip a sensitive stomach over the edge.

Osmolality and Concentration

Energy gels are essentially highly concentrated sugar water. The technical term is high osmolality, meaning the concentration of dissolved particles is significantly higher than your gut wall cells. When you consume something highly concentrated, your intestines pull water toward it to dilute it to a processable level.

This osmotic shift does two things:

1. It draws fluid away from the rest of your body, contributing to dehydration.
2. It leaves unabsorbed carbohydrate sitting in your intestines. 

That residual carbohydrate then ferments from gut bacteria, producing gas, bloating, cramping, and the urgent bathroom situation that has ruined more than a few days.

Taking gels without enough water significantly worsens this effect. The gel needs fluid to process properly, and if you're not providing it externally, your body pulls it from your GI tract instead. TL;DR - always drink water with your gels. 

High Fructose Content Without Gut Training

Most gels combine glucose and fructose to increase absorption. Glucose uses one intestinal transport pathway and fructose uses another, theoretically allowing higher carbohydrate absorption rates. The problem is that fructose absorption capacity varies significantly between individuals, and most people's guts haven't been trained to handle large amounts of fructose during exercise.

Your intestinal transport proteins are use-it-or-lose-it. If you rarely consume significant fructose during exercise, your gut doesn't maintain the transporters to absorb it efficiently. The result is unabsorbed fructose sitting in the intestines, fermenting, and causing exactly the cramping and diarrhea gel users frequently report.

Caffeine

Many popular gels contain 25 to 100mg of caffeine per serving. Caffeine is a well-known GI stimulant that increases intestinal motility, which is the last thing you want during a long run when your gut is already under stress. Research suggests consuming more than 6mg of caffeine per kilogram of body weight can significantly increase GI symptoms. For a 150-pound runner, that's 408mg total, reached in four caffeinated gels if you've also had pre-race coffee.

If you use caffeinated gels and experience GI problems, the caffeine is a prime suspect worth testing in isolation.

Too Much, Too Fast

A 2025 study confirmed that very high hourly carbohydrate intake, specifically around 120 grams per hour, correlates with moderate to severe nausea, abdominal cramping, and stomach fullness during running. For context, 120 grams per hour is approximately five standard gels. Most runners aren't hitting that number, but the principle scales down. Taking multiple gels in a short window, or taking them without adequate water, creates a concentrated load that outpaces your gut's ability to process it, unless your gut is adequately trained. 

What to Try Before Giving Up on Gels Entirely

If you want to make gels work before switching to alternatives, a few evidence-based adjustments can significantly reduce GI symptoms.

Take gels with water, not alone. The concentration issue is real and water helps. Most gel packaging says to take with water for this reason. It's not a suggestion, it's a functional requirement for proper absorption.

Practice in training. Your intestinal transport proteins adapt to consistent use. Runners who regularly fuel with carbohydrates during training develop better GI tolerance than those who only fuel on race day. If gels are new to you or you only use them in races, your gut hasn't had time to adapt.

Switch to isotonic gels. Isotonic formulas are pre-diluted to match the concentration of your gut wall cells, eliminating the osmotic issue. They require less water and are generally better tolerated than standard concentrated gels.

Try non-caffeinated versions first. If you use caffeinated gels and have stomach problems, test a non-caffeinated version in training. Isolating caffeine as a variable is the fastest way to know whether it's contributing to your issues.

Spread intake across the hour rather than taking multiple gels in quick succession. Smaller, more frequent amounts are better tolerated than large boluses.

If Gels Genuinely Don't Work for You

Some runners have tried every variation of gel and still find them intolerable. This is not a personal failure. It reflects real physiological differences in GI sensitivity, intestinal transport capacity, and how individual guts respond to highly concentrated carbohydrate during the mechanical stress of running.

For runners who have genuinely tried and failed with gels, real food alternatives deliver carbohydrates in a form that's closer to what your digestive system processes every day.

Smoothie pouches deliver carbohydrates from whole fruit rather than concentrated synthetic sugars. The sugars in banana and berries come packaged with the water content and micronutrients your gut expects from real food, which may explain why many athletes with gel-related GI issues tolerate whole-food pouches significantly better. Neve's Boysenberry Beet is built for moderate-intensity endurance efforts. It contains coconut cream alongside the fruit carbohydrates, making it sustained endurance fuel rather than a fast-absorption gel substitute. For sustained moderate-intensity hours that describe most long runs, real food works differently than gels.

Medjool dates are one of the most gut-friendly mid-run fueling options available. Natural glucose and fructose, no synthetic additives, easy to carry in a small zip bag. Many runners who can't tolerate gels handle dates without issue. Heads up, they do contain fiber.

Bananas at aid stations have been a staple in distance running for decades. Easy to digest, potassium-rich, and not coincidentally one of the lowest GI-distress foods in the running community.

The Bottom Line

Running gels upset stomachs for real, well-understood reasons: concentration, osmolality, fructose load, caffeine, and the underlying GI stress that running itself creates. For many runners, the fix is adjusting how and when they take gels, switching to isotonic formulas, or training the gut to handle higher carbohydrate loads.

For runners who have genuinely tried all of that and still find gels intolerable, the alternative isn't suffering through it or running under-fueled. It's finding fuel that works with your gut rather than against it, and for a lot of athletes, that means real food.