Every ultrarunner knows that an ultramarathon isn’t just a running contest—it’s a moving eating contest. To prevent hitting the dreaded “wall,” modern sports science tells us to aggressively fuel, with many athletes pushing anywhere from 60 to 90+ grams of carbohydrates per hour.
But there is a dark side to this heavy fueling strategy. Gastrointestinal (GI) distress is the leading cause of DNFs in ultrarunning. Among the most frustrating and painful symptoms is severe stomach bloating, which can make every step feel like you’re carrying a bowling ball.
While many runners blame “bad luck,” mid-race bloating usually comes down to two primary culprits: carbohydrate overload and aerophagia (swallowing too much air).
Culprit #1: Carbohydrate Overload & Osmotic Pressure
Carbohydrates are your muscles’ preferred fuel source, but your gut has a strict speed limit for how fast it can process them. When you exceed that limit, things go downhill quickly.
Why It Causes Bloating
During a long race, your body undergoes a process called splanchnic hypoperfusion, where up to 80% of blood flow is diverted away from your gut to supply your working muscles and cool your skin. With less blood flow, your stomach empties slower, and your intestinal carbohydrate transporters get sluggish.
If you dump highly concentrated carb mixes or rapid-fire gels into a shut-down gut, the unabsorbed sugars sit there. This creates an osmotic effect, pulling water out of your body and into your intestinal tract. At the same time, gut bacteria begin rapidly fermenting those stagnant sugars. The result? Excessive gas, fluid sloshing, and massive abdominal bloating.
How to Fix It
- Train Your Gut: Your digestive tract is adaptable. You must practice your race-day fueling strategy during long runs to increase your stomach’s volume tolerance and improve your small intestine’s absorption rates.
- Optimize the Sugar Ratio: Look for fuel blends that combine glucose (or maltodextrin) and fructose in a 1:0.8 ratio. Because they utilize different cellular transporters, they oxidize more efficiently and leave fewer undigested carbs behind to ferment.
- Watch the Osmolality: Never swallow thick gels without drinking adequate water. Highly concentrated sugars delay gastric emptying. Sip water alongside your fuel to help dilute the mixture so your stomach can pass it along.
Culprit #2: Aerophagia (Swallowing Air)
It sounds simple, but a surprising amount of mid-race bloating has nothing to do with what you are digesting, but rather how you are breathing and drinking. Swallowing air is medically known as aerophagia.
Why It Causes Bloating
When you are 50 miles deep, navigating technical terrain, and operating under high physical exertion, you are breathing heavily. If you gulp down fluids from a handheld bottle, straw, or hydration bladder while gasping for air, you inadvertently swallow massive pockets of oxygen and nitrogen.
Unlike the gas produced by carbohydrate fermentation lower in the GI tract, swallowed air traps itself right in the upper stomach. The physical bouncing and mechanical jostling of running compounds this, turning your stomach into a shaken-up soda bottle of trapped air.
How to Fix It
- Mindful Swallowing: Don’t chug your fluids or nutrition while actively panting. Take a conscious moment to exhale, slow your breathing rhythm down for a few seconds, and deliberately swallow your fluids smoothly.
- Purge Hydration Bladders: If you use a hydration backpack, flip the bladder upside down after filling it and suck the air out through the hose until fluid hits the bite valve. This prevents you from sucking in a mouthful of air every time you reach for a drink.
- Avoid Carbonation (Until the End): While ginger ale or cola at aid stations can feel like a lifesaver for nausea, the carbonation instantly introduces massive amounts of gas into an already compromised stomach. Save the fizzy drinks for the final miles when you just need to survive.
The Bottom Line
Mid-race bloating can derail a goal you’ve trained months for, but it doesn’t have to be inevitable. By systematically adapting your gut to handle high-carb loads in training, properly diluting your fuel with water, and paying active attention to how cleanly you swallow fluids at aid stations, you can keep your stomach flat, functional, and fueled all the way to the finish line.