You trained for months, dialed in your pacing, and then somewhere around mile 18 your stomach staged a revolt. Cramping, bloating, a sudden and non-negotiable need for a porta-potty. Most runners blame the heat, the pace, or last night’s dinner. Often the real culprit is sloshing around in your bottle.
Gastrointestinal trouble is one of the most common — and most preventable — reasons endurance athletes fall apart on race day. A 2025 study of 805 recreational long-distance runners in China found that 26.1% reported GI symptoms during races, with bloating (18.6%), an urgent need to defecate (17.8%), and stomach pain (16.5%) leading the list. In ultra-distance events the numbers climb higher, and GI distress is a leading cause of race withdrawal.
The good news: a large share of that misery traces back to one measurable property of your drink. It’s called osmolality, and once you understand it, choosing a sports drink stops being guesswork.
What Osmolality Actually Means
Osmolality is a measure of how many dissolved particles — sugars, salts, acids — are packed into a given amount of fluid. It’s expressed in milliosmoles per kilogram (mOsm/kg). Your blood sits at roughly 280 to 300 mOsm/kg. That number is the benchmark everything else gets compared against.
- Hypotonic (below ~280 mOsm/kg): fewer particles than blood. Moves out of the gut quickly.
- Isotonic (roughly 280–300 mOsm/kg): matched to blood. Empties efficiently and delivers both fluid and fuel.
- Hypertonic (above ~300 mOsm/kg, and especially above 500): more concentrated than blood. This is where problems start.
Here’s the part that matters at mile 18. When a hypertonic drink hits your small intestine, your body can’t absorb it as-is. It has to dilute the concentrate first — by pulling water out of your bloodstream and into your gut. You drank to hydrate, and the drink temporarily moved fluid the wrong direction. That extra water sitting in your intestine is what produces the sloshing, the bloating, and in bad cases osmotic diarrhea.
The 6-to-8 Percent Rule
Carbohydrate concentration is the biggest single driver of a drink’s osmolality, and decades of research have converged on a fairly narrow window. Beverages in the 5–8% carbohydrate range (5 to 8 grams per 100 mL) tend to land close to isotonic and empty from the stomach at a good clip. A classic 6% formula is the reason traditional sports drinks were built the way they were.
Push past that and gastric emptying slows measurably. Research has shown that repeated ingestion of an 8% carbohydrate beverage during exercise significantly reduces the rate at which the stomach clears fluid, while lower concentrations do not. Beyond 10 grams per 100 mL — the territory of most fruit juices and sodas, which run 10–15% — you’re both slowing emptying and dramatically raising the osmotic pull inside the intestine.
Practical thresholds worth memorizing: drinks above roughly 330 mOsm/kg carry elevated risk of GI symptoms during exercise, and most sports nutrition guidance advises avoiding anything above 500 mOsm/kg while you’re actually moving. Concentrated gels are frequently far above that — which is exactly why they’re meant to be chased with plain water rather than washed down with more sports drink.
Where Popular Products Land
Formulations change, so always check the current label. But the categories behave predictably:
- Classic 6% sports drinks (Gatorade Thirst Quencher, Powerade) sit near isotonic by design. They deliver carbohydrate and fluid together, which is their whole point — but the sugar load is significant if you’re drinking them all day.
- Hypotonic endurance formulas (Skratch Labs Sport Hydration, Precision Fuel & Hydration’s lower-strength mixes) deliberately run lighter on carbohydrate to prioritize fluid absorption over fueling.
- Zero-sugar electrolyte powders (LMNT, Nuun Sport, Liquid I.V. Sugar-Free) carry almost no carbohydrate, so osmolality is driven by sodium instead. LMNT’s 1,000 mg of sodium per stick is a lot of solute in 16 oz — some athletes tolerate it fine, others find it needs more dilution than the packet suggests.
- Concentrated shots take a different approach entirely. Pickle brine products like Fast Pickle deliver sodium in a 2–3 oz format with no added sugar, so there’s no carbohydrate osmotic load to manage — though brine is salty and concentrated by nature, so it’s still worth testing in training rather than debuting it on race morning.
- High-carb race fuels (Maurten, Gatorade Endurance Carb Energy) intentionally exceed the old rules using hydrogel or multi-transportable carbohydrate technology. They can work extremely well — but they demand gut training, not a blind first attempt.
Dilution Is a Legitimate Strategy
If a drink you otherwise like leaves you bloated, the simplest fix is usually not a new product — it’s more water. Mixing a scoop meant for 16 oz into 24 oz drops the concentration by roughly a third and pulls osmolality down with it. You’ll get less sodium and less carbohydrate per bottle, so plan to drink more volume or supplement elsewhere, but tolerance often improves immediately.
The same logic applies to gels and chews. Chase them with plain water, not with another mouthful of sports drink. Stacking two concentrated sources back-to-back is one of the fastest ways to overwhelm a gut that’s already working with reduced blood flow.
Why Exercise Makes Everything Worse
At rest, your gut handles a wide range of concentrations without complaint. During hard exercise, blood is redirected away from the digestive tract toward working muscle and skin. Digestion slows, the intestinal barrier gets stressed, and mechanical jostling — particularly in running — adds insult. Heat compounds all of it, and so does dehydration, which is why starting a race already fluid-depleted tends to amplify GI symptoms rather than cause them independently.
This is also why intensity matters. A drink you sip comfortably on an easy long run may feel completely different at threshold pace in 85-degree heat.
Train Your Gut Like You Train Your Legs
The most encouraging finding in this whole body of research: the gut adapts. Repeated exposure to carbohydrate during training improves absorption capacity and reduces symptoms over a period of weeks. Athletes who rehearse their exact race-day nutrition — same products, same concentrations, same timing, at goal intensity — consistently report fewer problems than those who wing it.
- Practice your fueling plan on at least three long sessions before race day.
- Change one variable at a time so you know what actually caused a problem.
- Weigh yourself before and after a long run to learn your real sweat rate, then match your fluid plan to it.
- Start races well hydrated — dehydration makes existing GI issues worse.
- If symptoms persist despite a sane fueling plan, a sports dietitian can help; low-FODMAP approaches have shown promise in recent case work.
The Short Version
Keep carbohydrate in the 5–8% range for drinks you’re consuming continuously. Treat anything above about 330 mOsm/kg with caution during exercise and avoid the 500-plus range in-race unless you’ve trained for it. Chase gels with water. Dilute anything that leaves you sloshing. And rehearse the whole plan before it counts.
Your stomach isn’t being dramatic. It’s responding predictably to chemistry you can control.
This article is for general informational purposes and is not medical advice. Individual tolerance varies considerably. Athletes with persistent or severe gastrointestinal symptoms should consult a physician or registered sports dietitian.