Every endurance athlete has experienced it at some point: the fuel that was supposed to sustain you starts sitting heavy in your stomach, sloshing, creating nausea, or worse. The instinct is to blame the brand, the flavour, or your own "weak stomach." But the reality is often simpler — and more scientific — than that. The answer, in many cases, comes down to osmolality.
What is osmolality?
Osmolality is a measure of the concentration of dissolved particles in a solution. In the context of sports nutrition, it describes how many solute molecules — sugars, electrolytes, flavouring agents — are present per unit of water in a drink or gel mixture once dissolved.
The body's own fluids — blood, interstitial fluid — have an osmolality of approximately 280–300 mOsm/kg. This is the baseline your gut is calibrated to work with. Solutions that closely match this range are considered isotonic. Solutions below it are hypotonic. Solutions above it are hypertonic.
This distinction matters because osmolality directly influences two critical processes during exercise: gastric emptying (how quickly fluid leaves the stomach) and intestinal absorption (how efficiently nutrients and water cross the intestinal wall into the bloodstream).
Gastric emptying: the first bottleneck
Before any nutrient can be absorbed, it must first leave the stomach. Gastric emptying rate is influenced by several factors: volume, temperature, caloric density, and — critically — osmolality.
Rehrer et al. (1990) demonstrated that solutions with very high osmolality slow gastric emptying significantly, particularly during prolonged exercise. The stomach effectively holds back hypertonic solutions, attempting to dilute them before releasing them into the small intestine. This creates the sensation of fullness, sloshing, and nausea that many athletes associate with "GI problems."
In practical terms: a highly concentrated carbohydrate drink may deliver a large amount of energy per sip, but if it empties from the stomach slowly, the actual delivery rate to the intestine may be no better — or even worse — than a less concentrated solution that empties faster.
Intestinal absorption: where it gets interesting
Once a solution reaches the small intestine, absorption depends on the osmotic gradient between the intestinal lumen and the blood. Shi et al. (1995) showed that beverage osmolality has a direct effect on the rate and completeness of fluid absorption.
Hypotonic and isotonic solutions are absorbed more rapidly because the osmotic gradient favours water movement from the intestinal lumen into the bloodstream. Hypertonic solutions, by contrast, can actually draw water into the intestine, exacerbating dehydration rather than addressing it, and increasing the risk of cramping and diarrhoea.
This is a crucial point that gets lost in the "more carbs is better" narrative. Beyond a certain concentration, adding more carbohydrate to a solution does not improve fuel delivery — it impairs it, by slowing gastric emptying and reducing net fluid absorption.
Why "more carbs" isn't always better
The trend in endurance nutrition over recent years has been toward higher and higher carbohydrate intake rates of 90, 100, even 120 grams per hour. The research supporting high carbohydrate oxidation rates (Jeukendrup, 2017) is sound. But oxidation rates measured in controlled laboratory settings do not always account for the gastrointestinal realities of sustained exercise in the field.
An athlete consuming 120 g/h of carbohydrate in a highly concentrated solution may achieve impressive intake numbers, but gastric distress can force them to stop drinking entirely. The athlete consuming 80–90 g/h from a well-formulated, osmolality-appropriate solution may actually absorb and utilise more carbohydrate over the course of a race, because they can sustain intake without GI failure.
Gut distress is not a sign of a "weak stomach." It is often a predictable physiological response to a solution that exceeds the gut's capacity to process it efficiently. The physics and physiology are clear.
Formulation discipline: the hidden variable
Understanding osmolality shifts the conversation from "how much can I cram in?" to "how do I optimise the balance between carbohydrate delivery and gut comfort?" This is a formulation question, not just a dosing question.
The choice of carbohydrate source matters. Maltodextrin, as a glucose polymer, has a significantly lower osmolality than equivalent amounts of free glucose — because fewer individual molecules are in solution (they're chained together). This is one reason maltodextrin is preferred in endurance formulations: it allows higher carbohydrate delivery at lower osmolality.
Similarly, the total solute load — including electrolytes, flavourings, and any other ingredients — contributes to osmolality. A formulation that delivers high carbohydrate rates while maintaining appropriate osmolality requires careful engineering, not just ingredient stacking. This is the design principle behind HYDRAX CORE — balancing carbohydrate delivery with gut compatibility.
Practical implications for endurance athletes
If you've experienced GI issues during training or racing, consider these factors before blaming your gut:
- Concentration matters. Mixing fuel more concentrated than recommended — or combining multiple concentrated sources — can push osmolality well above tolerable levels.
- Temperature and conditions affect tolerance. Heat reduces gastric emptying rate, meaning a solution that works in cooler conditions may cause problems in the heat.
- Gut training helps, but it has limits. Regular practice with your race-day nutrition improves tolerance, but it cannot override the physics of an excessively hypertonic solution.
- Sustained intake beats peak intake. The goal is not maximum intake in one hour — it's consistent, tolerable fueling across the entire event.
For a deeper look at how dual-source carbohydrate systems interact with absorption science, see our earlier article on high-carbohydrate fueling.
Summary
- Osmolality — the concentration of dissolved particles in a solution — directly affects gastric emptying and intestinal absorption during exercise.
- Hypertonic solutions slow gastric emptying and can draw water into the gut, worsening GI symptoms and impairing hydration.
- Higher carbohydrate intake is not always better. What matters is how much is actually absorbed and utilised, not how much is consumed.
- Formulation quality — carbohydrate type, solute load, concentration — determines real-world gut tolerance as much as training does.
- GI distress during endurance events is usually a physics problem, not a personal weakness.
References
- Rehrer NJ, Beckers EJ, Brouns F, ten Hoor F, Saris WH. Effects of dehydration on gastric emptying and gastrointestinal distress while running. Medicine & Science in Sports & Exercise. 1990;22(6):790–795.
- Jeukendrup AE. Training the Gut for Athletes. Sports Medicine. 2017;47(Suppl 1):101–110.
- Shi X, Summers RW, Schedl HP, Flanagan SW, Chang R, Gisolfi CV. Effects of carbohydrate type and concentration and solution osmolality on water absorption. Medicine & Science in Sports & Exercise. 1995;27(12):1607–1615.