Many endurance athletes have experienced stomach discomfort after consuming certain sports drinks during training or racing. While the cause is often blamed on specific ingredients, the underlying issue is frequently related to osmolality. Understanding this concept helps explain why some hydration strategies work well, while others lead to gastrointestinal distress.
What is osmolality?
Osmolality refers to the concentration of dissolved particles within a fluid. In sports drinks, these particles include carbohydrates, electrolytes, minerals, flavouring agents, and any other solutes present in the formulation.
The body's own fluids, including blood and interstitial fluid, have an osmolality of approximately 280 to 300 mOsm/kg. This is the baseline the 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.
The concentration of dissolved particles influences two critical processes during exercise: how quickly fluid leaves the stomach (gastric emptying) and how efficiently nutrients and water cross the intestinal wall into the bloodstream (intestinal absorption). Both of these processes are directly affected by the osmolality of what you consume.
Why osmolality matters during exercise
During endurance exercise, efficient fluid absorption is essential. The body is simultaneously losing fluid through sweat, redirecting blood flow to working muscles, and attempting to process ingested fuel. The digestive system operates under compromised conditions, and the margin for error is narrow.
Highly concentrated (hypertonic) drinks may slow gastric emptying significantly, causing fluid to remain in the stomach longer. Rehrer et al. (1990) demonstrated that solutions with very high osmolality delay stomach emptying, particularly during prolonged exercise. This creates the sensation of fullness, sloshing, and nausea that many athletes associate with "GI problems."
This can lead to a cascade of symptoms:
- Bloating and stomach distension
- Nausea and reduced appetite
- Stomach discomfort and cramping
- Reduced willingness to continue fueling
- Performance decline from both energy deficit and discomfort
Proper drink formulation balances carbohydrate availability with fluid absorption. This is a formulation challenge, not simply a dosing question.
Isotonic vs hypotonic drinks
Sports drinks are typically categorised into three categories based on their osmolality relative to blood plasma:
- Isotonic solutions have a similar concentration to blood plasma (approximately 280 to 300 mOsm/kg). They provide a balance of carbohydrate delivery and fluid absorption.
- Hypotonic solutions have a lower concentration, allowing faster absorption of water. They deliver less energy per unit of fluid but are better tolerated in conditions where hydration is the primary concern.
- Hypertonic solutions have a higher concentration. They can deliver more energy per sip but slow gastric emptying and can draw water into the intestine, potentially worsening dehydration and GI symptoms.
In endurance sports, drinks with moderate carbohydrate concentration and balanced electrolytes typically provide the best balance between energy delivery and digestive comfort. The choice between isotonic and hypotonic depends on conditions: hotter environments and higher sweat rates may favour more dilute solutions, while cooler conditions may tolerate higher concentrations.
Carbohydrates and absorption dynamics
Carbohydrate concentration strongly influences osmolality. Higher carbohydrate levels increase energy delivery but also raise solution concentration. This is where the choice of carbohydrate source becomes critical.
Maltodextrin, a glucose polymer, has a significantly lower osmolality than equivalent amounts of free glucose. Because the glucose units are chained together, fewer individual molecules are present in solution. This allows higher carbohydrate delivery at lower osmolality, which is one reason maltodextrin is preferred in endurance formulations.
Combining multiple carbohydrate sources, specifically glucose (or maltodextrin) with fructose, can improve absorption efficiency. These carbohydrates use different intestinal transporters (SGLT1 for glucose, GLUT5 for fructose), allowing total carbohydrate absorption to exceed the capacity of either transporter alone (Jeukendrup, 2010). This high-carbohydrate fueling strategy allows higher intake with improved tolerance when the formulation is properly engineered.
The sodium factor
Sodium plays a dual role in sports drink formulation. Beyond its function as an electrolyte replacement, sodium actively facilitates glucose absorption in the small intestine through the SGLT1 co-transporter. The presence of sodium in the intestinal lumen is required for glucose to be transported across the intestinal wall.
However, sodium also contributes to osmolality. Formulations must balance sufficient sodium for absorption support and electrolyte replacement without pushing osmolality into ranges that impair gastric emptying. Understanding how much sodium you actually need helps athletes make informed decisions about their hydration strategy.
Why some commercial drinks cause problems
Many commercial sports drinks prioritise taste and marketing over formulation science. Added sugars, artificial sweeteners, flavouring compounds, and unnecessary additives all contribute to the total solute load. A drink that tastes good on the shelf may have an osmolality well above optimal ranges for exercise.
Athletes who experience GI issues with commercial products often assume they have a "sensitive stomach." In many cases, the issue is not their gut but the formulation they are consuming. A solution engineered for osmolality-appropriate delivery, like Hydrax Core, approaches this differently: balancing carbohydrate delivery with gut compatibility through careful control of total solute concentration.
Practical hydration considerations
Athletes should consider several factors when developing hydration strategies for training and competition:
- Carbohydrate concentration. Follow recommended mixing instructions. Concentrating solutions beyond the intended ratio increases osmolality disproportionately.
- Environmental conditions. Heat reduces gastric emptying rates. A solution tolerated in cool conditions may cause problems when temperatures rise. Adjust concentration accordingly.
- Individual sweat rate. Higher sweat rates increase fluid replacement needs. You can calculate your sweat rate here to establish a baseline for your hydration planning.
- Exercise intensity. Higher intensities reduce splanchnic blood flow more aggressively, further compromising gut function and reducing tolerance for concentrated solutions.
- Gut training status. A practical approach to gut training can improve tolerance over time, but it cannot override the physics of an excessively hypertonic solution.
The mixing instruction problem
One of the most common causes of osmolality-related GI issues is incorrect mixing. Athletes frequently use less water than recommended, creating a solution that is significantly more concentrated than intended. A product designed to be mixed in 750ml of water will have a very different osmolality when mixed in 500ml.
Similarly, combining multiple carbohydrate sources (drink mix plus gels plus bars) without accounting for total carbohydrate load can push effective osmolality well above tolerable levels. The gut does not differentiate between sources. Total solute load reaching the stomach is what matters.
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.
- The choice of carbohydrate source (maltodextrin vs free glucose) significantly affects solution osmolality at equivalent carbohydrate loads.
- Sodium facilitates glucose absorption but also contributes to osmolality. Formulation requires precise balance.
- GI distress during endurance events is frequently a formulation problem, not a personal weakness. Understanding osmolality provides the framework for solving it.
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. Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care. 2010;13(4):452-457.
- 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.
- Jeukendrup AE. Training the Gut for Athletes. Sports Medicine. 2017;47(Suppl 1):101-110.
- Vist GE, Maughan RJ. The effect of osmolality and carbohydrate content on the rate of gastric emptying of liquids in man. The Journal of Physiology. 1995;486(2):523-531.