Osmolality is a buzzword in the hydration world, but you may have noticed it’s not always easy to find on a product label. The osmolality of a sports drink is similar to the concept of fluid concentration and is based on the number of particles (including carbohydrates, electrolytes, amino acids etc.) in the solution. If a drink is hypertonic, it is more concentrated than your plasma which is measured at 280-290 mOsmol/kg) and it will cause fluid to move from your body’s circulation into your gut to dilute the hypertonic solution, possibly leading to dehydration. If a drink is hypotonic or less concentrated than plasma, there is an increase in water transport from the gut to the body’s circulation, improving hydration. An isotonic drink is basically equal to the plasma osmolality. Osmolality influences gastric emptying rate and carbohydrate absorption in the gut, making hyperosmolality a possible contributing factor to GI distress. The “ideal” sports drink should empty rapidly from the stomach, enhance intestinal absorption, and promote fluid retention without causing extensive gastric distention or GI issues. However, there are other factors that also influence gastric emptying rate such as heat, dehydration, and exercise intensity, and mode of activity.
With so many different commercially available sports drinks on the market, it can be confusing when trying to navigate labels with various electrolytes, carbohydrate blends, and additional supplements. Most traditional sports drinks contain ~4 to 6% carbohydrate (40-60 g of carbohydrate per 1L of solution) plus electrolytes (e.g. sodium, potassium) in varying quantities. The addition of more particles (i.e. more simple carbohydrates in high concentration drinks, additional electrolytes in high sodium drinks) may slow gastric emptying and/or lead to dehydration compared to lower osmolality solutions. Interestingly, some traditional sports drinks are marketed as isotonic but have been found to be slightly hypertonic. While it is easy to determine the CHO and electrolyte concentration of a beverage, the only way to be certain of osmolality is to measure it in a laboratory setting. Since not all manufacturers provide the osmolality of the products on the label, you may need to ask or just test the product as part of your overall nutrition plan during your training.
Hypertonic –sports drinks or gels containing a higher number of particles in solution compared to plasma. These products, particularly high concentration drinks containing simple carbohydrates (glucose, fructose) can delay fluid absorption. When using a hypertonic solution such as a gel, water or some other very hypotonic fluid should be used to wash it down, thereby balancing out the tonicity, facilitating gastric emptying and absorption. When to use: For higher calorie intake before or during exercise in combination with water, after exercise to stimulate thirst for rehydration.
Isotonic – sports drinks with similar osmolality to plasma. When to use: During exercise (if using as your source of carbohydrates), can be alternated with water.
Hypotonic- sport drinks with a lower osmolality than plasma. Some of these options include complex carbohydrates like starches, maltodextrin, or cluster dextrin. Simple carbohydrates can also be part of hypotonic drinks and some products may also contain artificial sweeteners which may lead to unwanted GI side effects for some individuals. When to use: during exercise for fueling and hydration, and in combination with other carb sources when desired (e.g. bars, solid food, chews). Athletes with a sensitive stomach may respond better to a hypotonic beverage.
Finally, there are a lot of choices available on the market, so choose the sports drink that meets your goals and taste/GI preferences. If you are taking in > 60 g carb per hour, choose a beverage containing a combination of carbohydrates (i.e. glucose-fructose, maltodextrin-fructose, cluster dextrin-fructose) which can improve intestinal absorption and carb utilization. Drinks ranging from 4-8% CHO (40-80 g/L) of glucose polymers and multiple transportable carbs and 230-690 mg sodium/L are generally classified as hypotonic to isotonic, but should be tested for individual tolerance.
When testing products such as isotonic and hypertonic drinks or gels, alternate your sports drink with water to ensure rapid gastric emptying and fluid absorption. Start with lower carb intake rates and build up your gut tolerance with a regimen that suits your individual needs.
Mettler S, Rusch C, Colombani PC. Osmolality and pH of sport and other drinks. Schweizerische Zeitschrift für. 54 (3), 92–95, 2006
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