Written Summary

1. What is the background of this research? 

We wanted a clear understanding of which drinks are best for hydration during continuous exercise – hypertonic (for example, regular coke, Lucozade regular), isotonic (for example, Powerade, Lucozade Sport), hypotonic (for example, Torq Hydration, Lucozade Sport Lite) sports drinks or water?  Previously, research that has looked at various drink formulas on hydration status has used a wide range of techniques including triple-lumen based intubation direct intestinal uptake data, change in plasma volume outcomes, net central uptake of deuterated water, and urine balance. As you can imagine, the data from these various studies was quite contradictory and resulted in much ambiguity on the hydration effects of various drinks. 

In addition to this, there has been a lot of commercial marketing from the sports beverage industry around the benefits of isotonic drinks for hydration. The correct presentation of scientific results are often considered of lower importance when compared to increased revenue from beverage sales. After completing a narrative review, we did not have a clear answer on which drink would be best for hydration. However, we were able to generate a hypothesis that hypotonic beverages should hydrate the best, due to the combined osmotic and solvent drag effects of carbohydrates, salts, and fluid. This led us to complete a quantitative analysis, which turned out to be easier said than done.

2. What was the aim of the study? 

To determine the effects of ingested hypertonic, isotonic, and hypotonic drinks containing carbohydrate and electrolytes, and non-carbohydrate drinks/water on change in plasma volume relative to rest (dPV) during continuous exercise. We were unable to evaluate intermittent endurance exercise, as each rest period reset the plasma volume status making the analysis uninterpretable, at least on a change in plasma volume level. 

3. What assessment techniques did you use? 

Random effects meta-analysis using linear mixed models in SAS. We also chose to compare the magnitude-based inference against Bayesian and showed that the inferential outcomes were almost identical, at least with respect to magnitude and uncertainty. This demonstrated that magnitude-based inference, when interpreted correctly, is a valid statistical approach. 

4. What were your primary results? 

When discussing exercise performance, it is important to remember that small effects can be very beneficial to performance, especially in well-trained individuals. Our results showed that hypotonic drinks resulted in very likely better hydration outcomes measured by an attenuated decline in dPV, relative to isotonic drinks and likely relative to water. Surprisingly, hypertonic drinks outperformed isotonic drinks. Most brands of isotonic drinks use concentrated disaccharide solutions, typically made with sucrose and maltodextrin. When we analysed the effect of osmolality, we found that this formulation of isotonic drinks when in the small intestine appears to act more like a concentrated hypertonic solution, hence the less beneficial hydration effect. 

Key modifiers of change in plasma volume from rest during continuous exercise were found to be the intensity of exercise (higher intensities impaired isotonic effects on central hydration – dPV, further), drink ingestion rate (increasing ingestion rate improved the benefit of hypotonic drinks and impaired hypertonic), carbohydrate characteristics – multiple transportable (fructose, glucose) being superior, and electrolyte concentration (the higher-end of the normal range was generally better as exercise duration progressed, as seen in many medical rehydration solutions). 

5. What are the implications of your findings? 

In prolonged endurance exercise where we see elevated sweat rates, dehydration can reduce cardiac output, impair muscle blood flow, and thermoregulation, increase perceived exertion, and as a result could reduce exercise/sports performance. 

With more elite and age group endurance sporting events occurring in hot and challenging climates, having a better understanding of what drink is best for reducing the risk of dehydration and hyponatremia (change in plasma volume from rest) can contribute to the knowledge base of interventions that may support small but beneficial effect on performance. For athletes, it is recommended when providing a drink that maintains hydration during short to middle-distance events, a well-formulated hypotonic carbohydrate-electrolyte drink made with multiple transportable (e.g., 3-4% fructose:glucose/maltodextrin blend, or sucrose; 10-40 mmol/L, ~460-920 mg sodium, 120-220 mOsM/L) will be most beneficial compared to the more concentrated isotonic and hypertonic carbohydrate-electrolyte beverages. 

Examples that are currently available to athletes that would meet the above recommendations include:

• Torq hydration drink (16 g carbohydrates and 275 g of sodium per 18 g serving)
• SOS (3 g carbohydrates and 330 mg of sodium per 5 g serving) 
• Skratchlabs -Hydration Sport Drink Mix (21 g carbohydrates and 380 mg sodium per 22g serving). Alternatively, some other options for athletes include a higher carbohydrate formula delivered as a hydrogel. A good example is Maurten with 25 g of carbohydrates and 34 mg of sodium. 

Lastly, if athletes do not have access to these brands or would prefer to formulate a product to their taste preferences, we suggest diluting isotonic drinks such as Gatorade with 1/3 to ½ a cup of water (500-667 mL/litre Gatorade) to adjust the solution to 3-4% carbohydrates. To ensure sodium concentration is sufficient, we also suggest adding 0.5-0.8 g of table salt (NaCl) to 1 L of this dilute drink solution.