Seawater, The Ideal Rehydration Beverage?

You have probably seen the cool commercials showing sweating professional athletes making a legendary play, or training to their maximum level, while drinking their Gatorade or Powerade bottle. Many people, including the brands of these commercially available sport drinks, believe these sport drinks are good for you because they replenish the electrolytes you lose when sweating. However, certain nutritionists and other health and fitness guru’s tell you to stay away from them because they do more harm than good. How does this really work? Do we need to consume these sport drinks when exercising in order to replenish electrolytes?

It is well-known that we sweat during exercise, and that our sweat mainly consist of water and electrolytes. These electrolytes include sodium, potassium, calcium, and magnesium, sodium being the most abundant in our sweat. Sodium is the main cation in extracellular fluid, and its presence has a large effect on plasma osmolality. The amount of sweat and electrolytes lost during exercise varies greatly between individuals, showed by a study done on soccer players. They showed it is affected by factors such as fitness level, sweating rate, and prior diet. For example, sodium concentration in sweat can vary from 20-80 mmol/L. As sodium is the most important electrolyte in our body, it will be the main focus of this review.

Despite the variability within individuals as described above, several studies have researched the effect of sodium concentration in drinks on rehydration during and post exercise. A study published in the European Journal of Applied Physiology found that the addition of sodium to fluids consumed after exercise-induced dehydration has an effect on the rehydration process. More fluid was retained if the sodium concentrations were higher. Subjects did not remain in a positive fluid balance for more than 2 hours when the sodium concentration was low (20 mmol/L). However, drinking fluids with a volume of 1.5 times their sweat loss and a sodium concentration of 60 mmol/L did allow them to remain in a positive fluid balance (meaning that the fluid intake is greater than the fluid output). Another study from University Medical School in Scotland got similar results. They found that in order to sufficiently rehydrate after exercise, both the sodium concentration and volume of the beverage need to be high enough. It is important to note that both of these studies only looked at males, and included a minimum number of subjects. It should also be mentioned that these studies did not take effects of any other components into consideration that are present in many commercially available sports drinks, such as carbohydrates. According to a study about the maintenance of fluid and electrolyte balance during exercise, the total carbohydrate concentrations in drinks consume during exercise should be 5-10%, to avoid delay of fluid and electrolyte absorption. Therefore, sodium concentrations in sport drinks cannot simply be compared to these data in order to determine its efficiency on rehydration.

In addition to drinks, food is also a source of electrolytes. Maughan et al found that urine production was significantly less in subjects that consumed a meal with water after exercise, than in those that had a carbohydrate-electrolyte beverage (sport drink) after exercise. However, they did not give any data on the electrolyte content of the meal and sport drink. Melinda L. et al compared post heat- and exercise-rehydration when consuming chicken broth, soup, sport drinks, and water. They found that plasma volume was not significantly different from predehydration values in the chicken broth and soup trials, but remained significantly below predehydration values for the water and sport drinks trails. Considering the fact that chicken broth and soup had the highest sodium concentrations (109.5 and 333.8 mmol/L respectively versus 0.0 and 16.0 mmol/L in water and sport drinks), this might indicate that higher sodium concentrations have a positive effect on plasma volume recovery. However, it has to be taken into account these four products have large variation in substance (solid/fluid) and electrolytes and carbohydrates composition, so conclusions have to be drawn very carefully.

Analyzing various studies that have been performed on electrolyte and fluid replenishment after exercise, I think it is fair to say that electrolyte composition of fluids consumed after exercise does matter for a successful rehydration process. Looking at the results from University Medical School in Scotland and the article from the European Journal of Applied Physiology, we might be able to say that after intense exercise sport drinks might positively influence the rehydration process, especially compared to water. However, considering the study from Maughan et al and Melina L. et al, I do not think we should say that sport drinks are the ultimate way to rehydrate, as some commercials might suggest. According to this study, subjects remained in positive fluid balance when consuming beverages containing sodium concentrations of 60 mmol/L. This is equivalent to 1379 mg/L, which is not close to the 423 mg/L that for example Gatorade contains. The sodium content of sport drinks is relatively low, while the carbohydrate concentration is relatively high. This makes sense, as a very salty drink most likely does not have a preferable taste. However, if you were to replenish your water and electrolytes purely with sport drinks, you would consume a lot of (unnecessary) sugars. Therefore, I think that consuming sport drinks in order to rehydrate is mostly beneficial during longer training sessions, as the carbohydrates function as a fuel source. However, after exercising I think the calories from the sport drink might counteract the purpose of exercising, and should therefore mostly be consumed after exercise if you don’t have access to food containing high concentrations of sodium. Otherwise, the combination of consuming water and a “salty meal” might be the most efficient way to rehydrate.


Recommended Further Reading:

Rachael Rettner, Are Sports Drinks Better of Worse Than Water? April 25, 2016. Available from:

Christie Wilcox, Sport Drinks: More Harm Than Good? 2009 [cited 2017Feb19]. Available from:

Gatorade [Internet]. c2016 [cited 2017Feb19]. Available from:

Maughan R.J., Shirreffs S.M., Rehydration and Recovery After Exercise. Science & Sports, 2004, 19;234-238. Available from:

Maughan R.J., Leiper J.B., Sodium intake and post-exercise rehydration in man. European Journal of Applied Physiology and Occupational Physiology, 1995, 71;4:311. Available from:

Gethin H. Evans, Susan M. Shirreffs, Ronald J. Maughan, Postexercise rehydration in man: The effects of osmolality and carbohydrate content of ingested drinks. Nutrition, 2009:905-913. Available from:

Eric T. Wittbrodt, Maintaining Fluid and Electrolyte Balance During Exercise. Journal of Pharmacy Practice, 2003, 16;1:45–50. Available from:

Melinda L. et al, Effect of sodium in a rehydration beverage when consumed as a fluid or meal. Journal of Applied Physiology, 1998, 85;4:1329–1336. Available from:


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One thought on “Seawater, The Ideal Rehydration Beverage?

  1. Nice discussion of some of the prior literature in the area of electrolytes and recovery from exercise. You mentioned pros/cons of carbohydrates (sugars) in sports drinks. I’m wondering, with the craze in sugar-free drinks/foods, how that may impact the benefits or drawbacks of sports drinks? Do you stand by your conclusions, even for sugar-free drinks?

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