Rehydration is a great challenge for strenuously endurance training athletes, especially when exercise takes place in a warm environment. One reason for this is the perceived difficulty to drink as much fluid as is lost by sweat and evaporation. Adding to that challenge is choosing a drink of such a quality that it facilitates the absorption of fluid from the intestines as well as contributes to retaining it in the body. Using a drink that also tastes well in order to increase spontaneous fluid intake therefore improves chances of quick and efficient rehydration.

Sweat (as well as evaporation through breath) loss is often greater than the fluid intake and/or uptake during training in endurance athletes, which leads to a state of dehydration. This condition affects the performance noticeably. When you train regularly, perhaps even multiple training sessions per day, a water deficit that arises during training can also influence your next training given you do not rehydrate properly between sessions. It is not uncommon for elite athletes to lose 1,5 liter of fluid through sweating and breathing processes. However, it is still not useful to drink more than 1 liter per hour, as this matches the stomach emptying rate. This is why athletes who train in warm environment conditions need to drink before, during and after exercise to maintain a proper fluid balance.

Drink more than you lose!

Keeping in mind that many athletes have difficulties matching their fluid intake with their fluid losses during exercise, it is valuable to choose the right drink for your sports activity. It is obvious that the electrolyte content in a drink plays a major role in the rehydration process, as the uptake and retention of  fluid is influenced by the quantity of sodium in that same fluid. (It has also been shown that potassium may contribute to the rehydration processes.)
 The amount of fluid that you need to drink after strenuous exercise should also exceed the total volume of water lost. This is due to the fact that urine production continues even in the dehydrated state, which means that part of the fluid intake will always be lost through urine. A large amount of a low-sodium drink will result in an even higher urine production.
Therefore, fluid intake corresponding to 150% of lost body weight is often recommended to ensure proper rehydration.
 An effective sports drink or foodstuff that you drink to facilitate rehydration should contain both minerals and carbohydrates. The minerals can be a blend, but the main electrolyte should be sodium, in a concentration of approximately 0, 1 percent (of the drink). For proper rehydration, a reasonable carbohydrate concentration would be 6-8 percent, maybe even a little lower. On the other hand, carbohydrate concentration in an energy drink would best be higher than in a sports drink, as it is not meant for rehydration only but rather energy supply.

Advantages and disadvantages of some drinks

Water

One of the greatest advantages of water is that it is easy to refill and store in a bottle for quite a long time. However, the low content of minerals and complete lack of carbohydrates means it is not the optimal choice for endurance exercise (over 90 minutes). It would then be more suitable to choose a sports drink that facilitates fluid uptake and retention.

Milk

Milk contains both electrolytes and carbohydrates in quantities that resemble the quantities in many commercially available sports drinks. In one study (1), eleven test subjects where asked to exercise in a warm environment up to the point where they had lost 1,8 percent of their body weight in fluid. Subsequently, they were given a drink corresponding to 150% of that volume in the form of water, sports drink, skimmed milk, or skimmed milk with added salt (20 mM NaCl). Urine samples were collected in the five following hours. It was shown that the best rehydration drink was in fact milk. With milk and milk+salt, the average urine output was 611 ml and 550 ml, respectively. With water and sports drink, urine output was twice as great (1205 and 1184 ml, respectively). These results indicate that milk is an effective rehydration drink after training and could be used as such by anyone, except in cases of milk allergy or lactose intolerance.

Chocolate milk

One great advantage of both chocolate milk and ordinary milk is the relatively high content of protein, which is important for the restitution process. (However, protein has not been shown to be of importance for the rehydration process as such, given that the electrolyte and carbohydrate content is sufficient.) As it contains both carbohydrates and electrolytes, chocolate milk has further been suggested an excellent sports or energy drink (2). The taste is perceived as appealing to most, which would facilitate spontaneous intake. However, chocolate milk (or plain milk) is not suitably kept in warm temperatures unless specially processed and bottled. Further, many endurance athletes prefer a fresher taste to quench their thirst.

Orange juice

Orange juice contains a lot of carbohydrates, which could mean a glycogen-saving effect in the training situation. A somewhat diluted juice would even possibly facilitate the rehydration process. Another advantage of juice is that it can easily be kept in a bottle, staying fresh for quite a long time. However, its electrolyte content is probably too low to be an effective rehydration drink during and after long and intensive exercise.

Maxim Energy Drink Electrolyte

Maxim Energy Drink Electrolyte is designed as a sports drink, facilitating rehydration during and after training, as well as providing extra carbohydrates. The sodium content in Maxim Energy Drink Electrolyte is high, indicating it is an effective rehydration product.

Maxim Active Hydration

Maxim Active Hydration is a rehydration drink designed to be a thirst quencher before, during and after endurance exercise. The carbohydrates in this drink function mainly as an absorption aid for the fluid itself. This product also contains more sodium than milk, chocolate milk or orange juice per serving. The content of potassium is lower than in milk or chocolate milk. However, it is above all the sodium content that is related to the absorption and retention processes. It is therefore very likely that this drink indeed facilitates the rehydration process.

Which drink is the better?

To be able to answer the question of which drink is the better one for optimal rehydration, that would require a scientific study where the drinks be tested under the same circumstances. An obvious advantage of the Maxim products is that they very easily can be brought with you. The products will stay fresh in a bottle, as opposed to dairy drinks. They also contain more sodium than milk, chocolate milk, and orange juice, which contributes to the rehydration process.
 The drink of your choice should be a drink that provides salt as well as carbohydrates. It should also offer a taste of your liking in order to encourage your spontaneous intake in sufficient amounts. These qualities combined are likely to facilitate your rehydration process.

Conclusion

There are advantages and disadvantages for every drink discussed in this article. Water would be the drink least suitable as a rehydration drink during and after long and intensive endurance exercises. Orange juice contains too little sodium to be optimal. Milk and chocolate milk, however, seem to be of more interest in this matter. These drinks contain sodium as well as potassium, a fair amount of carbohydrates, and even a considerable amount of protein, which makes them suitable as recovery drinks after exercise. Chocolate milk especially would be popular due to its taste, also containing more carbohydrates than plain milk. However, it is difficult to keep milk and chocolate milk fresh in a bottle unless specially processed and bottled. Maxim Energy Drink Electrolyte or Active Hydration may be a better alternative in that aspect, as they come in tablet or powder form for convenient transport and are easy to mix with water. The Maxim products also contain a high amount of sodium, which facilitates rehydration.
Choose a drink that suits both your training regime and your liking, in order to optimize your rehydration process.

References:

1: Shirreffs SM, Watson P, Maughan RJ. Milk as an effective post-exercise rehydration drink. Br J Nutr 2007;98:173–180.

2: Karp JR et al: Chocolate milk as a post-exercise recovery aid. Int J Sport Nutr Exerc Metab. 2006 Feb;16(1):78-91.