Fluids and electrolytes during exercise

Maintaining water balance is an important consideration during exercise. Physical activity results in increased heat production, and evaporation of sweat from the skin allows the body to dissipate this heat and maintain a normal body temperature. The amount of fluid lost as sweat varies according to factors such as the intensity and duration of activity and the air temperature.

In hot weather, long distance runners can lose up to two litres of water per hour.  However, on average, sweat losses equate to approximately one litre of fluid for each hour of exercise. Failure to replace lost fluid results in dehydration. Severe dehydration can lead to heat stroke and can be fatal but even mild dehydration (having lost as little as just over 2% of body weight) affects exercise performance. Fluid losses in excess of 5% of body weight can reduce exercise capacity by as much as 30%. It is therefore important to keep well-hydrated by drinking before, during and after exercise.

Electrolytes such as sodium, and to a lesser extent magnesium and potassium, are also lost from the body with sweat. However, except in extreme cases, losses are small and replacement during exercise is not a priority. 'Sports drinks' often contain electrolytes, particularly sodium. These have the effect of stimulating water absorption from the small intestine, which is beneficial during exercise. In addition, after exercise, replacing lost sodium is essential for full recovery and rehydration.

Before exercise

It is important to begin an exercise session well-hydrated. Drinking 400 to 500ml of fluid 10 to 15 minutes prior to exercise is recommended.

During exercise

To offset fluid losses, it is suggested that 150 to 250ml of fluid should be drunk every 15 minutes. The choice of drink depends on the intensity and duration of the activity. For exercise sessions lasting less than one hour, which are of low to moderate intensity, water is suitable. However, if the activity is more intense or lasts longer than one hour, specially formulated drinks containing carbohydrate and/or electrolytes may be more appropriate.

Adding carbohydrate to drinks, usually in the form of glucose polymers, is a useful way of increasing the fuel supply to the working muscles and can delay fatigue during endurance exercise. However, the higher the carbohydrate concentration of a drink, the slower the rate at which it leaves the stomach (the rate of gastric emptying) and therefore the slower the speed at which fluid from the drink is likely to get into the body. The addition of electrolytes, especially sodium, promotes absorption from the intestine and encourages fluid retention.

The type and duration of the activity, and, in particular, whether supplying fuel or fluid is the main priority will therefore determine the optimal carbohydrate and electrolyte composition of the drink. Commercial sports drinks generally fall into one of three categories - isotonic, hypotonic and hypertonic - based on their carbohydrate and electrolyte concentrations. Isotonic and hypotonic drinks are the most usually consumed during exercise.

Sports drinks

Isotonic drinks have the same osmolarity as plasma (the liquid part of blood) so are absorbed relatively quickly. The electrolyte content encourages fluid absorption and the carbohydrate content of 4% to 8% is sufficient to provide a useful amount of energy yet not so high as to hinder fluid absorption. Therefore, during exercise, this formulation strikes a balance between fluid replacement and fuel supply and is the most popular of the commercially available sports drinks.

If more rapid fluid replacement is a priority, hypotonic drinks are the most appropriate. These contain very low levels of carbohydrates and electrolytes and are suitable for athletes such as gymnasts who require fluid rather than energy provision.

After exercise

After exercise, rehydration and full recovery can only be achieved if the electrolytes which have been lost in sweat are replaced as well as the water. The small amount of sodium added to some commercial sports drinks therefore means these can have a beneficial rehydrating effect compared with plain water. However, salt supplements are not normally necessary.

Drinks containing carbohydrate can also help replenish depleted muscle glycogen stores (see above) and the high levels of carbohydrate (around 10% to 15%) in hypertonic drinks make them suitable for this purpose. However, this concentration of carbohydrate inhibits fluid absorption and, consequently, hypertonic drinks are not usually suitable for rehydration during exercise.

Milk and hydration

There is increasing interest in the use of milk as a rehydration solution since milk is a natural provider of water, sugar and electrolytes.
A study from Loughborough University found that milk may be more effective than water, and commercially produced sports drinks at rehydrating the body after exercise.
The presence of water, sugar (in the form of lactose) and electrolytes (sodium and potassium) in milk make it a great candidate as a recovery solution after exercise.
The results of this study are supported by another study by the same researchers that found that the subjects became sufficiently rehydrated with both skimmed milk and a carbohydrate-electrolyte solution after exercise-induced dehydration. Interestingly skimmed milk ensured they were in a positive fluid balance at the end of the recovery period, but the carbohydrate-electrolyte solution ensured normal fluid balance.