FLUIDS 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
TYPE CONTENT USE IN SPORTS DRINKS
Isotonic Fluid, electrolytes, 4 -8% carbohydrate Fluid replacement during and after exercise. Fuel supply during exercise.
Hypotonic Fluid, very low electrolyte and carbohydrate content (<4%) Rapid fluid replacement without energy provision.
Hypertonic Fluid, electrolytes, high level of carbohydrate (>10%) Post-exercise glycogen replenishment. Not suitable for rehydration during exercise.
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, 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.