5 Hydration: Water and sports drinks
It is essential for health that good levels of hydration are maintained. Water is the most important aspect of our diet because we can’t manage without daily supplies, and fluid losses must be replaced quickly to avoid long-term damage, for example, to the brain and vital organs. The body only has a small reserve of water, and dehydration of tissues results in weakness, headaches, tiredness and loss of concentration, followed by collapse and eventually death. Water losses through urine, from the skin, from breath and in faeces average about two litres per day, and they must be replaced. During physical activity dehydration results from sweating, and so rehydration is necessary to maintain physical and mental performance levels. It is important not to rely on your thirst as an indicator of dehydration, because by then it is already too late as the effects of dehydration are already impacting on performance. Even a loss of water representing 1 to 2 per cent of your body weight can result in a lack of concentration and loss of performance.
Figure 3 illustrates the damage caused by increasing levels of dehydration on the body. It is necessary to drink plenty of fluids before, during and after exercise, especially in warm weather. As a general rule, you are advised to drink about 0.5 litres (500 millilitres) about two hours before exercise and 150 millilitres every ten to fifteen minutes during exercise. For events of a long duration, hypotonic or isotonic sports drinks, which supply some carbohydrate, may be preferred to water. These sports drinks also contain the electrolytes, sodium and potassium, which control the flow of water in and out of cells and are vital to ensure absorption.
Water is an excellent sports drink and may well be the preferred drink for activities of less than one hour, but isotonic sports drinks are considered beneficial for use by athletes for events of one hour or more duration. Isotonic drinks are so-called because they have the same number of particles per 100 millilitres as plasma and therefore are quickly absorbed. They contain 4 to 8 grams of carbohydrate per 100 millilitres and some electrolytes (including sodium) that aid absorption. Isotonic drinks are absorbed as fast, or faster, than water alone. These drinks are beneficial for endurance sports because they provide a source of fuel (usually glucose sucrose, glucose polymer or maltodextrin) in addition to water to reduce dehydration. Colouring and flavouring are usually added. You may have come across other sports drinks that are described as hypotonic or hypertonic. The decision to use these drinks will depend upon whether the goal is promoting hydration or promoting carbohydrate uptake from the gut. Hypertonic drinks contain more than 8 grams of carbohydrate per 100 millilitres. They increase the amount of carbohydrate fuel that can be supplied to the muscles, but that will also decrease the rate at which water is available because it slows the rate at which the contents of the stomach empty into the small intestine. When the provision of water for fluid replacement is the first priority, then hypotonic drinks (less than 4 grams of carbohydrate per 100 millilitres) or isotonic drinks should be used, not hypertonic drinks (Figure 4).