Swimmers Tips
Dehydration and Sport
Introduction
Water is essential for numerous functions in the body. When the body's fluid levels become depleted, through increased sweating and/or inadequate fluid intake, dehydration can occur. Almost all levels of dehydration are associated with a reduction in sports performance. During activity, heat is generated by the exercising muscles, which raises the body's core temperature. To avoid the core temperature increasing excessively, the body employs a system known as thermoregulation. During exercise, heat is transferred from the muscles to the blood and blood flow to the skin is increased so that heat can escape into the atmosphere. The production of sweat and its evaporation from the skin, also helps keep the body core temperature low. High temperatures, humidity and strenuous exercise will all increase the amount of fluid lost as sweat. Replacing these fluid losses is particularly important to prevent dehydration and its associated dangers.
Prevention
To prevent dehydration, attention to fluid intake before, during and after exercise is important.
BEFORE EXERCISE - suitable fluids should be consumed in the hours before exercise to produce a lightly coloured urine of normal or above volume. For most people, between approximately 500 to 1000 mls of fluid during the 2 hours prior to exercise will be suitable. Experimenting with timing of fluid intake will help counter the problem of needing to urinate during exercise. Those athletes who are likely to dehydrate significantly during exercise may benefit from experimenting with the consumption of as large a volume of fluid as can be comfortably tolerated e.g., between 300 to 500ml, 15 minutes before commencing exercise. In addition to providing fluid, this strategy helps to prime the stomach to stimulate a more rapid gastric emptying of subsequent fluid intake during exercise.
DURING EXERCISE - it is important to drink early and at regular intervals, ideally at a rate that matches sweat loss. Since thirst does not provide a guide to fluid needs, fluid intake needs to be planned for each activity depending on factors such as duration of the event and conditions. Monitoring weight before and after exercise is a useful way of estimating fluid needs. After accounting for food and fluid consumed and toilet losses, each kilogram of weight lost is equivalent to a litre of fluid. Fluid losses should be replaced before, during and after exercise.
It is important to find a fluid that is pleasant to drink and encourages regular consumption. Water and sports drinks are good choices. Carbohydrate-containing beverages are particularly useful during moderate to high intensity exercise of greater than 60 to 90 minutes duration, and high intensity exercise of approximately 1 hour. A carbohydrate intake of between 30 to 60 gms per hour will provide additional fuel when muscle carbohydrate stores become depleted, thus delaying fatigue. This can be achieved by ingesting between 600 to 1200 ml/hour of drinks containing between 4 to 8% carbohydrate concentrations. Drinks within this range are able to provide useful rates of delivery of fluid and carbohydrate simultaneously. It is also recommended that sodium (between 0.5 to 0.7 g/l or 20 to 30 mmol/l) be included in rehydration fluids consumed during exercise lasting more than 1 hour. Sodium will enhance the taste of the drink and promote the retention of fluids consumed.
AFTER EXERCISE - a deliberate effort needs to be made to replace fluids lost during exercising. Drinks containing carbohydrate and electrolytes (sodium and potassium) are useful for recovery, providing carbohydrate for glycogen storage and promoting fluid retention. Recent research suggests that the optimal sodium concentration in rehydration fluids appears to be higher than that for drinks consumed during exercise and is probably in the order of between 50 to 90 mmol/l. It is particularly important to replace fluid and carbohydrate losses when exercise is being repeated in a short period of time.
Fluids containing caffeine and alcohol are less suitable as rehydration beverages as these promote increased urine losses. Alcoholic beverages are best avoided close to exercise as these not only increase dehydration but also impair co-ordination and adversely affect sporting performance.
Signs and symptoms
Common symptoms of dehydration include lethargy, nausea, loss of appetite, anxiety, inability to concentrate and in later stages, dizziness, vomiting, mental confusion and increasing weakness. Eventually this can lead to exhaustion, heat stroke and in some cases, can be fatal. Thirst is not a reliable indicator of fluid needs. Dehydration can impair performance, although the exact mechanisms for this are still unclear. Some studies have indicated that dehydration decreases the blood plasma volume, which results in less oxygen being delivered to the exercising muscle and increases breathing and heart rates.
Health care
As with all medical conditions your Doctor should be consulted. Your Doctor will diagnose and treat dehydration. Your Doctor may refer you to a Sports Dietitian for the latest advice about preventing dehydration through adequate fluid intake.
