Hydration & Performance
|In this issue we delve into the archives of Sports Dietitians Australia to revisit the Consensus Statement on Fluid Replacement, developed by Australia’s leading sports dietitians.|
Many expert bodies have recognised the importance of hydration in promoting safety, enjoyment and performance during exercise and sporting activities. Dehydration can lead to a decrease in exercise performance, including impairment of skill and concentration. Fluid replacement during exercise can reduce these problems.
Comprehensive literature reviews* of fluid and energy replacement for physical activity have been undertaken. Based on this information, Sports Dietitians Australia makes the following recommendations, the benefits of which apply equally to athletes and those undertaking recreational exercise activities.
Active people should follow a well-balanced high-carbohydrate diet to maintain adequate fuel stores for their sport and exercise activities. Most people will need to drink more than their thirst dictates to achieve optimal hydration before exercise, especially in warm climates. Replenishing carbohydrate and fluid stores between exercise sessions is essential for recovery, and for safe and optimal participation in subsequent activity.
Suitable fluids should be consumed in the hours before exercise to produce a lightly-coloured urine of normal or above volume. The addition of sodium in a hydration beverage will help the body retain water, and experimenting with the timing of fluid intake will also help counter the problem of needing to urinate during exercise.
Pre-exercise hyperhydration may be considered for athletes who are likely to have high levels of dehydration during exercise from inadequate opportunity to match fluid intake to sweat losses. These athletes may benefit from consuming as large a volume of fluid as they can comfortably tolerate (e.g. 300-500 mL) just prior to exercise (e.g. 15 minutes before starting). This may help an athlete maximise fluid replacement during exercise in which sweat losses are extremely high. The athlete must experiment to ensure that they can tolerate such a fluid bolus without discomfort.
Athletes competing in weight division sports often commence exercise in a dehydrated and/or fuel depleted state because of strategies to ‘make weight’. These individuals should be guided to compete in an appropriate weight division, and to achieve weight loss by a combination of sound dietary and training strategies. Mild dehydration (1 to 2 per cent of body mass) may be an acceptable pre-competition strategy to assist some elite athletes to make weight, provided fluid balance is restored before the commencement of competition.
During exercise, the athlete should drink early and at regular intervals to minimise dehydration. Since thirst does not provide a guide to fluid needs, fluid intake should be a planned event for each athlete and each situation. Athletes can gain a clearer understanding of their sweat losses under various exercise and environmental conditions by monitoring changes in body mass over a typical exercise session. After accounting for the mass of fluid/food consumed during exercise, and any toilet visits, each kilogram of body mass lost is equivalent to a litre of fluid. The athlete can then judge total sweat rates, their success in replacing fluid during the session, and residual fluid losses that must be replaced during recovery. This information can be used to plan fluid intake strategies, or to periodically assess their success.
Fluids consumed during and after exercise must be palatable to encourage intake. The inclusion of flavour and sodium in a rehydration beverage generally encourages greater fluid consumption through taste perceptions. In fact, carbohydrate-electrolyte beverages (i.e. sports drinks) have been shown to be consumed in greater quantities than flavoured or plain water. Cool drinks (15-20oC) are refreshing and can be consumed quickly in large volumes. This is important when athletes have only a short time to drink (e.g. at an aid station, or during a pause in play).
The consumption of carbohydrate has been proven to improve performance during moderate to high intensity exercise of longer than 60-90 minutes. A carbohydrate intake of 30-60g per hour will provide additional fuel when muscle carbohydrate stores become depleted, and thus delay fatigue. This goal can be achieved by ingesting 600-1200 mL/hr of drinks containing 4-8 per cent (g/100mL) carbohydrate in the form of sugars (sucrose, glucose) or malto-dextrins, or a mixture of these. Drinks within this range of concentration can deliver fluid and carbohydrate at rates that are likely to benefit performance.
Water has often been described as the ‘best’ choice of fluid during exercise as the activities of most athletes and recreational exercisers do not deplete body carbohydrate stores. However, studies on intermittent high intensity exercise, and high-intensity exercise of about one hour, show that a carbohydrate-electrolyte beverage may also enhance performance in exercise of these types.
Sodium (0.5-0.7 g/l or 20-30 mmol/L) should be included in rehydration fluids consumed during prolonged exercise (greater than one hour). Sodium enhances the palatability of the drink, and promotes retention of the fluid consumed. In endurance events, sodium may help to prevent hyponatraemia in susceptible individuals. Although sodium is known to facilitate intestinal absorption, the amount of sodium that is palatable to consume during exercise is unlikely to provide this function.
Carbohydrate-electrolyte drinks address the goals of post-exercise rehydration and recovery by promoting fluid intake, providing a source of carbohydrate for glycogen storage, and promoting fluid retention. Although sodium replacement will occur in the diet over time, immediate replacement is important when the athlete needs to rehydrate quickly from a significant fluid deficit (for example greater than 2 per cent of body mass). Oral rehydration solutions used in the clinical treatment of dehydration contain a higher concentration of sodium (50-90 mmol/L) than found in commercial sports drinks. While the higher sodium concentrations may promote optimal electrolyte and fluid recovery, the salty taste of these drinks may be considered less palatable and cause lower rates of voluntary fluid consumption.
Overall, the athlete may replace salt losses using a combination of sodium-containing drinks and the sodium found in post-exercise recovery meals and snacks, either as a component of the food or added to the meal. Fluids containing caffeine (such as coffee or cola drinks) and alcohol are less suitable as rehydration beverages since they promote increased urine losses.
American College of Sports Medicine. Position stand on exercise and fluid replacement. Medicine and Science in Sports and Exercise, 28: i-vii, 1996.
Hargreaves M (ed). Fluid and energy replacement for physical activity. Australian Journal of Nutrition and Dietetics. 53 (4:suppl): S2-S48, 1996