All athletes, whether it be trained or untrained, should ingest fluids before, during and after training or competition in order to achieve optimal euhydration. Fluids are required to prevent dehydration (loss of >2% body weight) and excessive electrolyte loss which leads to impaired exercise performance, fatigue and physiologic function, with >3% dehydration increasing the risk of developing an exertional heat illness (e.g. heat cramps, exhaustion or heat stroke (Casa, 2000). Fluids also provide a carbohydrate source to prevent the depletion of the body’s stores (Maughan 1994) – thus the composition of fluids is crucial. The amount and rate of fluid replacement will depend on a variety of factors – individual athlete’s sweat rate; exercise duration; exercise intensity; environmental factors; acclimatization state and the sport dynamics for example opportunities to drink and access to fluids (Casa, 2000). The absorption of fluid into the body depends on the speed at which it is emptied from the stomach and the rate at which it is absorbed through the walls of the small intestine. The higher the level of fluid in the stomach, the more gastric emptying is encouraged.
Aim of Sports Drinks
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Sports drinks aim to provide a number of different functions including provision of substrate, prevention of dehydration, electrolyte replacement, pre-exercise hydration and post-exercise rehydration (Maughan, 1998). The functional characteristics of a sports drink can be manipulated by altering the different variables which include carbohydrate content: concentration and type; osmolality; electrolyte composition and concentration; flavouring components; and other active ingredients (Maughan, 1998). Composition of Sports Drinks
The major components of sports drinks are carbohydrates and electrolytes. Evidence suggests that the only electrolyte that should be added to sports drinks is sodium (Maughan, 1994). Sodium does not have a direct impact on physical performance but has a number of essential functions including maintaining plasma osmolality (Below et al, 1995), encouraging voluntary fluid intake (Passe, 2001) and reducing urine output (Vrijens & Rehrer, 1999). Present in small amounts, sodium helps to quicken the rate of gastric emptying and increases the rate of fluid absorption into the intestine. Sodium concentration in sweat and plasma are 10.0-70.0mEq/l and 135.0-148.0mEq/l respectively (Latzka & Montain, 1999, Costill, 1984). Athletes have a sweat rate of around 1litre/hour during exercise which increases in higher temperatures (Brouns F & Kovacs E). It is important to remember that sweat rates and sweat electrolyte content vary between each individual athlete (Sawka, 2007). Carbohydrates are the main energy source during exercise and are stored as glucose in the liver and muscles. Consuming sports drinks containing a carbohydrate source helps to maintain the body’s glycogen stores and prevent glucose levels from falling too low. However, high concentrations of carbohydrate in the drink slow the rate of gastric emptying (Merson el al, 2002).
Types of Sports Drinks
There are three types of sports drinks, all of which contain various levels of fluid, carbohydrates and electrolytes (usually 10-25mmol/l of Sodium) – (i) Isotonic drinks contain fluid, electrolytes and 6 to 8% of a carbohydrate source. These empty from the stomach at a rate similar to water and quickly replace fluids lost by sweating and supply a boost of carbohydrate. Isotonic drinks are the drink of choice for most athletes for example in middle and long distance running and team sports. In the Position Stand by the American College of Sports Medicine Position 2007, isotonic drinks are recommended for events lasting longer than one hour .(ii) Hypotonic drinks contain fluid, electrolytes and a low level of carbohydrate and quickly replace fluids lost by sweating. This is suitable for athletes who need fluid without the boost of carbohydrate e.g. jockeys and gymnasts. A study by Bonneti and Hopkins 2010, found that the ingestion of a hypotonic drink enhanced performance similar to that of an isotonic drink, suggesting that hypotonic drinks may be the preferred option for endurance performance of longer than an hour. More evidence is needed to prove this, but it seems that water is absorbed into the intestine more rapidly when using a hypotonic drink over an isotonic drink ( Bonneti & Hopkins 2010) (iii) Hypertonic drinks contain fluid and high levels of carbohydrate. They are used to supplement daily carbohydrate intake to meet energy demands, particularly in ultra distance events were high levels of energy are required. They need to be used in conjunction with isotonic drinks to replace fluids.
Studies on Carbohydrate-Electrolyte Sports Drink
In an area of on-going research, many studies have been conducted with carbohydrate-electrolyte sports drinks and their effects on performance. Table 1 summarises the effect of these drinks when compared with placebos. Studies lasting longer than one hour, with the addition of carbohydrate to the drink, have shown to increase performance (Coyle, 2004). Events lasting less than one hour have shown to have no need for carbohydrate electrolyte sports drinks (Bonen et al 1981, Powers et al 1990). However, a study by Murray et al 1989, found that drinks containing 8-10% carbohydrate delayed fluid absorption and gastric emptying whilst drinks containing 6% carbohydrate solution enhanced exercise performance after 1 hour. When comparing a 15% carbohydrate-electrolyte solution to a 2% carbohydrate-electrolyte solution in a hot climate, it is in fact the 2% solution that increased performance and prevented fatigue (Galloway & Maughan). There has been numerous studies conducted with high carbohydrate content solutions (above 10 %,) but this amount of carbohydrate content should ideally be used in carbohydrate loading only and not when looking to increase performance.
There is clear evidence that drinks, containing an energy source in the form of a carbohydrate and an electrolyte, have beneficial effects for athletes and improve performance (Convertino et al, 1996; Casa 2000) provided exercise duration is long enough to allow empting of the drink from the stomach followed by absorption into the intestine (Shirreffs, 2003). It is important to remember, that every individual is different, and what suits one person in a given situation may not suit another, therefore customized fluid replacement programmes are advised (Maughan 1993).
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