Professor John Brewer of St Mary’s University, Twickenham, investigates the use of scientifically formulated sports drinks to help replace fluid and energy stores.
Effects of exercise on the body
It has been well documented that the two biggest challenges facing anyone taking part in endurance sport are the loss of fluid and a depletion of the body’s stores of energy (1,2). Evidence suggests that the loss of fluid, and consequent onset of dehydration, can have a more rapid impact on performance than the depletion of energy stores. Depending on the intensity of exercise and climatic conditions, sweat rates of up to 4 litres per hour have been recorded during intensive sporting activities in hot and humid conditions (3), although rates of between 1 and 2 litres per hour are more normal. A position statement from the American College of Sports Medicine (4) recommended that hydration strategies should be adopted to ensure that exercising individuals lose no more than 2-3% of body weight from fluid loss, since amounts in excess of this can impair both physical and mental performance. It is therefore clear that failure to replace fluids that are lost as a result of sweating and other thermoregulatory processes can have a rapid negative impact on performance. For example, a 75kg individual losing fluid at a rate of 2 litres per hour will lose more than 2% of body weight (1.5 litres of fluid) after less than 1 hour of exercise. The challenge of dehydration during exercise is exacerbated when the environmental conditions exhibit high heat and humidity (5). In such circumstances, sweat rates are high and sweating becomes a less efficient means of maintaining core body temperature, with the potential for thermal stress and even hyperthermia.
The body’s preferred fuel for moderate and high intensity exercise is carbohydrate, which is stored in the muscle and liver as glycogen. Whereas a loss of fluid can quickly negatively impact on performance during moderately strenuous exercise, the body has sufficient glycogen stores for approximately 90-150 minutes of exercise, and possibly longer if the exercise intensity is low. Hence whilst it is true that loss of fuel and fluid remain major challenges for those taking part in sport, it is the loss of fluid that may be most problematic in the majority of sporting events, including those that do not last longer than 90 minutes. This alsosuggests that the impact of fluid loss on performance in ‘traditional’ endurance sports, including long distance running and cycling, is also a factor in other sports, such as soccer, rugby, hockey and tennis. Endurance activities, such as marathons, will require approximately 2500-3000 calories of energy expenditure, since running generally requires an energy expenditure of 100-120 calories per mile and as a consequence endurance running results in significant glycogen depletion. However running shorter distances or team sports, such as soccer, where energy expenditure is likely to be around 15 calories per minute for a duration of 90 minutes, will result in a total energy cost that is likely to be less than 1500 calories. This will place only a limited challenge on the body’s total reserves of glycogen, which should provide the equivalent of 2000 calories of energy. Similarly the energy cost of playing a rugby match has been estimated at approximately 1900 calories (6), a hockey match 1200 calories (7) and a women’s basketball match approximately 600 calories (8). Hence these studies would suggest that whilst the loss of energy and fluid are a challenge in many sports, the onset of dehydration as a result of fluid loss and subsequent impact on performance occurs earlier, and in more sports, than impact of the loss of energy stores.
Sports drinks
Studies have shown that the optimal means of replacing the fuel and fluid lost during exercise is through consumption of sports drinks, in particular those that are ‘isotonic’ (9). These have a carbohydrate content of between 4% and 8%, as well as electrolytes – primarily sodium and potassium – which aid in the absorption of fluid and carbohydrate, whilst also replacing electrolytes that are lost as a result of sweating. The combination of carbohydrate and electrolytes affects the concentration (or osmolality) of a drink. Isotonic drinks have an osmolality of between 270-330 mmol/l; those with an osmolality less than this are hypotonic solutions, whilst those with a higher concentration are hypertonic solutions. Drinks that are hypotonic will result in rapid fluid absorption, but limited energy replacement, whilst the high osmolality of hypertonic solutions may slow the process of emptying from the stomach into the gut and the transport of fluid from the gut to body tissues.![Image]()
Influence of taste
Many nutritional companies across the world have responded to research on the benefits of isotonic drinks by producing a wide range of products specifically developed to optimise the rate of fluid and fuel absorption. But whilst the ingredients of such a drink may be optimised for technical performance, it has also been shown that the more fundamental variable of taste, has an impact on the desire to drink, and hence the quantity of fluid that is ingested (10,11). Voluntary consumption of a flavoured drink was shown to be greater than that of unflavoured water during cycling exercise at 50% of maximum oxygen uptake suggesting that a palatable flavour can increase consumption of a drink (4). This is particularly an issue when a sports drink contains the electrolyte sodium, which is required to enhance the absorption of fluid from the gut and to replace sodium that is lost from sweating, particularly in hot and humid environments (4). The ‘salty’ taste that results from the inclusion of sodium in a drink can be off-putting for users, and restrict the volumes that are consumed. One option is to mask this taste with additional carbohydrate to provide a sweeter taste. However this will increase the calorific content of the drink and can quite easily increase the drink’s osmolality to a level that changes its scientific properties, such that drinks designed to be either hypotonic or isotonic no longer fit into these categories.
Low calorie sweeteners
An alternative to the addition of carbohydrate to optimise taste is to add a low calorie sweetener, which has minimal effect on the osmolality of a drink, controls the calorific content and enables the scientific formulation and integrity to be maintained. A sub optimal taste or high calorie content is unlikely to be popular with both manufacturers and consumers and is far harder to underpin with science. Whilst low calorie sweeteners can be used to support the development of science based sports drinks for optimum hydration, their ability to control and reduce calorie content should not be over-looked. Although many people take part in sport and exercise for competitive reasons and to enhance their performance, there are also many who exercise for other goals, including weight loss and weight maintenance. Moderate intensity exercise in a gymnasium environment can use approximately 10 calories per minute, or 300 calories in total for a 30 minute session. A 500ml bottle of isotonic drink consisting of 6.5% carbohydrate (i.e. 6.5 grammes of carbohydrate per 100ml of fluid) will contain approximately 130 calories of energy or almost half of the energy burnt during the session, which is equivalent to the energy expended during a 1 mile run for an average person. Where the main priority is to replace fluid and electrolytes and not fuel, a low calorie hypotonic solution, flavoured water or water, may be the best option. In this circumstance, regulation of taste can be achieved with low calorie sweeteners, sustaining taste whilst simultaneously reducing the energy content of a drink. A recent review (13) highlighted the potential impact of the acidity and sugar content of a drink on dental erosion (acidity) and caries (sugar). Along with good oral hygiene and appropriate usage, a reduction in sugar content, through the use of low calorie sweeteners, can help to mitigate much of this risk.
Summary
The challenge of energy and fluid replacement is one that has to be met in many sports. Evidence suggests that the fluid loss challenge occurs earlier, and across a broader range of sports, than that of energy loss. It is accepted that scientifically formulated sports drinks can help to meet this dual challenge by replacing fluid and energy stores along with electrolytes, such as sodium, that are excreted when sweating. Low calorie sweeteners can be used to maintain the scientific integrity and taste of sports drinks, optimising the uptake of both fluid and energy. The use of low calorie sweeteners can help to promote hydration during exercise, whilst supporting the maintenance or loss of body weight, and contribute to a reduction in the risk of dental caries.
References
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