Exercise scientists have known for many years that the two main challenges facing the majority of people involved in sustained sport and exercise are a loss of energy, and a loss of fluid. While the Ancient Greeks relied heavily on a diet of protein rich foods, in the assumption that these would replace energy lost during exercise and enhance the quality of the muscles, scientists now know that the primary fuel for most sporting activities is in fact carbohydrate, stored in the body as a substance called glycogen, in the liver and muscles.
While glycogen is a readily available fuel for both endurance and high intensity activities, for example, marathon running and high intensity sprinting, common in games such as soccer and handball, unfortunately the body’s stores of glycogen will only last for around two hours of moderate intensity exercise. After that, the body has to rely upon its less efficient stores of fat to provide energy at a slower rate, which results in fatigue.
Studies undertaken across a range of sports and environmental conditions have shown that a loss of fluid as a result of sweating during exercise, can quickly impair performance. Sweating is essential, since it is the body’s main defence mechanism against a dangerous rise in core temperature.
Sweat rates of up to four litres per hour have been recorded (although rates of one to two litres per hour are more common) in conditions where the exercise or sport is intensive, and particularly when conditions are hot and humid.
There is strong evidence to suggest that when dehydration—as a result of a loss of body fluid—exceeds two percent of body weight, both physical and mental performance will start to suffer. For an individual weighing 75kg, a two percent weight loss is equivalent to 1.5kg, or 1.5 litres of sweat, which can be easily ‘achieved’ in less than an hour when sweat rates are moderately high.
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The early awareness of the importance of fluid and fuel as a means of sustaining performance was initially focused on endurance sports, such as long distance running and cycling. These sports place an obvious major challenge on the cardio-vascular system, creating high rates of energy and fluid loss.
In recent years, scientists have shown that other sports can also place a significant challenge on the body’s energy and fluid stores, including many team sports. As a result, athletes and coaches have become increasingly aware that any small differences that can be made through the consumption of drinks to replace either fluid or energy before, during and after training and competing, can be the difference between success or failure and winning or losing.
Development Of Sports Drinks
Turn the clock back 100 years or so and the main fluid consumed during sport was either water or alcohol. The former, the most natural source of fluid, is an obvious means of replacing fluid lost from sweating. The latter, used more as a means of dulling the pain of intensive effort, has long been discarded as a sensible and effective method of rehydration for (most!) sportsmen and women.
Water was, and still remains, a valid means of replacing lost fluid, especially for individuals are only exercising for moderate amounts of times and who may also want to lose weight. Crucially, however, sweating also results in a loss of essential electrolytes, in particular sodium and potassium, which if left unchecked can disrupt the function of the muscles and may possibly lead to muscle cramps.
In countries where the debilitating effects of diseases such as cholera included dehydration and loss of life, it was found that adding small amounts of electrolytes to water would actually improve the rate of fluid absorption and combat dehydration.
Many manufacturers of sports drinks will invest significant sums to refine the taste of their products, which has been shown to enhance fluid consumption.
The presence of electrolytes in a drink, particularly sodium, has been shown to increase the rate of transport of fluid across the gut membrane into the body, thus enhancing the rate of fluid absorption and combating dehydration.
Therefore, in the 1970s, scientists and manufacturers started to apply the science underpinning the drinks used to fight dehydration during illness, to the design of products aimed at fighting dehydration during sport and exercise.
At the same time, muscle biopsy studies, taken from athletes taking part in a range of sporting activities, were revealing the importance of glycogen in both sustaining performance and offsetting fatigue. While diet was seen as one means of providing the carbohydrate required for conversion into muscle and liver glycogen, solid foods are often impractical and unpalatable, and have minimal impact on exercise-induced dehydration.
As a result, scientists began to investigate the optimal concentration of carbohydrate within a sports drink that would quickly provide energy needed for sport and exercise. This knowledge, combined with an insight into the importance of electrolytes, helped to create the multi-million dollar industry that now sells sports drinks across the world.
Isotonic, Hypotonic & Hypertonic
At the forefront of this market is the sale of isotonic drinks—sold with a concentration of carbohydrate and electrolytes that optimises absorption of the fluid from the stomach to the gut and subsequently through the wall of the intestine into the body tissues. The carbohydrate concentration of isotonic drinks is normally between four to eight percent, or four to eight g of carbohydrate per 100 ml of fluid.
Of course, there are many occasions when energy replacement during sporting activities is not essential. For example, when exercise lasts for less than 90 minutes or when an individual is exercising to promote weight loss. The body’s reserves of glycogen are sufficient to meet the energy requirements of most activities of this duration, although dehydration can still be an issue.
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To meet this demand, many manufacturers have produced hypotonic sports drinks—these still contain electrolytes to promote the transfer of fluid through the gut wall, but a lesser concentration of carbohydrate and hence, fewer calories.
Hypotonic drinks normally have a carbohydrate concentration of around two percent or less and are best used in environments when hydration, not energy replacement, is the primary goal. Of course, water is the ultimate hypotonic drink and it would be wrong not to recognise the role of this most natural of fluids as a means of rehydrating.
Drinks with a higher carbohydrate concentration (above eight percent) are known as hypertonic energy drinks and are not sports drinks. Their carbohydrate concentration is too high to promote rapid energy absorption during exercise and their consumption runs the risk of sickness and stomach upset if consumed during vigorous exercise.
A better alternative is to use hypertonic drinks as part of a recovery strategy to replace carbohydrate stores after exercise. Some of them are carbonated, but carbonation is not ideal for drinks designed to enhance sports performance, simply because the feeling of ‘bloating’ can reduce the desire to replace the required volume of fluid.
There is an increasingly large market for the manufacture and promotion of protein drinks—often in the form of powdered ‘shakes’ for sportsmen and women. While these certainly have a role to play, protein replacement during exercise is not performance-enhancing and as such, are not essential before or during exercise.
There are, however, scientific studies which suggest that consuming high concentrations of protein during the post-exercise phase may help to boost the recovery process and the development of muscle strength, particularly after resistance training. Other studies are showing that a combined protein-carbohydrate drink after exercise enhances recovery and the body’s re-synthesis of muscle glycogen.
Sports and energy drinks containing caffeine are promoted and used widely. However, the association of the word ‘energy’ with caffeine is misleading, since caffeine itself contains no calories or energy value.
Caffeine is a stimulant and as such, it gives the consumer a mental ‘boost’ and a feeling of extra energy and alertness. Therefore, drinks containing caffeine may be of more value in sports where focus, concentration and co-ordination are crucial. High concentrations of caffeine are known to have a diuretic effect on the body, increasing urine production, which can be detrimental during sporting activity.
Many manufacturers of sports drinks will invest significant sums to refine the taste of their products, which in some studies has been shown to enhance fluid consumption. Clearly, when individuals are involved in activities where high sweat rates are prevalent, encouraging appropriate fluid consumption is important.
However, in situations where sweat rates are low, excessive fluid consumption can result in a condition known as hyponatremia, where the body tissues become ‘flooded’ with excess fluid. Side effects include a loss of muscle function, coordination, nausea and collapse, which can have serious and potentially fatal consequences.
The use of low calorie sweeteners, such as aspartame, is common in many sports drinks. These are used to ensure that palatability is retained, without compromising the carbohydrate content of the product, which would otherwise need to be increased. While there are those who would cite research suggesting the potential harm of sweeteners, this remains an area of controversy and the levels found in sports drinks are well below the minimum recommended by food regulatory authorities.
The growth of the internet has enabled many products to be sold to a global market. Unfortunately, this has opened up the opportunity for sales of sports nutrition products to less scrupulous manufacturers, often making claims about the efficacy of their products that have not been proven by good science.
Worse still, they may contain products which are harmful, and banned by food regulators and the World Anti-Doping Association (WADA). Recently, in the UK, a marathon runner died after consuming a sports drink towards the end of a race, which was subsequently found to contain the banned substance DMAA, raising awareness of the need for appropriate regulation and informed decision making for consumers.
Sports drinks can play a major role in enhancing performance during a range of sports and exercise activities. By definition, all will contain fluid, which can offset dehydration, but other ingredients, particularly carbohydrate and electrolytes, will enhance the efficacy of the product and can be changed to reflect the need-state for which the drink is designed.
With the growing concerns over product claims, consumers would be more wary of products with extravagant claims that have no scientific backing and the purchase of products from unregulated sources, where the brands cannot be verified.