?Sports drinks? ? what are they and for whom?

The first generation of sports drinks was developed in 1965 after the coach of the University of Florida Gators, an American football team, asked Dr Robert Cade, a renal specialist, whether his players? extreme weight loss (18lbs or more in the course of a game) and their inability to urinate was connected with their poor performances in the second half of games and their frequent hospital admissions! Cade soon realised that the Gators were suffering severe dehydration, and, after discussion with colleagues, his solution was to mix water with salt and some sugar, in amounts that would be tolerated by the exercising gut, and offer it to the flagging players.

The first experimental batch tasted so bad that nobody could tolerate it, but ? the story goes ? Cade?s wife suggested adding lemon juice, and the US sports drink Gatorade was born. The worldwide demand for sports drinks is now immense: the market is worth billions of dollars and further growth is predicted. Alongside Gatorade (Pepsico-owned) there are Powerade (owned by Coca-Cola), the UK brand leader Lucozade Sport (GSK) and others including Isostar, SiS Go, High5 Isotonic and Maxim Electrolyte. They are all designed predominantly to provide carbohydrate and salt to prevent the problems associated with dehydration, to supply carbohydrates to augment the limited stores of muscle and liver glycogen, and to replace electrolytes lost due to perspiration. The popular brands typically provide 6-8 per cent carbohydrate and 17-21mmol/l sodium, and are flavoured and coloured for palatability.

While such carbohydrate-electrolyte drinks have been the main sports nutrition tool for the past decade, recent years have seen the growth of sports drinks reportedly designed for other performance enhancing effects. These include beverages intended to provide high levels of carbohydrate for “carbo-loading” or stimulants including caffeine and guarana for mental alertness, as well as those that contain certain amino acids that are beneficial to immune function, and which offset mental fatigue or start the process of muscle recovery. We are now seeing the emergence of new “sports waters” ? flavoured low carbohydrate, low salt drinks aimed at the fitness, weight-consciousness market.

Carbohydrate-electrolyte drinks and effective hydration

The major reason why anyone drinks fluid before, during and after physical activity is to replace the water that is lost through sweat. If the water is not replaced, dehydration will occur and performance will be impaired, even at fluid losses of as little as 2 per cent of body mass¹. Progressive dehydration leads to further adverse physiological effects on both health and performance and can eventually be fatal. The many reports over the years of hyperthermia and dehydration in sportsmen and women clearly illustrate this.

How much carbohydrate should there be in a sports drink?

Drinks that aim to promote rapid rehydration tend to have a carbohydrate content in the range of 2-8 per cent ie lower than many popular soft drinks and fruit juices. For the optimum effect, a drink has to move quickly from the stomach and be absorbed through the intestine in order to maintain homeostasis during exercise. It seems that there is an inverse relationship between carbohydrate content and the rate of gastric emptying. Increasing the amount of carbohydrate in a beverage enables more exogenous substrate to be provided to the exercising athlete, but reduces the availability of water. Where fluid, rather than carbohydrate availability, is the priority for the exercising individual, a lower carbohydrate content is vital.

Noakes et al² looked at the effect on gastric emptying of carbohydrate levels in drinks and found that a content of up to 8 per cent has no effect, so manufacturers are expected formulate a brand containing 6-8 per cent carbohydrate, enabling the provision of both water and carbohydrate.

Once emptied from the stomach, fluids have to be absorbed. Although a carbohydrate concentration of 6-8 per cent does not significantly alter the absorption of water from beverages, when the luminal concentration of glucose is greater than 10 per cent, fluid enters the intestine from the vascular space, promoting dehydration. Again, manufacturers have taken up the results of this research to keep the carbohydrate fairly low.

What type of carbohydrate should be used in a sports drink?

Glucose, sucrose, fructose and maltodextrins are the carbohydrates commonly used in commercial energy-hydration drinks. When it comes to gastric emptying, it seems that glucose, sucrose and maltodextrins are very similar. Fructose solutions, however, seem to empty the quickest. In addition, by adding 2-3 per cent fructose to solutions that also contain glucose, gastric emptying is enhanced³. Many brands include a small amount of fructose in their formula, but too high a concentration is associated with gastrointestinal distress. The type of carbohydrate can also influence intestinal absorption. Overall it seems that solutions containing two transportable carbohydrates, eg glucose plus fructose, show greater absorption of water, sodium and carbohydrate than those containing a single carbohydrate source⁴.

Why is sodium needed in a sports drink?

Effective rehydration requires that the beverage consumed is retained by the body. The inclusion of sodium ? which plays a role in exertional heat cramps, heat exhaustion and hyponatraemia⁵ ? in carbohydrate-electrolyte drinks helps with fluid retention as well as stimulating glucose absorption in the small bowel through the active co-transport of glucose and sodium. Sodium also helps maintain the extra-cellular fluid volume and stimulates the desire to drink, whereas plain water “turns off” thirst before an athlete is completely rehydrated. Sodium helps maintain the drive to drink, so an athlete is more likely to consume the larger volumes needed after training or competing.

Research shows that plain water can be effective in rehydration when consumed with (solid) food ? drinking with meals helps the retention of fluid, probably because of the presence of salt and other electrolytes in the meal.

Most elite athletes, particularly those competing in endurance events and training for 2-5 hours a day, should not try to follow a low salt diet, because their daily sodium losses through sweating could be high. A typical British adult gets about 9 g of salt a day, and the suggested healthy level of 6 g a day should not be applied by serious athletes. To illustrate this ? 5 litres of sweat at an “average” sweat sodium concentration of 50mmol/l (the usual range is 20-80mmol/l) would contain about 15g of salt (3 teaspoons). Replacement of sodium through food and drink is therefore essential if full fluid balance is to be achieved. Plain water taken post-exercise promotes high urine flow, and positive fluid balance is maintained for only a short period. A correctly formulated sports drink that includes sodium will encourage positive fluid balance thus limiting dehydration, heat illness and performance reduction.

How much sodium should be in a sports drink?

It has been argued that, in comparison with those found in diarrhoeal conditions, sodium deficits during exercise are not great, justifying the use in fluid-replacement beverages of lower sodium concentrations ? typically 10-25mmol/l ? rather than the typical 60mmol/l of oral rehydration solutions prescribed for diarrhoea.

However, during exercise sodium is drawn from both the extracellular and the intracellular spaces. If the extracellular sodium is not replaced, water is lost from the intracellular space, and the athlete is unable to maintain plasma volume at an optimal level. While the absolute amount is still a moot point, the general consensus is that a drink should provide 20-30mmol/l, although some sports scientists believe that 40-60mmol/l may be best, although the palatability of the drink then becomes an issue.

Should the drink be isotonic, hypertonic or hypotonic?

The caloric density of a beverage and volume consumed are considered the main influences on the rate of gastric emptying, while its osmolality has more influence on intestinal water absorption. Both hypo- and isotonic drinks (with less than 280mOsm/kg) promote water absorption, but hypertonic drinks stimulate less water absorption and more secretion into the gastrointestinal lumen, leading to potential for dehydration⁶. A good carbohydrate-electrolyte drink for hydration will therefore be iso- or hypotonic.

Why is flavour important?

Research shows that people will drink more of a lightly flavoured beverage than they will of plain water and will therefore be better hydrated by such a drink. The most important elements in making a sports drink taste good are flavour type, flavour intensity, and sweetness. These elements combine with others, for example tartness and saltiness, to form an overall flavour profile that optimises the palatability of the beverage. The flavour of a sports drink may be even more important in the young athlete. This is important because at a given level of hypohydration children’s core temperatures rise faster than those of adults, and with their immature sweat glands they are more at risk of heat-stress. Getting them to drink when exercising in hot conditions is therefore imperative. In the US, it appears that pre-pubertal and early pubertal children prefer a grape flavour to apple and orange flavours or water⁷.

At what temperature should the drink be served?

The temperature of the drink is very important in determining how much athletes will consume during exercise, an effect suggested many years ago when researchers noticed that soldiers working under warm conditions in the field dramatically increased their voluntary fluid consumption when the water that was available to them was chilled. More systematic research conducted since then has revealed that chilling water provided can lead to an increase in uptake of 40-80 per cent.   However, it is worth noting that a cooled beverage has only a trivial affect on core body temperature, quickly reaching body temperature in the stomach in any case.

While evidence of the benefits of carbohydrate-electrolyte drinks for many sportsmen and women engaged in high intensity, high volume training is clear, the benefits for the weight-conscious occasional gym user are less well-defined. For most an organised meal and snack pattern with chilled water, and an awareness of the need to drink slightly more with meals would be most appropriate from a health point of view. Perhaps of more concern is the fact the largest market for sport beverages is in the non-exercising population. Such individuals may chose sports drinks as an alternative to soft drinks; to associate themselves with their sporting heroes or to foster the image of a “sporting lifestyle”.

The emergence of sports drinks as a beverage of choice, rather than as an aid to athletic training, has an number of public health implications. Firstly, a 500ml bottle of a typical sports drink will provide 120-160kcal of simple carbohydrates and will be high in acidity to ensure product stability and safety. They have therefore, like most soft drinks, the potential to harm teeth, and while sports dieticians working with athletes educate them on ways to minimise the risk to teeth, it is uncertain whether such messages are filtering through to other consumers. Secondly, a typical serving of a sports drink provides 0.6g of salt at a time when there are calls to lower the salt content of food in the diet; clearly there is a need for consumer education in the advantages and disadvantages of using sports drinks.

While the days of a cup of tea, or orange segments, at half time are long gone, modern sports drinks may still fall short of what is optimal for an elite athlete under all exercise conditions. At times, eg during winter training, an athlete may want a drink to offer more than 6-8 per cent carbohydrate, and less fluid. On the other hand, when sweat losses are large, and fluid and sodium provision may be more important, a drink providing 240-320kcal per litre may lead to an inadvertent excess calorie intake. Some manufacturers are now producing highly specific products, not for sale to the general public, but aimed at elite (endurance) athletes. Commercially these are a non-starter, but they offer athletes a more scientifically appropriate drink and in return, the companies can maintain good public relations with the elite, which helps sells their product to lesser mortals.

The bottom line, however, is that a sports drink, not matter how scientifically formulated, is only effective if it is consumed. Despite the convenience factor, it is the palatability and cost of such branded products which seem to be a barrier for many athletes who would otherwise benefit from their use. Many elite squads now hold lucrative sports drinks deals which overcome the cost element, while for others, a home-made version using flavoured, sucrose-containing cordials and/or glucose and salt, whilst not scientifically tested, could confer many of the basic performance-enhancing effects.

1 Walsh RM, Noakes TD, Hawley JA et al. Impaired high-intensity cycling performance time on low levels of dehydration. Int J Sports Med 1994; 15:392-397

2 Noakes TD, Rehrer NJ, Maughan RJ. The importance of volume in regulating gastric emptying. Med Sci Sports Exerc 1991;23:307-313

3 Neufer PD, Costill DL, Fink WJ et al. Effects of exercise and carbohydrate composition on gastic emptying. Med Sci Sports Exerc 1986:18(6):658-662

4 Shi X, Summers RW, Schedl HP, et al. Effects of carbohydrate type and concentration and solution osmolality on water absorption. Med Sci Sports Exerc 1995; 27(12): 1607-1615

5 Shirreffs SM, Armstrong LE and Cheuvront SN. Fluid and electrolyte needs for preparation and recovery from training and competition. J Sports Sci 2004;22:57-63

6 Wapnir RA, Lifshitz F. Osmolality and solute concentration: their relationship with an oral hydration solution effectiveness. An experimental assessment. Pediatr Res 1985; 19: 894-898

7 Meyer F, Bar-Or O, Salsberg A, and Passe D . Hypohydration during exercise in children: Effect on thirst, drink preferences, and rehydration. Int. J. Sports Nutr. 1994; 1:22-35.

Jacqueline Boorman RD is an Accredited Sports Dietician with World Class Performance Programmes