The role of sports drinks in an athletes diet
Wander into any sports or health food shop and the chances are that energy drinks will be the most ubiquitous sports nutrition product on the shelves. But what exactly are they, what are the benefits and how should they be used? Andrew Hamilton explains.The primary purpose of an energy drink is to supply carbohydrate in a rapidly absorbable form in order to help provide fuel for hard-working muscles during vigorous exercise. A secondary use of energy drinks is to help refuel muscles after exercise, when it might not be possible to consume adequate carbohydrate in the form of a meal or snacks.
Most energy drinks contain a combination of quick-releasing simple sugars and slower-releasing longer chain sugars in order to provide a quick acting, yet sustained increase in blood sugar, which in turn helps keep muscles fuelled. The most common sugars used are as follows:
- Glucose – a sweet-tasting simple sugar that is easily and rapidly absorbed into the bloodstream, making it ideal for rapid transport into working muscles;
- Maltodextrin – less sweet-tasting glucose that is also readily absorbed, but which provides a gentler and more sustained rise in blood sugar;
- Fructose – a simple sugar derived from fruit sugar, which when combined with glucose, enhances the maximal uptake of carbohydrate into working muscles thus providing an additional performance benefit (more later).
In addition to carbohydrate and water, many energy drinks now supply electrolyte minerals such as sodium, potassium, calcium, magnesium and chloride. There are three reasons why replacement of these minerals via an electrolyte mineral-containing drink may be better then drinking pure water alone:
- Prolonged heavy sweating can lead to significant mineral losses (particularly of sodium), which has been linked with such side effects as cramping. Drinking a formulation without electrolyte minerals for a long period of time can dilute the concentration of electrolyte minerals in the blood, which can impair a number of normal physiological processes;
- Drinks containing electrolyte minerals – particularly sodium – are known to stimulate thirst, thereby encouraging a greater voluntary intake of fluid (1). There is also evidence that drinks containing sodium stimulate the rate and completeness of rehydration after a bout of exercise (2);
- When the electrolyte minerals – again, particularly sodium – are present in appropriate concentrations, the rate of fluid absorption from the small intestine into the rest of the body appears to be enhanced, especially in conjunction with small amounts of glucose (3). This is particularly important when rapid uptake of fluid is required, such as during strenuous exercise in the heat.
The benefits of using energy drinks
The benefits of using energy drinks stem mainly from the conservation and replenishment of stored muscle carbohydrate, which is also known as glycogen. Although the human body can use fat and carbohydrate as the principal fuels to provide energy for exercise, it’s carbohydrate as muscle glycogen that is the preferred or ‘premium grade’ fuel for sporting activity. There are two main reasons for this:
- Firstly, carbohydrate is more ‘oxygen-efficient’ than fat, liberating more energy for muscle contraction per oxygen molecule from the air you breathe than when fat is oxidised. This is important because the amount of oxygen available to working muscles isn’t unlimited – it’s determined by your maximum oxygen uptake (VO2max);
- Secondly and more importantly, unlike fat (and protein), carbohydrate can be broken down very rapidly without oxygen, via a process known as glycolysis, to provide large amounts of extra energy during intense training (anaerobic training). And since many athletes tend to include intense training in their schedules, this additional energy route provided by carbohydrate is vital for maximal performance.
Carbohydrate can be stored in muscles in the form of muscle glycogen – ready for use when muscles need to work vigorously. However, valuable as these glycogen stores are, they are often insufficient to supply the energy needs during longer events. This explains why during long events, when your muscle glycogen supplies run low, you sometimes feel as though you’ve hit a ‘wall’ and have to drop your pace significantly from that sustained when glycogen stores were higher.
Studies have shown that muscles can only store enough muscle glycogen to fuel around 1½ to 2 hours of high-intensity exercise. For example, a trained marathon runner can oxidise carbohydrate at around 200-250g per hour at racing pace; even if he or she begins the race with fully loaded glycogen stores, muscles would become depleted before the end of the race. The body can store up to 375g of glycogen.
The problem of premature depletion can be an even bigger problem in longer events such as triathlon or endurance cycling events and can even be a problem for athletes whose events last 90 minutes or less and who have not been able to fully load glycogen stores beforehand. Long distance runners call this performance drop ‘hitting the wall’, while cyclists refer to it as ‘the bonk’!
Given that stores of precious muscle glycogen are limited, ingesting carbohydrate drinks during exercise can help offset the effects of glycogen depletion by providing working muscles with another source of glucose.
But why not just eat carbohydrate-rich foods, I hear you ask? Well, trying to replenish carbohydrate using conventional high-carbohydrate foods (eg bread, pasta, potatoes etc.) while on the move is almost impossible; not only does digestion slow down the rate at which the released carbohydrate comes ‘on tap’, most people also find it impossible to consume solid food during vigorous exercise without suffering from stomach cramps, abdominal bloating, etc.
Energy drinks, by contrast, can be taken on the move without causing abdominal distress, and can therefore help prevent glycogen depletion during endurance training/events. Moreover, as we’ve already seen, prolonged exercise is invariably associated with loss of fluid and (when sweating occurs) electrolytes; well-formulated energy drinks can replace this lost fluid and minerals.
Who will benefit from energy drinks?
Even if you have a high-carbohydrate diet, large volumes of training (more than seven hours per week) or workouts that last 90 minutes or more can make it hard to adequately replenish muscle glycogen – energy drinks can therefore aid performance. In reality, this tends to apply to endurance athletes, such as runners, cyclists, swimmers, triathletes, rowers, etc. Energy drinks can also help those who need to train for shorter periods, but more than once a day, by helping to replenish carbohydrate. However, if you have long periods of rest in between these sessions, you may be better off using a ‘recovery drink’ (look out for this article on PPP). The key point about energy drinks is that they are designed to supply rapidly and easily absorbed carbohydrate!
What makes a good energy drink?
Carbohydrate blend – look for a mixture of carbohydrates – eg maltodextrins, dextrose, fructose etc. Of these the slower releasing maltodextrins should constitute the major component (ie be listed first on the ingredients label).
Electrolyte minerals – although energy drinks aren’t designed to keep you hydrated per se, the addition of the electrolyte minerals (sodium, chloride, magnesium, potassium) can be useful, espeically if you’re training in warm conditions where sweating is likely.
Vitamins and other nutrients – all natural unprocessed foods come with a good slug of nutrients. However, the sugars in energy drinks don’t naturally contain nutrients, so if you’re a regular user of energy drinks and are using them to obtain a significant proportion of daily calorie intake, added nutrients (eg B-vitamins) may be helpful.
Taste – last but not least, taste is incredibly important. No matter how hi-tech the drink formulation, if it tastes like dishwater, you’ll struggle to drink enough of the stuff to actually make a difference, especially when trying to drink during exercise, when your thirst response may be blunted anyway.
Recent developments in carbohydrate drink technology
It’s not often that real breakthroughs occur in sports nutrition, but the development of glucose/fructose energy drinks seems to be just such an example. These drinks combine approximately two parts of glucose and maltodextrins to one part of fructose, and are claimed to improve the rate at which the sugars leave the stomach, enter the bloodstream and then reach working muscles. This in turn improves endurance by supplying more ingested (exogenous) carbohydrate to working muscles, thus sparing stored (endogenous) muscle glycogen. A landmark study comparing the benefits of traditional glucose/maltodextrin drinks to those with added fructose showed the following (4):
- During the last hour of exercise, the oxidation rate of ingested carbohydrate was 36% higher with glucose/fructose than with pure glucose (figure 1);
- During the same time period, the oxidation rate of endogenous (ie stored) carbohydrate was significantly less with glucose/fructose than with pure glucose (figure 1);
- The rate of water uptake from the gut into the bloodstream was significantly higher with glucose/fructose than with pure glucose (figure 2);
- The perception of stomach fullness was reduced with the glucose/fructose drink, compared to pure glucose;
- Perceived rates of exertion in the later stages of the trial were lower with glucose/fructose than with pure glucose.
Figures 1 and 2: Benefits of glucose/fructose drinks
Subsequent studies seem to have confirmed these potential benefits, so athletes looking for maximum performance from an energy drink may wish to consider a combined glucose and maltodextrin/fructose drink rather than a pure glucose and maltodextrin drink.
Using an energy drink
There’s always a degree of personal preference (how concentrated you mix the drink and how much you drink), but as a rule of thumb (and for optimum performance), you should mix the drink as per the manufacturer’s instructions and aim to consume enough of the drink during training/competition to provide the following:
- 60kg body weight – 60 grams of carbohydrate per hour;
- 70kg body weight – 70 grams of carbohydrate per hour;
- 80kg body weight – 80 grams of carbohydrate per hour.
So, for example, if the directions say you should add 50g of powder to 500ml of water to make up the drink, a 70kg person should consume 700ml per hour of the drink.
- Appl. Physiol. 80:1112-1117, 1996
- Int. J. Sports Nutr. 7:104-116, 1997
- Amer. J. Physiol. 258 (Gastrointest. Liver Physiol.) 21: G216-G222, 1990
- J Appl Physiol 2006; 100:807-816