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This article originally appeared in the sports science newsletter, Peak Performance
Keeping hydrated is one of the first things you learn in any sport, but what are you supposed to keep hydrated with? There’s so many potions and ‘sports-cocktails’ out there it’s hard to know where to look. Many bemused athletes have now returned to water, but is this the correct path to follow?
Because even small losses of water can cause a drop in performance, optimum hydration is extremely important to athletes. However, replacing fluid lost in sweat and urine is not the only justification for boosting fluid intake. Glycogen (stored muscle carbohydrate) is the body’s principle fuel for high intensity activities, and replenishing glycogen stores with dietary carbohydrate is vital to continuing high performance.
But the process of ‘fixing’ carbohydrate into muscles in the form of glycogen also requires water; each gram of glycogen fixed into muscle fibres requires around 3g of water, which is why you often feel thirsty after a high-carbohydrate post-training meal. If you don’t drink to aid this process, water is simply drawn out of the bloodstream, leading to dehydration.
Fluid, then, is vital for adequate recovery – not just to replace water lost through sweating, but also to help replenish lost glycogen.
A comprehensive hydration strategy involves ensuring good hydration before training/ competition, maintaining it during exercise and then replacing any shortfall as soon as possible afterwards. However, hydration isn’t just about water: fluid loss via urine and, especially, sweating involves the loss of electrolyte minerals – calcium, magnesium, sodium, potassium and chloride. Although the composition varies from person to person (partly as a function of acclimatisation) a litre of sweat typically contains the following:
• Calcium – 0.02g
• Magnesium – 0.05g
• Sodium – 1.15g
• Potassium – 0.23g
• Chloride – 1.48g
There are three reasons why replacing these minerals by means of an electrolyte mineral- containing drink may be better then drinking pure water alone:
1. Although the amounts lost in sweat are generally small in proportion to total body stores, prolonged heavy sweating can lead to significant mineral losses (particularly of sodium). Drinking pure water effectively dilutes the concentration of electrolyte minerals in the blood, which can impair a number of normal physiological processes. An extreme example of such an impairment is ‘hyponatraemia’, when low plasma sodium levels can be literally life threatening.
2. Drinks containing electrolyte minerals – particularly sodium – are known to promote thirst, thereby stimulating a greater voluntary intake of fluid. There is also evidence that drinks containing sodium enhance the rate and completeness of rehydration after a bout of exercise.
3. 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. This is particularly important when rapid uptake of fluid is required, such as during strenuous exercise in the heat.
Properly formulated carbohydrate/electrolyte drinks can and do increase hydration (and, as a bonus, supply extra carbohydrate to working muscles), so it’s hardly surprising that they really do enhance performance when fluid loss is an issue (6). But what’s the best strategy for individual athletes? And how do you decide on the best drinks for you? Here are some simple guidelines derived from the evidence referred to in this article:
Pre-exercise
• Make sure your normal diet contains plenty of water and a minimum of other substances known to impair hydration;
• Drink ample (but not excessive) water in the run-up to a training session or event;
• Consider using glycerol for hyper-hydration only if you are an elite athletes undertaking long endurance events in extremely hot conditions. Even then it has its drawbacks.
Post-exercise
• Follow Ron Maughan’s advice on replacing lost fluid in terms of volume;
• Drinks containing electrolytes (especially sodium) stimulate the desire to drink and may therefore be preferable to plain water. There’s also evidence that these drinks are absorbed more efficiently from the small intestine, especially when carbohydrate is present;
• Remember that you’ll need to absorb extra fluid for glycogen replenishment – about 300ml for every 100g of carbohydrate consumed.
Mid-exercise
• For events lasting less than 30 minutes, mid- exercise fluid replacement isn’t necessary, since it’s not possible to lose enough fluid to affect performance in such a short space of time;
• Weather and exercise intensity affect fluid needs; the higher the temperature, humidity and exercise intensity, the greater the rate of fluid replacement required;
• Gastric emptying is most efficient when there is a high fluid volume in the stomach, so your best strategy is to start exercise with fluid on board and drink little and often to keep it topped up;
• Gastric emptying is also affected by the concentration of a drink. As a rule, the more concentrated the drink, the slower it empties;
• Plain water empties rapidly as do low concentration (hypotonic) drinks and isotonic drinks. More recent research also suggests that energy drinks containing up to 4-5% glucose also empty as rapidly as water. However, drinks containing glucose and sodium are absorbed from the intestine more rapidly than plain water;
• More concentrated drinks (more than 6%) leave the stomach more slowly, but still manage to deliver more carbohydrate. Where energy replacement is the priority, these drinks are recommended, although they are less efficient for hydration;
• Where hydration is the priority, water, isotonic or low concentration glucose drinks will all suffice, though hypotonic/isotonic electrolyte/glucose containing drinks may be absorbed more rapidly from the intestine;
• Whatever sports drink you choose, ensure it contains electrolyte minerals;
• Where hydration is your goal, water is okay but high volumes of plain water are not recommended where profuse and prolonged sweating occurs (more than 3-4 litres lost) because of the risk of sodium dilution. If water is your preferred drink, consider using salt tablets in these circumstances;
• The evidence in favour of glucose polymer drinks is mixed. Overall, they may confer a slight advantage in terms of gastric emptying, but be prepared to pay more!
• Fructose or pure fruit juice drinks are not absorbed rapidly and may cause abdominal distress;
• Never experiment with a new drink during competition. Try it in training first to see how your body tolerates it!
• Choose a drink you find palatable. If it doesn’t taste nice, you won’t drink it, no matter how advanced the formula!
Hyponatraemia – the dangers of fluid overload
Hyponatraemia is a disorder in fluid-electrolyte balance that results in an abnormally low plasma sodium concentration (less than 135mmol per litre compared with a normal range of 138-142mmol/L. A sustained decrease in plasma sodium concentration disrupts the dynamics of water exchange (osmotic balance) across the blood-brain barrier, resulting in a rapid influx of water into the brain. This can cause swelling in the brain, leading to a series of increasingly severe neurological responses, such as confusion, seizure, coma – even death.
The lower the blood sodium and the faster it falls, the greater the risk of life-threatening consequences. A drop in plasma sodium concentration to 125-135mmol/L often results in little more than gastrointestinal symptoms, such as bloating and nausea. Below 125mmol/L, the symptoms become more severe and can include confusion, throbbing headache, wheezy breathing, swollen hands and feet, unusual fatigue and reduced coordination. Below 120mmol/L, the risk of seizure, coma and death is increased.
Hyponatraemia in athletes is often, although not always, caused by excessive drinking. During exercise, urine production is decreased, reducing the body’s ability to excrete excess water, while at the same time sodium losses are increased through sweating. The combined effect makes it much more likely that the body’s sodium content will be significantly diluted.
Glycerol myths and reality
For most people, taking a glycerol/water solution before an event produces an increase in total body water (hyper-hydration). The question, however, is whether this extra water in the body actually enhances performance, and to date there is no clear-cut evidence to suggest that it does. It is true that after ingestion glycerol stays in the body and holds water with it, but the unanswered question is whether this extra water increases hydration within the cells or simply increases the amount of water swilling around in general circulation?
Overall, the current weight of evidence is tilted slightly in favour of a glycerol hyper-hydration protocol, but only in events where substantial dehydration is likely to be a problem. Moreover, there is still no agreement about the best way to take glycerol solution, or about whether certain kinds of plain water hyper-hydration protocols might offer similar benefits.
Unless your event is long and taking place in hot/humid conditions, resulting in unavoidable dehydration, there is probably little point in using glycerol. Not only are there unlikely to be any performance benefits, but glycerol ingestion can cause stomach upsets, together with headaches and blurred vision at higher doses. If you are tempted to try glycerol, make sure you’ve tried other hydration methods first. Glycerol should be considered only as a last resort.
Sports drinks jargon buster
With so many sports drinks on the market, it’s easy to become confused about which type is best suited to your needs. Isotonic, energy and recovery drinks can all be used to promote hydration, but tend to have slightly differing effects, which are explained below. It’s important to understand, though, that these categories can overlap – eg energy drinks containing relatively small amounts of carbohydrate can be almost isotonic – so the distinctions here should serve as a guide only.
• Isotonic drinks provide the body with water, energy and electrolytes in a form enabling the water to be absorbed as rapidly as possible. Studies have shown that fluid is rapidly emptied from the stomach when it contains roughly the same concentration of dissolved substances as that of blood serum – a value of 280 milli-osmoles/kg for you technophiles out there! At this concentration, a drink is said to be ‘isotonic’ or at the same concentration as your body fluids. During exercise, energy in the form of carbohydrate, and electrolyte minerals, such as sodium, potassium, calcium and magnesium, are lost along with water. When these substances are dissolved in water at an isotonic concentration they not only help replace lost fluid more rapidly than even plain water but also help replace some of the lost energy and minerals. However, research has demonstrated that drinks containing dissolved glucose at higher than isotonic concentrations (up to 5%) can be emptied from the stomach just as rapidly, and can therefore replace lost energy more rapidly. Although not strictly isotonic, these drinks offer all the fluid replacement benefits of isotonic drinks and are often marketed as such.
• Energy drinks are less about replacing lost fluid and more about keeping the working muscles supplied with energy during very long and sustained workouts. Energy drinks need to contain much higher concentrations of soluble carbohydrates than isotonic drinks, because an isotonic solution of carbohydrate struggles to provide energy at a sufficient rate to replace what is lost during intense exercise. The disadvantage of energy drinks is that their high carbohydrate concentrations tend to slow down the rate of water absorption, particularly during hard exercise. They are therefore best reserved for longer endurance events performed in more temperate conditions, where a very high rate of fluid replacement is not quite so critical.
• Recovery drinks, as the name suggests, are taken after training to supply the muscles with everything they need for recovery, including water, carbohydrate and amino acids. These drinks often contain such additional nutrients as electrolyte minerals, vitamins needed to aid metabolism of the ingested carbohydrate, and protein and more exotic co-factors designed to accelerate recovery. Because they’re taken after training, rapid gastric emptying and absorption is not a priority.
Andrew Hamilton
References
1. Geigy Scientific Tables, 8th Ed, Ciba-Geigy Ltd, 1981
2. Human Physiology, 2nd Ed, Springer-Verlag, Berlin, 1989
3. Appl Physiol 80:1112-1117, 1996
4. Int J Sports Nutr 7:104-116, 1997
5. Amer J Physiol 258 (Gastrointest. Liver Physiol) 21:G216-G222, 1990
6. International Journal of Sports Medicine, 13:399-406, 1992
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