Getting enough fluids while you exercise

(Article written by Shelly Meltzer & Associates, dietitians associated with the Sports Science Institute of South Africa.)

For many years, athletes have been encouraged to drink lots of fluid to prevent exhaustion, heat stroke and numerous other alleged consequences that have been associated with dehydration. The fact that minor dehydration affects many athletes every day in different sports without serious consequences and that top runners have won endurance events 'dehydrated', seems inconsistent.

The previous fluid guidelines were based on assumptions and on studies done in controlled laboratories rather than on real life situations and field studies. It with much tenacity that Professor Tim Noakes eventually convinced many sporting bodies that guidelines had to be changed and that over-hydration (which could be a consequence of adhering to previous guidelines especially when slower smaller athletes over-consumed fluid causing sodium levels in the body to drop ie hyponatraemia) can in fact be more dangerous than dehydration.

Essentially, what Prof Tim Noakes has shown is that previous guidelines incorrectly assumed that all weight lost during exercise must be replaced, that thirst is not an accurate determinant of fluid requirements, that a universal guideline is appropriate for all athletes, that high rates of fluid intake can do no harm and that heat stroke is always a result of dehydration. 

Important points to remember

· Both severe dehydration AND over-hydration should be avoided during exercise

· Fluid requirements are an individual matter and differ between individuals and between exercise situations. The type, intensity and duration of exercise, weather conditions (ie hot and humid vs cold and raining), the volume of fluid a person can tolerate in the gut and factors that determine a persons' sweat rate ie body size, training status and acclimatisation must be taken into consideration.

There is no 'blue print' for the amount of fluid an athlete should consume while exercising. It boils down to finding the balance between dehydration and over-hydration. An elite athlete running a fast 2 1/2 - hour marathon in hot, humid conditions will probably sweat more than a recreational cyclist, doing a slow 2 1/2 hour fun ride in cool conditions. The elite runner must drink more than the cyclist to maintain body water content during exercise and to replace sweat losses after exercise.

· There is a limit to how much fluid can be absorbed from the gut during exercise. So, in some situations it may be appropriate to introduce other methods of cooling to reduce sweat losses such as increased recovery/rest periods, cooler clothing etc

· Be aware of the differences in the symptoms of dehydration and over-hydration. Over-hydration appears to be most common amongst slow, smaller marathon runners, drinking more than they sweat out.

Symptoms of over-hydration (or specifically water intoxication hyponatraemia) include an increase in body weight as a result of oedema (rings and watches may feel tight), extreme fatigue, mental confusion, seizures and even death, if left untreated. Symptoms of dehydration include a decrease in body weight, dark concentrated urine, increased body temperature, increased cardiovascular strain, fatigue and difficulty to concentrate. 

· "Don't let all this discourage you from practising good fluid habits. For example, always take your own drinks with you when you are exercising and make sure it is accessible (ie not left in the change rooms). The drink should be cool, palatable and provide you with the optimal amount of energy (carbohydrate) for your event. Use a fluid replacement plan that you have tried out in training. Drink as much as is practical and comfortable in attempting to match sweat losses

· It is important to be hydrated before exercising, especially when exercising intensely in hot conditions for more than 60 minutes at a time. Drinking 400 to 600 ml of water two to three hours before an event gives the kidneys enough time to regulate body water volume and excrete excess fluid.

It is also important to top up glycogen stores before an endurance event. By drinking another 250 to 500 ml of a carbohydrate drink, 15 to 30 minutes before exercise, both fluid and fuel needs can be met. Experiment with volumes to determine what's comfortable for you. If participating in an endurance event, you can consider a drink that contains some sodium (not too much as this would not be palatable and may cause nausea and vomiting).

· It is not compulsory to force yourself to ingest fluids during exercise if the session is less than 40 to 60 minutes. However, it is important to consume fluids during exercise (drink small amounts at regular intervals to quench thirst) when you are exercising in hot, humid conditions and/or at a high intensity for more than an hour.

The volume of fluid you consume during exercise should NOT exceed sweat rates so drinking when you feel thirsty is a safe way to ensure you ingest fluids without drinking too much. Drinking small volumes (150 ml) of fluid frequently (every 15 to 20 minutes depending on thirst) is better tolerated than a big volume once an hour and allows for better absorption.

A sports drink with a 6 to 10% carbohydrate concentration is ideal for during exercise, unless of course you don't need the carbohydrates in which case you could choose a sports water (water with some electrolytes) or plain water. 

There is little risk of running into deficiencies of potassium, magnesium or calcium during exercise and there is also no conclusive evidence to suggest that magnesium intake during exercise will prevent muscle cramps.

The majority of sports drinks do contain some sodium (with or without small amounts of other electrolytes). The amount of sodium though is limited and a lot less than actual amounts lost in sweat, as too much sodium in the drink would be unpalatable. Sodium aids glucose absorption and can be important in endurance events. If you think you need the extra sodium, include some salty snacks like pretzels.

· Drinking after exercise is an important part of the recovery process to replace sweat losses (fluids and electrolytes), especially in hot, humid conditions and when competing in more than one exercise session per day.

After exercise, you will continue to lose sweat for some time. Fluid replacement after one exercise session can be seen as hydration before the next exercise session. A post-exercise drink should be high in carbohydrates and should contain sodium which helps restore fluid levels.

Sodium also stimulates glucose absorption in the small intestine through the active co-transport of sodium and glucose. Plain water is not the ideal post-exercise drink when rapid and complete restoration of fluid balance is necessary.

Putting different drinks in perspective

Sports drinks

Sports drinks are designed specifically for exercise to help maintain normal body water levels and supply fuel for energy and to maintain normal blood sugar levels. 

The 'ideal' sports drink should provide optimal amounts of carbohydrate (this may differ depending on the situation), specifically glucose or glucose polymers, with no or very little fat and protein and some electrolytes (especially sodium).

Sports drinks can come in a ready-to-drink, concentrated or powdered form. Well-known sport drinks include Energade, Powerade, Game and Fast Fuel.

Sports waters

Many different 'fitness waters' or 'sport waters', that contain electrolytes and even some vitamins but very few calories have hit the market. These do not supply fuel (ie they have no carbohydrate), but they can help to maintain fluid levels during short events where there is a lot of sweating involved and where carbohydrate is not a limiting factor. 

Energy drinks

There are many energy drinks on the market that are not primarily aimed at exercise and that may or may not contain high amounts of carbohydrate. 

These drinks may contain other ingredients like guarana, caffeine, taurine and vitamins that claim to provide energy. Caffeine (and guarana) can potentially mask fatigue during exercise and rest by stimulating the central nervous system and leaving one feeling 'awake'. However, caffeine also has diuretic properties and should be avoided when trying to replace fluids after exercise.

Examples of energy drinks include Sportron Energy Drink (contains no carbohydrates) and Red Bull. A 250 ml can of Red Bull contains 28g of carbohydrate (compared to 19 g for 250 ml Powerade) as well as 78 mg caffeine (similar to the amount youd get in a 500 ml Coke) and small amounts of taurine and vitamins. However, Red Bull contains no electrolytes.

Fruit juices, cold drinks, cordials and iced teas

Although these drinks may contain carbohydrates they are not always ideal to drink before or during exercise, because of their fructose content (ie fruit juice with high fructose content can cause gut discomfort during exercise). Fizzy drinks may leave you feel bloated and uncomfortable and these drinks may also be very low in electrolytes (eg cordials). However, they can be consumed after exercise too help replenish carbohydrates (but then choose caffeine-free options).