Water is essential to life. Not only does it serve as the medium in which all chemical reactions in the body take place, but it also makes up an essential part of the cell fluid, helps control the electrolyte balance in the body, provides protection to key tissues, serves as a transport medium and is crucial for proper functioning of the special senses.
Water constitutes the major part of sweat and so helps to regulate body temperature. The evaporation of sweat from the skin’s surface is a powerful cooling mechanism that allows the release of the heat produced by working muscles. However, together with water, some electrolytes are also lost through sweat. To prevent dehydration it is therefore important to replace both fluid and electrolytes and athletes have long been advised to consume “as much fluid as possible” to ward off the demons of dehydration.
More recently, though, athletes and medical staff at sporting events have been told to limit hydration owing to the potential dangers associated with overhydrating during long periods of physical activity.
The advice may seem contradictory: drink enough fluids during activity to prevent performance-limiting dehydration, yet do not consume so much fluid that hyponatremia (low salt level in blood) develops. This may leave the athlete confused, because how much is then just right?
Dehydration is defined as the sudden drop of fluid stores below the level that is required to support a steady-state condition of normal body water levels. If such low body water stores persist for an extended period of time, hypohydration can arise, which is a steady-state condition of decreased body water.
Risk factors for dehydration
Dehydration sets in when the amount of fluid lost through sweat, urine and respiration is not matched by sufficient intake. The risk for dehydration increases when the sweat rate increases, for example in hot conditions, during high-intensity activity or when participating in multiple training sessions on the same day. Inadequate access to fluids or food, poor compliance to an individualised hydration plan or failure to attain proper hydration at the start of the activity can also increase the risk for dehydration. Individual factors such as sweat rate, body weight, personal preference for flavour or temperature of a drink, fluid tolerance and illness also need to be considered as possible contributors to dehydration.
Complications of dehydration
"When dehydrated, the athlete’s perceived effort increases greatly and concentration and mental functioning diminish."
Dehydration can cause the core body temperature to rise; in short, the body overheats. It further also puts extra strain on the heart because the loss of water thickens the blood, which requires the heart rate to increase to sustain a specific workload. When dehydrated, the athlete’s perceived effort increases greatly and concentration and mental functioning diminish. To regain a proper fluid balance may prove difficult following dehydration owing to gastrointestinal discomfort and upsets that are often experienced.
A change in body weight is a good indication of an athlete’s hydration status following exercise.
Early visible signs of dehydration include :
- headache
- fatigue and muscle cramps
- loss of appetite
- flushed skin
- heat intolerance
- dizziness
- dry mouth and eyes
- dark urine and infrequent urination
Advanced visible signs of dehydration include:
- difficulty in swallowing
- clumsiness
- shrivelled skin
- sunken eyes and dim vision
- painful urination
- numb skin
- muscle spasms
- delirium
Fluid balance and sweat loss calculation
Any weight loss reflects a mismatch between fluid intake and fluid loss during exercise. A deficit of 1 kg indicates failure to replace approximately 1 l of fluid during exercise.
STEP 1: Change in body mass: Measure body mass before exercise in minimum clothing and immediately after exercise in the same clothing, towel dried.
STEP 2: Fluid-intake: Measure mass or volume of drink bottle/s before exercise and immediately after exercise.
STEP 3: Urine or toilet losses: Measure difference in mass before and after going to the toilet.
Calculations:
- Fluid deficit (ml) = Change in body mass after exercise x 1000
- Fluid intake (ml) = Change in mass of fluid bottle after exercise
- Urine losses = Change in body mass before and after toileting x 1000
- Total sweat loss = Fluid deficit + Fluid intake – Urine losses
- Hourly sweat rate = Simply divide total sweat loss during exercise by the duration of the exercise
- Percentage dehydration = Total fluid deficit (kg) divided by pre-exercise mass (kg) x 100
The main principle for preventing dehydration is that fluid intake should match fluid losses. Also remember that the sweat rate generally increases after 10 – 14 days of heat exposure, which means that a greater fluid intake will be required for a similar bout of exercise.
Overhydration refers not only to excessive fluid intake during exercise but also to the resulting low blood sodium levels. Overhydration is therefore also known as hyponatremia.
Risk factors for overhydration
Athletes participating in events of more than four hours are at risk of overhydrating. Prolonged sweating encourages fluid intake over a long period, yet large amounts of sodium are lost. This is especially applicable to athletes who drink large volumes of water but without replenishing sodium, or slower athletes who are overzealous about fluid consumption. Athletes participating in lower-intensity endurance activities are also at risk, because more fluid may be consumed over the duration of the event than is lost through sweating.
Signs of overhydration
Visible signs of overhydration include:
- dizziness
- nausea
- extreme fatigue
- respiratory distress
- confusion and/or disorientation
- swollen hands or feet (oedema); rings, watches and shoes may feel too tight
Overhydration left untreated can have serious consequences, such as seizures, coma and even death.
Preventing overhydration
Athletes should be sensitive to onset of thirst as the signal to drink, rather than “staying ahead of thirst”. It is also important to ensure that the fluids consumed include adequate amounts of salt (sodium), and during continuous days of exercise in hot weather to consume a little extra salt with meals and snacks. Body weight can be used to gauge hydration status. Weighing more after training than before is a sign of developing water overload. Non-steroid, anti-inflammatory drugs and pain relievers can contribute to developing a water overload.
General guidelines and tips for fluid intake
- Athletes should drink enough before exercising, so that they can start training in a well-hydrated condition. With the help of a structured plan, athletes can learn to tolerate up to 5 ml fluid per kilogram of body weight in the warm-up before an event. This equates to 350 ml for an athlete weighing 70 kg.
- Use the same fluid replacement plan during a competition as during training sessions. Drink as much as is practical and comfortable in attempting to match sweat losses. Note that sweating occurs even when exercising in water or in air-conditioned venues. Start fluid replacement early and top up frequently as this will maintain gastric volume and increase fluid absorption.
- Make the most of opportunities to drink fluids (intervals or injury time in team sports).
- Adopt a pattern of drinking small amounts of fluid at regular intervals during exercise rather than trying to drink large volumes all at once.
- Drinks should be cool (not cold), palatable and contain the optimal amount of carbohydrates applicable for the specific event. Sports drinks (5 – 7% carbohydrates) are good options since they empty from the stomach faster than soft drinks (which generally contain 10% carbohydrates), while also helping to replace sodium losses. Drinks with carbohydrate concentrations greater than 10% may promote gastric cramps.
- Make sure that drinks are easily accessible. They should be in a container that allows easy drinking with minimal interruption of exercise.
- Replace fluid losses as completely as possible between competition sessions. Sweating continues for some time after exercise and replacement fluids should therefore also include adequate amounts of electrolytes (sodium and potassium).
- Active cooling should be part of the recovery plan. Avoid long, hot baths or prolonged sauna or spa sessions. If such activities are necessary, drink extra fluid.
- For fluid deficits of more than 1.5 – 2 l and short recovery time, rehydration solutions with a sodium content of 60 – 80 mmol/l (see label information) can be used. Pretzels, soup or potatoes dipped in salt, taken with water, are also good sources of sodium.
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