When we exercise our body core temperate raises so we produce sweat to help cool us down. Sweat contains water and electrolytes. Electrolytes have important functions from regulating your heartbeat to allowing your muscles to contract so you can move. These electrolytes include calcium, magnesium, potassium, sodium, phosphate and chloride. Water and electrolytes need to be replaced otherwise dehydration and hyponatremia (low blood sodium) can develop. This can directly impact performance and lead to medical problems.
The amount of water and electrolyte losses vary for each individual so each person will need to customise recommendations accordingly.
Factors effecting sweat loss include:
- The Individual – body weight, genetics, metabolism
- Duration & intensity of exercise
- Environmental conditions — temperature, humidity, sun & wind exposure
- Type of clothing
Hydration assessment
You can monitor your hydration status by simply measuring your weight. Taking a nude weight first thing in the morning for several days can be used to establish base-line body weights that represent normal hydration. Body weight changes can reflect sweat losses during exercise and can be used to calculate your fluid replacement needs for specific exercise and environmental conditions. Any weight lost after you exercise is the amount of fluid you need to replace to go back to normal hydration.
Dehydration
Dehydration increases physiologic strain and perceived effort to perform the same exercise task, and is heightened in warm-hot weather. The greater the dehydration level the greater the physiologic strain and exercise performance impairment. The magnitude of water loss and performance impairment is related to the heat stress, exercise task, and the individual`s unique biological characteristics.
Complications of dehydration include:
- Heat exhaustion and heat stroke
- Rahabdomyolysis (syndrome causing release of skeletal muscle contents) associated with acute renal failure
- Dehydration with low sodium can lead to muscle cramps
Over hydration
Over hydration can be more of a disadvantage then an advantage to performance. Consuming more fluid then that that is lost from sweat can lead to hyponatremia. Large sweat sodium losses and small body weight contribute to the hyponatremia. Women generally have lower sweating rates than men and have a greater risk of developing hyponatremia. In general, dehydration is more common, but over drinking with hyponatremia is more dangerous.
Diet
Eating food is crucial to promote full hydration as it promotes fluid intake and retention. Caffeine intake does not markedly affect hydration status however; alcohol can increase urine output and delay full rehydration.
Fluid replacement before exercise:
The goal of pre-hydration is to start an activity fully hydrated. Just by consuming sufficient beverages with meals, and a recovery period of 8-12 hours after the last exercise session, should leave you close to full hydration.
However, if you need to rehydrate quickly before a next activity, or have suffered substantial fluid loss you can follow a rehydration program:
- Pre-hydrating should be initiated at least several hours before exercise
- Slowly drink beverages at least 4 hours before exercise.
- If you don’t produce urin or your urin is dark and concentrated you should slowly drink more fluid about 2 hours before the event.
- Consuming beverages with sodium or having a salt containing food at meals will stimulate thirst and retain fluid.
During Exercise
The amount of fluid that needs to be replaced depends on sweat rate, exercise duration and opportunities to drink. There are no set recommendations because of the variability between exercise tasks, environment and individuals. Drinking at regular time intervals will help to prevent dehydration.
- Drinking at regular intervals according to a fluid replacement program will help prevent dehydration
- The routine measurement of pre and post exercise body weights is useful for determining sweat rates and developing customised fluid replacement programs.
- Consumption of beverages containing electrolytes (sodium & potassium) and carbohydrates can help sustain fluid electrolyte balance and exercise performance.
A possible starting point suggested for marathon runners (who are fully hydrated at the start) is they drink according to thirst from 0.4 to 0.8 litres per hour, with the higher rates for faster, heavier individuals competing in warm environments and the lower rates for the slower, lighter persons competing in cooler environments.
The table below provides approximate sweat rate of people of different sizes, running at different speeds in different environmental conditions.
After Exercise
The goal after exercise is to replace any fluid and electrolytes lost.
- If time permits, consumption of normal meals and beverages will restore normal hydration.
- Individuals needing rapid and complete recovery from excessive dehydration can drink ~1.5 L of fluid for each kilogram of body weight lost.
- Consuming beverages and snacks with sodium will help expedite rapid and complete recovery by stimulating thirst and fluid retention.
- Intravenous fluid replacement is generally not recommended, unless medically indicated.
Reference: Sawka, Burke, Eichner, Maughan, Montain, Stachenfeld (2007). Position Stand Exercise and Fluid Replacement. American Collage Of Sports Medicine.
