Sport Nutrition Education No. IV - Water - The Fountain of Life
Functions of Water
- Regulates body temperature
- Transports nutrients (e.g. glucose, electrolytes, oxygen, iron)
- Carries waste products away (e.g. lactate, urea)
Fluid is Lost Through
- Evaporation from skin
Fluid is Gained By
- Fluids (e.g. water, juice, soup, milk etc.)
- Foods (e.g. fruits, vegetables, congee etc.)
- Metabolism of nutrients (e.g. carbohydrate, fat, protein) releases water
Symptoms of Dehydration
- Flushed skin
- Shrivelled skin
- Light headedness
- Dark coloured urine
- Heat stroke
The Relationship between Dehydration & Fatigue
Fluid loss of >2% body weight can negatively affect aerobic exercise performance especially in warm hot weather. Dehydration can result in increased body core temperature, increased heart rate, increased glycogen utilization whhich can further cause fatigue. Excess fluid loss may also degrade cognitive performance which is important for sports requiring skill, concentration and agility.
Guidelines to Fluid Supplementation
According to ACSM guideline in 2007
- Before exercise
Drink 5-7 mL of fluids per kg body weight at least four hours before exercise. If there is no urine production or urine output is minimal, drink 3-5 mL of fluids about 2 hours before exercise.
- During exercise
The goal of drinking during exercise is to prevent excessive dehydration (>2% body weight loss from water deficit). Drinking sports beverages can help to sustain fluids/electrolytes balance and enhance performance for long duration and high-intensity exercise. Fluid replacement depends upon individual variability and environmnetal conditions. Thers factors include body weigth, weather condition, duration and intensity of exercise and type of sports. For example, sports require the wearing of protective clothing such as fencing and american football, exercises undertaking in hot humid weather condition and outdoor activities will increase seating rates of athletes. It is important for athletes to establish individualized fluid replacement startegies.
- After exercise
Fluids and electrolytes loos should be fully replenished post-exercise events. The change in body weight after exercise can determine sweat rates and is useful for establishing effective individualized fluid replacement strategies. for rapid and complete recovery, it is recommended to drink 1.5L (6 cups) of fluid for each kilogram (2 pounds) of body weight lost.
* Weight loss after exercise is primarily due to loss of body fluids and not fat because you need to burn 3500 kcal for losing a pound of fat.
Fluid Loss Assessment
Hydration status assessment
Good Hydration Status
|Total Body water||< 2%|
|Plasma osmolality||< 290 mOsmol|
|Urine specific gravity||< 1.020 g/mL|
|Urine osmolality||< 700 mOsmol|
|body weight||< 1%|
Among different assessment methods, body weight measurement is the simplest tool for athletes to assess fluid balance. Body weight changes before and after exercise reflects the amount of fluid loss from sweat. Athletes can monitor their hydration status by using body weight measurement.
|Pre-exercise body weight 80kg
Post-exercise body weight 78kg
Body weight loss -2kg
According to post exercise fluid replacement guideline:
1.5L fluid / kg body weight loss x 2kg
->Replace 3L fluid
Although adequate fluid replacement is important to maintain normal physiological function, excessive fluid replacement may not be beneficial. Drinking large amount of hypotonic fluid after exercise will reduce plasma concentration and result in hyponatremia. It affects the replenishment of body fluid and stimulates urine production. Athletes may think that adequate fluid has been taken, but they are actually more dehydrated. Athletes are suggested to drink sodium-containing fluid after exercise which can help to restore plasma volume.
Choosing a Suitable Fluid Replacement Drink
- For exercise whcih lasts 90 minutes or less, replacing with water or sports drink (contains carbohydrate, electrolytes and fluid).
- For exercise which lasts longer than 90 minutes, sports drink is a better choice for the following reasons:
- Carbohydrates to supply energy
- Commercial sports drinks available in the market normally contain 6-8% carbohydrates, and are absorbed as quickly as water
- Contains sodium, potassium and other electrolytes to replace loss from sweating and maintain fluid and electrolyte balance
Beverages to Avoid During Exercise
- Alcoholic beverages (e.g. beer, wine, cocktail)
* Alcohol is diuretic which promotes fluid loss.
- Carbonated beverages (e.g. soft drinks, soda water)
* Carbonation leads to sensation of stomach fullness and burping which decrease the desire to drink.
Dehydration can have detrimental effects, it may lead to heat stroke and even death!
- Sawka, M.N., L. Burke, E. Eichner, et al. AmericanCollege of Sports Medicine position stand: Exercise and fluid replacement. Medicine and Science in sports and Exercise. 39:377-390, 2007.