Hydration for Endurance Events

The content below is from Steve Bird's recent presentation.

Temperature Control

  • Essential to maintain our core temperature at approximately 37°C
  • Skin temperature can vary
  • Changes in core temperature of more than a few degrees affect performance
  • Core temperature changes greater than plus or minus 4°C can be fatal

Temperature Control Whilst at Rest

  • Maintain core temperature by achieving a balance between:
    • Heat gained through metabolism
    • Heat lost through conduction, convection, radiation and evaporation
  • Achieve balance by adjusting heat loss through
    • Blood flow to skin
    • Altering skin temperature
Temperature Control During Exercise

Metabolism increases between 6 and 15 times during excercise

  • Heat production increases accordingly
  • Need to lose excess heat
  • During exercise, the main process for heat loss is usually through evaporation of sweat
  • In warm conditions, this is the only method of cooling


  • Sweating per se does not cool you
  • The evaporation of the sweat (or other liquids) off the skin provides the cooling
  • Must allow evaporation to take place
    • If evaporation is prevented, you will over-heat
    • Wear appropriate clothing
    • Be sufficiently hydrated to allow sweat production


  • Sweat is a filtrate of the blood
  • Fluid and electrolytes are taken from the blood to form sweat
  • If you become dehydrated:
    • Sweating becomes inhibited and may cease
    • Core temperature will increase
  • In hot conditioins, swat loss is up to 3 litres per hour
  • A 1% reduction in body mass (~0.7L) = 3% reduction in performance
  • A 4% reduction in body mass (~3L) = 22% reduction in performance
  • Fatigue is caused by:
    • Reduced blood volume
    • Increase in core temperature
  • Risk of heat illness and hyperthermia

Consequences of Dehydration

  • Heat exhaustion - symptoms:
    • High heart rate
    • Dizziness
    • Confusion
    • Nausea
    • Concentrated urine
    • Cool, but pale skin
    • Cramps
    • Loss of skill/motor coordination

Gabriela Anderson-Scheiss staggers to the finish of the 1984 Olympic marathon severely dehydrated






Gabriela Andersen-Scheiss takes 5 minutes and 44 seconds to complete the lap of the stadium at the 1984 Olympic marathon.

Response to Dehydration

  • Sit or lie in a cool shaded environment
  • Drink
  • If unable to drink, seek medical help

Heat Stroke - symptoms

  • Dry skin
    • Sweating has stopped to conserve fluids, despite high core temperature
  • Confusion
  • May collapse
  • Response:
    • As for heat exhaustion and seek medical assistance

Strategies for Avoiding Dehydration

  • Be fully hydrated before the start
    • Consume 400-800 ml 2-3 hours prior to the event
  • In reality
    • Not just the morning of the event
    • Every day, but particular attention commencing the day before
  • During the event:
    • Replenish during the event - little and often
    • 150-300 ml every 15 minutes
    • Electrolytes in drinks assist retention of fluid
    • Most sports drinks also have glucose to reduce the risk of fatigue from glycogen depletion - these help prevent fatigue from energy depletion

What to drink?

  • Concentration is important:
    • Too weak - will not reatin as much of the fluid as it causes temporary dilution of the blood and is urinated out before it gets into other body tissues
    • Too strong - inhibits absorption of the fluid into the bloodstream
  • Recipe
    • 6-8g glucose per 100ml sodium and potassium
  • Hyponatremia
    • Results from depleted electrolytes due to the consumption of large volumes of hypotonic fluids (usually straight water)
    • Incidence - approx 3 in 10,000 competitors

Post-event Recovery

  • Drink lots of fluids: ~1.5 x weight lost
  • Eat lots of carbohydrates

The Effect of Humidity

  • Humid environments inhibit evaporation
    • Prevents cooling
    • Increases risk of hyperthermia
    • Increases risk of dehydration as you sweat profusely to try and cool the body, despite its lack of effectiveness 

Children at Risk

  • Children are at greater risk of heat illness
    • Temperatures of >34°C in non-humid conditions
    • Temperatures of >26°C in very humid conditions


  • Perspire at lower core temperature
    • More rapid and sensitive response
  • Rapid adaptation in 305 days, but can take 10-14 days
  • Note: issues when travelling interstate etc
  • But cannot improve ability to function without water, so:
    • There is no point in being deliberately dehydrated in training
    • You must drink

It is essential to maintain hydration