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

• 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

Dehydration

• 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 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

Acclimatisation

• 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

 

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