Thermoregulation and hydration

14 important questions on Thermoregulation and hydration

What happens to our body in the heat? (so to loose heat):

  • Vasodilation
  • sweat loss (very powerful) > nude body is the best
  • behavior change (reduce heat gain)
  • powerful acclimatization

What happens to our body in the cold (so to stay warm):

  • Vasoconstriction (retain heat)
  • shivering (increase heat, a little)
  • behavior mechanism: exercise (>heat production), clothing allow extending the habitat behind physiological limits (retain heat)
  • poor acclimatization

4 ways the body loses heat:

  1. Radiation : emission of electromagnetic radiation
  2. Conduction : direct transfer by contact
  3. Convection : moving air removes radiated heat
  4. Evaporation : loss of heat by evaporation of water

1, 2, and 3 can also go the other direction.

How can you measure the heat balance during exercise?

  • Metabolism: indirect calorimetry
  • Storage: change in body temp
  • Radiation: difference Tskin and ambient
  • Convection: air flow (wind)
  • Conduction: difference Tskin and ambien (very limited)
  • Evaporatnio: body weight change, corrected for fluid loss

Name some techniques to reduce performance decrease in the heat:

  • Selection e.g. Afrikan vs europeans
  • physiological adaptations - acclimatization
  • physical training (cross acclimatization)
  • cooling (prior, during and after exercise)
  • drinking > cold drinks/ice slurries
  • pacing strategy

What are symptoms?

  • Hyperthermia
  • increased risk of...
    • initiate dehydrated
    • multiple training
    • diuretics
  • decreased endurance performance (particular in the heat)
    • cardiovascular and thermoregulation
  • less effect on...
    • short term high intensity exercise performance
    • muscle strength
    • power (jumping)

Symptoms: nausea, vomiting, headache, dizziness, muscle cramping

Link dehydration with thermoregulation

  • Hyperthermia or heat stress increases dehydration
  • hypohydration increases risk:
    • heat illness: heat exhaustion heat stroke
  • dehydration increases risk of hyperthermia
    • increased threshold for sweating and cutaneous vasodilation
    • reduces sweat rate due to low circulating volume
  • cooling down: per cooling/cool beverages

What are reasons for decreased exercise performance?

  • Increase temperature, heat storage (cognitive)
  • reduced exercise-induced heat strain
    • blood flow, sweat rate
  • increase cardiovascular strain, reduced CO
  • increase glycogen utilization
  • altered metabolic function
    • muscle blood flow

Hyperhydration =
Euhydration =
Hypohydration =
Dehydration =
Rehydration =

Hyperhydration = state of overhydration
Euhydration = normal hydration state
Hypohydration = state of underhydration
Dehydration = process of loss of fluid
Rehydration = process of gain of fluid

Fluid balanec during exercise is influenced by:

  • Evaporation
    • depends on sex
  • respiratory loss
  • environmental conditions
    • heat acclimatization
    • altitude
  • intensity/duration
  • fitness level
  • body composition
  • medication (diuretics)

Fluid balance/sweat rate = preBW - post BW + fluid - urine

A strategy to reduce hyperhydration - pre exercise is...

  • Euhydration - sodium containing fluid
  • hyperhydration no thermoregulatory advantage, delays dehydration
  • fluid overloading: BUT
    • urine production
    • stomach/gut problems
    • body weight higher
  • glycerol hyperhydration:
    • osmotic pressure/expands extra-cellular space
    • advantage? / hyponatremia complaints

Name the issues in (post-exercise) rehydration process:

  • Practical: opporunities, conscious decisions
  • voluntary intake: 30-70% of sweat losses
  • > people fail to rehydrate
  • difficult after high levels of hypohydration (2-5% BM) or interval between exercises short (6-8h)
  • GI physiology:
    • gastric emptying
    • intestinal absorption

When to drink post-exercise?
What to drink post-exercise?

  • Prevent > 2-3% body water loss
    • small advantage of lower BW in some sports
  • thirst sensation:
    • delayed compared to other physiological indicators of hypohydration
  • overdrinking:
    • GI problems might occur
    • hyponatremia

  • water ingestion:
    • dilution of plasma osmolarity and sodium content
    • results in increased diuresis and reduced thirst
  • + sodium:
    • water absorption
    • osmolarity > water retenion/reabsorption
  • amount sodium:
    • ORS: 90 mmol/L
    • post exercise: ~50 mmol/L (2-3 g/L)
    • commercial sport drinks: 10-25 mmol/L

How about drinking caffeine and alcohol?

Caffeine: no clear evidence
  • increased voluntary intake
Alcohol: increases urinary losses (ADH)
  • effect of low dose alcohol is blurred when dehydrated
  • interfering with ability/interest post exercise nutrition 

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