Thermoregulation and exercise in heat

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Thermoregulation and exercise in heat

• For every L O2 consumed during exercise
• 4 kcal (16000 J) heat generated
• 1 kcal used to perform mechanical work
• Exercise at 4 L/min O2 consumption
• Heat production 16000 x 4 /60 800 J/s (W), or
  16 x 4 x 60/1000 3.84 MJ/h (917 kcal/h)
• Most heat passed to body core via venous blood
  returning to heart
• temperature?1 C/min in belly of quadriceps
  muscle during initial moments of high-intensity
  cycling
• Body core temperature sensed by thermoreceptors
  in hypothalamus
• Also receive input from skin thermoreceptor
• Response ? blood flow to skin, initiate sweat

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Heat storage during exercise

• Exercise at 80-90VO2max can potentially ? 1C
  every 4-5 min
• Heat capacity of human tissues 3.21 kJ/kg/C
  3.47 kJ/kg/C for lean tissue, 1.73 kJ/kg/C for
  fat tissue
• Heat storage during exercise
• brain (not muscle) temperature 39.5C result in
  central fatigue
• Body temperature 36-38C at rest, 38-40C exercise

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Environmental heat stress and heat loss

• Heat loss via evaporation of sweat largely
  determined by water vapor pressure (humidity)
• Local humidity may be high if inappropriate,
  poorly ventilated clothing
• Body temperature rise faster in dehydrated state
• Radiation transfer of energy waves by emission
  from 1 object and absorption by another
• Convection exchange of heat between solid medium
  (body) and on that moves (air, water)
• Body surface available for heat exchange,
  temperature gradient

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Regulation of body temperature

• Input deep body and peripheral thermoreceptors
• Central thermoreceptors in hypothalamus
  sensitive to core temperature, monitor
  temperature of blood flowing to brain
• Central thermoreceptors more effective in
  eliciting appropriate response
• Osmoreceptors and pressure receptors change in
  plasma osmolarity and blood volume
• Effect cutaneous vasodilation, sweating

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Exercise Training, Acclimatization, and
Temperature Regulation

• Exercise training improves temperature regulation
  during exercise at same absolute work
• Must training at sufficiently high intensity,
  such as 70-100 VO2max to increase body temp gt
  39C
• Marathon runners lower resting body temp, lower
  sweating threshold (?set-point temperature)
• Training induce earlier onset of sweat, ?total
  amount of sweat, more diluted sweat
• enlarged sweat gland
• Training ? total blood volume and maximal cardiac
  output
• Maintain blood flow in muscle and skin
• Dehydration inhibit thermoregulation, result in
  hyperthermia and earlier onset of fatigue

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Effect of Dehydration on Exercise Performance

• Dehydration impaired endurance performance
• Dehydration of 2 BW ?exercise performance
• Dehydration of 5 BW?ex capacity by 30
• Dehydration of 3 BW?VO2max by 5
• Dehydration of 2 BW ?5000 and 10000 M by 5,
  ?1500 M by 3
• Dehydration of 7 BW?time to exhaustion at 25
  VO2max by gt50
• Dehydration of 8 BW?time to exhaustion at 47
  VO2max by gt50
• Dehydration impaired high-intensity performance
• Dehydration of 2.5 BW?high-intensity exercise
  capacity by 45
• Travel to compete in hot climates

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Dehydration impaired performance

• Mechanisms
• ? blood volume, ? maximal cardiac output, ?
  VO2max because less filling of heart during
  diastole
• ? skin blood flow, ?sweat rate, ? ? heat
  dissipation
• ?cored temperature, negate thermoregulatory
  advantages conferred by high aerobic fitness and
  heat acclimatization
• ?cored temperature ??catecholamine response ??
  rate of muscle glycogen use
• Dehydration by 5 BW?gastric emptying rate by
  20-25
• Fluid consumption should begin during early
  stages of exercise in heat

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Mechanisms of Heat Illness

• Heat injury most common during exhaustion
  exercise in hot, humid environment
• Particularly if dehydrated
• Heat syncope (fainting), organ injury
• ?blood volume? vasoconstriction in skin and guts
  ? cellular hypoxia in GI tract
• Reactive oxygen species (ROS) cause cell membranc
  damage, lead to passage of endotoxins from gut
  into systemic circulation ? endotoxemia (?????,
  blood poisoning)
• ?NO ? hypotension

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Effect of Fluid intake on Exercise Performance
pre-exercise

• Pre-exercise hyperhydration may be helpful
• Expansion of blood volume by 450-500 ml ?time
  trial by 10
• Water-electrolyte solutions 1-3 hr before
  exercise, rapidly excreted
• Glycerol ? water retention
• 1 g/kg glycerol 21.4 ml/kg water ? ?sweat rate,
  ?core temperature
• 1 g/kg glycerol 1-2 L water in dry hot condition
  ? ?core temperature, ?time trial performance
• Some studies showed no effect in performance

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Effect of Fluid intake on Exercise Performance
post exercise

• May not meet fluid loss
• Sweat rate can reach 2-3 L/h
• Stomach uncomfortable with 1 L inside
• Sweat rate vary significantly ? difficult to
  recommend
• Thirst is NOT a good indicator of body-water
  requirement or degree of hydration
• Regular water intake during prolonged exercise
  effective in improving exercise capacity and
  performance
• Too much added CHO in drinks ?water absorption
• Hypertonic drink delay restoration of plasma
  volume
• Small amount of glucose and Na ? water absorption

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Effect of Fluid intake on Exercise Performance

• Electrolytes (mostly Na) added to sport drink
• Increase palatability
• Maintain thirst, maintain desire to drink
• Prevent hyponatremia
• ? rate of water uptake
• ? the retention of fluid
• Replacement of water more important than
  electrolytes during exercise
• Sweat hypotonic compared to plasma
• Replacement of electrolytes in postexercise
  recovery period
• Fluid during strenuous exercise lt 30 min no
  advantage
• Exercise gt 60 min in hot/humid condition
  CHO-electrolyte drink

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Plasma volume change during exercise

• Plasma volume?10-15 acutely at 60-80 VO2max
• Uptake of water into active muscle
• Further ?plasma volume, ?plasma osmolarity if no
  fluid replacement
• Hypotonic drink more effective in restoring
  plasma volume, compared to hypertonic
• Higher exercising-limb perfusion after 2
  CHO-electrolyte drink
• Cool fluid additional small benefit during
  exercise in heat
• Add to bodys heat-storage capacity

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Daily water balance

• Water in food major contribution to water intake
• Bodys water balance tightly regulated
• Plasma osmolarity around 290 mOsmol/L
• Closely coupled to intake/excretion of Na, major
  electrolyte in extracellular fluids
• Sensation of thirst evoked by ?plasma osmolarity
  (most important) and ?plasma volume
• Distention of stomach wall ?thirst, stop fluid
  ingestion prematurely
• Water intoxication, hyponatremia
• Occasionally in endurance athletes
• Very high water intake (8-10 L) and loss of Na in
  sweat

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Daily Water Balance
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Fluid Requirement for Athletes

• Ensuring Adequate hydration before exercise
• Urine color clear
• Urine osmolality 900 mOsmol/kg dehydrated,
  100-300 mOsmol/kg hydrated
• Body weight change
• Ensuring hydration during exercise
• Pre exercise 500ml - 2hr before 500ml 15 min
  before
• During exercise 120-180 ml/ 15-20 min
• Should accustomed to consuming fluid at regular
  intervals (with or without thirst) during
  training
• Exercise 30-60 min in moderate temperature cool
  water

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Composition of sport drinks

• When rehydration is main priority (e.g. prolonged
  exercise in heat)
• Contain glucose/glucose polymer 20-60 g/L
• Contain Na 20-60 mM
• Isotonic, 290 mOsmol/L
• CHO ?gastric emptying, but?water absorption in
  intestine
• When substrate provision is main priority (e.g.
  in cool environment)
• 100-150 g/L glucose, mostly polymer
• Frequent ingestion of small amount, keep fluid
  volume in stomach as high as possible to maintain
  gastric emptying
• Importance of practicing drinking during training
• Weighing before and after exercise to measure
  sweat rate
• Idea sport drink during exercise taste good, no
  GI discomfort when ingestion in large amount,
  promote rapid gastric emptying, provide CHO

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Composition of sports drinks
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Rehydration after exercise

• ? intracellular volume after exercise??glycogen
  and protein synthesis
• ?intracellular volume ?these processes
• Rehydration after exercise contain Na, CHO
• Ingestion water only after exercise ??plasma Na
  and osmolarity
• ?thirst, ?urine output
• Add 77 mM NaCl to water rapidly and completely
  restore plasma volume, more Na than most sport
  drink
• Need to restore Na loss from sweat
• K enhance rehydration of intracellular water
• Fluid composition after exercise
• Drink 150 of body weight loss, compensate for
  urine loss
• 1-2 alcohol seem no detrimental effect on
  rehydration

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ACSM guidelines on fluid intake for exercise

• Nutritionally balanced diet, drink adequate
  fluids during 24-hr before event
• 500 ml 2 hr before exercise
• During exercise, start drinking early and at
  regular intervals
• Cooler and flavored fluids, easy-to-drink
  container
• Contain proper amount of CHO/electrolytes
• Intense exercise gt 60 min, CHO 30-60 g/h in
  600-1200 ml/h
• Na 0.5-0.7 g/L in exercise gt 60 min