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3. Thermoregulation and exercise in heat
Ex Nutr c8-water
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: 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

4. Heat storage during exercise
Ex Nutr c8-water
Heat storage during exercise
Exercise at 80-90%VO2max can potentially ↑ 1°C 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 39.5°C result in central fatigue
Body temperature 36-38°C at rest, 38-40°C exercise

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6. Environmental heat stress and heat loss
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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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10. Regulation of body temperature
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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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12. Exercise Training, Acclimatization, and Temperature Regulation
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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 % VO2max to increase body temp > 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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14. Effect of Dehydration on Exercise Performance
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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 M by 5%, ↓1500 M by 3%
Dehydration of 7% BW↓time to exhaustion at 25% VO2max by >50%
Dehydration of 8% BW↓time to exhaustion at 47% VO2max by >50%
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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17. Dehydration impaired performance
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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
↑core temperature, negate thermoregulatory advantages conferred by high aerobic fitness and heat acclimatization
↑core 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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21. Mechanisms of Heat Illness
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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 membrane damage, lead to passage of endotoxins from gut into systemic circulation  endotoxemia (內毒素血症, blood poisoning)
↑NO  hypotension

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23. Effect of Fluid intake on Exercise Performance –pre-exercise
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Effect of Fluid intake on Exercise Performance –pre-exercise
Use pre-exercise urine color (or osmolality) as indicator for fluid status
Study: 1/3 football players ↑urine osmolality prior to competition
Pre-exercise hyperhydration: may be helpful
Expansion of blood volume by ml ↑time trial by 10%
Water-electrolyte solutions 1-3 hr before exercise, rapidly excreted
Glycerol ↑ water retention
1 g/kg glycerol 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
Banned by WADA in 2010 (plasma expanders)

24. Effect of Fluid intake on Exercise Performance –during exercise
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Effect of Fluid intake on Exercise Performance –during 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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27. Effect of Fluid intake on Exercise Performance
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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 < 30 min: no advantage
Exercise > 60 min in hot/humid condition: CHO-electrolyte drink

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31. Plasma volume change during exercise
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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 body’s heat-storage capacity

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37. Daily water balance Water in food major contribution to water intake
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Daily water balance
Water in food major contribution to water intake
Body’s 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

39. Fluid Requirement for Athletes
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Fluid Requirement for Athletes
Ensuring Adequate hydration before exercise
Urine color: clear
Urine osmolality: 900 mOsmol/kg – dehydrated, mOsmol/kg – hydrated
Body weight change
Ensuring hydration during exercise
Pre exercise: 500ml - 2hr before + 500ml – 15 min before
During exercise: ml/ min
Should accustomed to consuming fluid at regular intervals (with or without thirst) during training
Exercise min in moderate temperature: cool water

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41. Composition of sport drinks
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Composition of sport drinks
When rehydration is main priority (e.g. prolonged exercise in heat)
Contain glucose/glucose polymer g/L
Contain Na mM
Isotonic, ~290 mOsmol/L
CHO ↓gastric emptying, but↑water absorption in intestine
When substrate provision is main priority (e.g. in cool environment)
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

42. Composition of sports drinks
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Composition of sports drinks

43. Sport drink osmolarity
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Sport drink osmolarity
NaCl 20 mM = 0.02 x 58.5 = 1.17 g/L (0.117%)
Compare to saline 0.9% NaCl
Glucose 60 g/L = 60/180 = 0.3 M = 300 mM
~300 mOsmol/kg

44. Rehydration after exercise
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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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48. ACSM guidelines on fluid intake for exercise
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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 > 60 min, CHO g/h in ml/h
Na g/L in exercise > 60 min