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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 80-90%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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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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Ex Nutr c8-water 12Exercise 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 > 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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Ex Nutr c8-water 14Effect 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 >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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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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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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Ex Nutr c8-water 23Effect 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 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 Banned by WADA in 2010 (plasma expanders)

Ex Nutr c8-water 24Effect 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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Ex Nutr c8-water 27Effect 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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Ex Nutr c8-water 31Plasma 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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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

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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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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

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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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Ex Nutr c8-water 48ACSM 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 30-60 g/h in 600-

1200 ml/h Na 0.5-0.7 g/L in exercise > 60 min