Ex Nutr c8-water

Ex Nutr c8-water

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: 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

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

Ex Nutr c8-water

Environmental heat stress and heat loss Ex Nutr c8-water 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

Ex Nutr c8-water

Ex Nutr c8-water

Ex Nutr c8-water

Regulation of body temperature Ex Nutr c8-water 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

Ex Nutr c8-water

Exercise Training, Acclimatization, and Temperature Regulation Ex Nutr c8-water 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 > 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

Ex Nutr c8-water

Effect of Dehydration on Exercise Performance Ex Nutr c8-water 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 >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

Ex Nutr c8-water

Ex Nutr c8-water

Dehydration impaired performance Ex Nutr c8-water 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

Ex Nutr c8-water

Ex Nutr c8-water

Ex Nutr c8-water

Mechanisms of Heat Illness Ex Nutr c8-water 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

Ex Nutr c8-water

Effect of Fluid intake on Exercise Performance –pre-exercise Ex Nutr c8-water 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 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)

Effect of Fluid intake on Exercise Performance –during exercise Ex Nutr c8-water 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

Ex Nutr c8-water

Ex Nutr c8-water

Effect of Fluid intake on Exercise Performance Ex Nutr c8-water 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

Ex Nutr c8-water

Ex Nutr c8-water

Ex Nutr c8-water

Plasma volume change during exercise Ex Nutr c8-water 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

Ex Nutr c8-water

Ex Nutr c8-water

Ex Nutr c8-water

Ex Nutr c8-water

Ex Nutr c8-water

Daily water balance Water in food major contribution to water intake Ex Nutr c8-water 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

Ex Nutr c8-water Daily Water Balance

Fluid Requirement for Athletes Ex Nutr c8-water 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

Ex Nutr c8-water

Composition of sport drinks Ex Nutr c8-water 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

Composition of sports drinks Ex Nutr c8-water Composition of sports drinks

Sport drink osmolarity Ex Nutr c8-water 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

Rehydration after exercise Ex Nutr c8-water 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

Ex Nutr c8-water

Ex Nutr c8-water

Ex Nutr c8-water

ACSM guidelines on fluid intake for exercise Ex Nutr c8-water 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 30-60 g/h in 600-1200 ml/h Na 0.5-0.7 g/L in exercise > 60 min