Hydration (fluid and electrolyte requirements for exercise)

Question 1

body water basics

Answer 1

• adult body = 50-60% water
  
  • lean body tissues = ~75% water
  • fat mass = ~5-10% water
• functions
  
  • nutrient transport
  • protection
  • temperature regulation
  • biochemical reactions
  • medium for reactions

Question 2

distribution of water in a young 70kg man

Answer 2

• intracellular fluid - 28L - 40% of body mass - 62.5% of total body water
• extracellular fluid - 14L - 20% of body mass - 37.5% of total body water
• interstitial fluid - 10.5L - 15% of body mass - 30% of total body water
• blood plasma - 3.5L - 5% of body mass - 7.5% of total body water
  total body-water volume is 42L or 60% of body mass

Question 3

daily body water balance for a sedentary adult

Answer 3

fluid loss
- respiration 200ml
- skin 600ml
- feces 100ml
- urine 1300ml
- total =2200ml
fluid gain
- drink 1200ml
- food 700ml
- metabolic 300ml
- total = 2200ml

Question 4

net body water balance

Answer 4

• net body water balance is the difference between fluid water gain (intake + production) and fluid loss
• typically, well maintained on a day-to-day basis (within 1% of body mass)
  
  • thirst and hunger drives
• net body water balance can be challenged during period of high sweat rates

Question 5

dehydration

Answer 5

• dehydration, sometimes referred to as hypohydration, is defined as a body water deficit greater than normal daily fluctuation
• change in body mass provides the most sensitive and simplest measure to determine acute changes in body water
• acutely: >2% body mass reduction = 90% chance that body water deficits are sufficient to be classed as dehydration

Question 6

why do we sweat

Answer 6

• heat production during exercise
  
  • for every L of oxygen consumed 4kcal of heat is produced and only about 1kcal is used to perform mechanical work
  • most of this heat is passed to the body core
  • hypothalamus sense increase in body (and skin) temperature
  • responds to increase blood flow to skin and initiate sweating response
• sweating is the body’s principle means of preventing excessive rises in body temperature during exercise (hyperthermia)
• normal body temperature is 36-38 degrees celcius
• during exercise is 38-40 degrees celcius
• evaporative cooling
  
  • 1L sweat evaporated will remove 573kcal of heat from the body

Question 7

sweat loss

Answer 7

• water loss from the water component of blood (plasma) = reduce plasma and blood volume
• sweat is hypotonic as compared to plasma, therefore plasma osmolality increases
  
  • sodium = 24.1 +- 15.0 mmol/l
  • chloride = 18.6 +- 15.9 mmol/l
  • potassium = 3.25 +- 0.62 mmol/l
  • lactate = 5.87 +-0.66 mmol/l

Question 8

dehydration and its development

Answer 8

• dehydration, sometimes referred to as hypohydration, is defined as a body deficit greater than normal daily fluctuations
• it largely develops due to inadequate fluid intake relative to fluid losses, and during exercise fluid losses are largely attributable to sweating
• sweating, and evaporating heat loss, is the main mechanism to control body temperature during exercise, preventing development of hypothermia
• we would typically define exercise-associated dehydration as an acute body mass loss >/=2%

Question 9

does hydration impair performance

Answer 9

• 1-2% BM loss = minimal impact
• 2-3% BM loss = may degrade aerobic performance (moreso in heat), deterioration of sport-specific skills, minimal impact on sprint running, minimal cognitive function impact (without heat stress), reduced cognitive function (with heat stress)
• > 3% BM loss = impaired cognitive function (even in temperature)
• 3-4% BM loss = minimal impact on muscle strength and power

Question 10

impact of ‘blinded’ dehydration on aerobic performance

Answer 10

• blinded dehydration impairs endurance performance
• similar performance decline with blinded vs unblinded dehydration

Question 11

potential mechanisms of impaired aerobic performance in warm-hot conditions

Answer 11

• cardiovascular: blood pressure and blood flow, oxygen delivery, metabolite removal
• central nervous system: brain metabolism, brain temperature
• peripheral muscular factors: temperature, metabolic, afferent feedback
• psychological: thermal comfort, perceived exertion
• respiration: breathing sensations
  dehydration exacerbates these proposed mechanisms, although cardiovascular strain (high skin blood flow + reduced blood volume) is likely a critical primary factor

Question 12

dehydration and performance

Answer 12

• inadequate maintenance of fluid balance and the development of hypohydration can negatively impact physiology and exercise performance in some, but not all exercise contexts
• the mechanisms by which hypohydration affects performance are multifactorial, although cardiovascular strain appears to be the most prominent
• environment can play a big role in the impact of hypohydration

Question 13

ACSM guidelines

Answer 13

• 2-4h pre event = 5-10ml fluid/kg body weight (sodium, salt snacks or small meals may help), more fluid if no/dark urine
• during exercise = sufficient fluid to limit body mass losses to <2% and limit excessive electrolyte imbalance (pre-post weighing can help in estimation)(typically 0.4-0.8L/h [cool, flavour])
  
  • > 2h (or salty swaters) = add sodium
  • > 1h - may consider CHO addition
• post exercise = rapid recovery (<12h): consume 1.25-1.5L for each kg BM loss + sodium, more recovery time: resume dietary practices + extra plain water

Question 14

why sodium?

Answer 14

• pre exercise = fluid retention
• during exercise = limit electrolyte losses (risk of hyponatremia), stimulate thirst
• post exercise = restoration of sodium and fluid balance

Question 15

how do we characterise a CHO-E solution

Answer 15

• A CHO-E should
  
  • supply CHO as the major energy source
  • be effective in maintaining hydration status
• energy: 80-350kcal/L from CHO, >=75% of energy from CHOs that induce high glycaemic response
  
  • e.g., glucose, glucose polymers, sucrose
• sodium: 20-50mmol/L (460-1150mg/L)

Question 16

sodium content of ‘sports’ drinks

Answer 16

• body fuel
  
  • sodium = 50mg/100ml
• fit water
  
  • sodium = 21mg/100ml

Question 17

hyponatremia

Answer 17

• low blood sodium (<135mmol/L)
• can arise from over drinking fluids more than fluid losses (exacerbated with high sweat sodium losses + low sodium beverages)
• recreational athletes and women at greater risk
• dangerous, immediate and accurate medical attention

Question 18

calculating sweat rate

Answer 18

sweat rate (L/h) = (weight loss +volume consumed - urine loss)/duration of exercise (h)