Hydration (fluid and electrolyte requirements for exercise) Flashcards
1
Q
body water basics
A
- 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
2
Q
distribution of water in a young 70kg man
A
- 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
3
Q
daily body water balance for a sedentary adult
A
fluid loss
- respiration 200ml
- skin 600ml
- feces 100ml
- urine 1300ml
- total =2200ml
fluid gain
- drink 1200ml
- food 700ml
- metabolic 300ml
- total = 2200ml
4
Q
net body water balance
A
- 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
5
Q
dehydration
A
- 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
6
Q
why do we sweat
A
- 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
7
Q
sweat loss
A
- 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
8
Q
dehydration and its development
A
- 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%
9
Q
does hydration impair performance
A
- 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
10
Q
impact of ‘blinded’ dehydration on aerobic performance
A
- blinded dehydration impairs endurance performance
- similar performance decline with blinded vs unblinded dehydration
11
Q
potential mechanisms of impaired aerobic performance in warm-hot conditions
A
- 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
12
Q
dehydration and performance
A
- 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
13
Q
ACSM guidelines
A
- 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
14
Q
why sodium?
A
- pre exercise = fluid retention
- during exercise = limit electrolyte losses (risk of hyponatremia), stimulate thirst
- post exercise = restoration of sodium and fluid balance
15
Q
how do we characterise a CHO-E solution
A
- 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)
