chapter
chapter
88
Fluid
Fluidand
and
Electrolyte
Electrolyte Balance
Balance
Prof Jennifer Broxterman, RD, MSc
FN3373: Nutrition for Physical Activity
Lecture 6
Author name here for Edited books
Introduction
• Water is an essential nutrient
– It is the most abundant constituent of the body
– Can go weeks without eating food, while death from
dehydration can occur with 3-4 days
– Water loss can impair exercise performance and the
ability and desire to perform work
• Water offers more than just H2O
– Contains numerous electrolytes: Na, Cl, Mg, K, Fl, Ca
Water & Electrolyte
Balance
Body Fluids
• Total body water
– Comprises about 60% of total body weight (range: 4570% water)
– People with more muscle mass have a higher overall
% of body water than those with more body fat
– Men tend to have higher %’s of water weight than
women (lean men < 40 yr = 60-65% water; lean
women < 40 yr = 50-55% water)
– As people age, muscle mass usually declines,
decreasing total body water content
Functions of Water
• Water has a number of biological functions:
–
–
–
–
–
–
–
Lubricant
Transport medium
Removal of waste materials
Solvent
Chemical reactions
Structural part of body tissues
Regulation of body temperature
Body Water Compartments & Composition
• Intracellular water: includes all water enclosed
within cell membranes
– The medium in which all chemical reactions of
cellular metabolism occur
– Makes up 2/3 of total body water
• Extracellular water: is all the water outside cells
– The medium through which all metabolic exchanges
occur
– Makes up 1/3 of total body water
Fluid Balance
• Osmotic pressure: the force necessary to exactly
oppose osmosis (movement) of water into a solution
across a semipermeable membrane
– Depends on: (1) the total # of solute particles in the
solution and (2) the permeability characteristics of the
membrane through which the solute must pass
• Osmolarity: measure of the total number of solute
particles (moles) per unit volume
– The more solutes found in a fluid, the greater its
osmolarity or concentration
Fluid Balance
• Osmolality: refers to the # of moles or particles per
kilogram of water (1 L of water weighs 1 kg)
• Electrolytes:
– Help maintain fluid balance by keeping water within
a particular body water compartment
– E.g. Na, Cl, K, Ca, Mg, other small molecules
Fluid Balance
Body Water & Electrolyte Losses
• Normal daily body water losses come from:
– Urine, feces, sweat, respiration
• Exercise conditions:
– Amount of water lost in sweat can dramatically
increase to 1-2 L/hr (compared to 100-200 mL/day
typically lost on non-exercise days or in a cool or
temperate environment)
– Electrolytes lost in sweat: Na, Cl, K, Mg, Ca, Fe
• Primarily electrolytes lost: Na (10-80 mmol/L) & K (5-10
mmol/L)
• Na is reabsorbed by sweat glands
Sweating
During Exercise
• Amount of sweat lost
depends on:
1.
2.
3.
4.
5.
Environmental conditions
Clothing
Exercise intensity
Level of physical conditioning
Acclimation to the environment
Table 8.2
Figure 8.1
Regulation of Water &
Electrolyte Balance
• Regulation of body water
– Intricate, constant regulation of fluid status
– Stimulation of thirst to increase intake of water
– Regulation of fluid loss through the kidneys to
increase or decrease output as necessary
– These responses help control blood pressure and
restore fluid balance in the various body
compartments
Gastric Emptying &
Intestinal Absorption
• Oral rehydration
– Affects optimal fluid balance, especially during exercise
– Depends on:
1.
2.
3.
4.
Rate of fluid ingestion
Gastric emptying
Intestinal fluid absorption
Individual variability
• Fluid recommendations for athletes:
– Need to be individualized based on GI complaints, past
experience with fluid intake during exercise, sweat
rates, and fluid needs
Factors that Affect Gastric Emptying
(Figure 8.2)
Gastric Volume
• Gastric volume’s effect on gastric emptying:
– One of the strongest regulators of gastric emptying
– The greater the volume of fluid consumed, the greater
the rate of gastric emptying (up to 600 mL)
• Average rate:
– 40 mL of water per minute
– Or 2.4 L of water per hour
Osmolality
• Osmolality and gastric emptying:
– Negatively correlated with gastric emptying
– Most sport beverages are low in Na (132-416 mg/L)
and in osmolality (<400 mOsm/L) they do not inhibit
water absorption
– Glucose polymers have a lower osmolality than
glucose
Carbohydrate Concentration & Type
• Carbohydrate concentration:
– As the CHO concentration of a fluid increases, gastric
emptying decreases
– Solutions containing 4-8% CHO are ideal
• Type of carbohydrate:
– Variety of sugars (i.e. glucose, fructose), compared to
one type of sugar in a similar dose (i.e. glucose only) is
more readily absorbed from the GI tract
• Carbonation:
– Noncarbonated CHO-electrolyte beverages and water
leave the gut sooner than lightly carbonated CHOelectrolyte beverages of carbonated pop
Exercise Intensity & Type
• Low vs. high-intensity exercise
– High-intensity exercise (>70% VO2 max) delays
gastric emptying
• Type of sport
– Different types of physical activity can alter the
ability to consume fluids, the ease of consuming
them, and the way in which they are emptied from
the stomach
– Runners vs. cyclists
Euhydration, Hypohydration,
Dehydration, & Hyponatremia
Definitions for States of Hydration
• Euhydration: normal hydration
• Dehydration: excessive loss of body water
• Hypohydration: refers to a reduction of body
water as the body progresses from a normally
hydrated (euhydrated) to a dehydrated state
• Hyponatremia: electrolyte disturbance in
which the sodium concentration is the blood is
lower than normal (normal = 135 – 145 mEq/L)
Dehydration
• Effects on athletic performance:
– Dehydration impairs performance in most events
(i.e. aerobic capacity, cognitive function)
– Greater levels of dehydration affect performance
more significantly, although performance effects are
dependent on:
• the environment (i.e. heat, humidity)
• the exercise task
• the individual’s tolerance to dehydration
– Athletes should consume adequate fluid to limit
dehydration to <2% of body weight
Figure 8.3
Hypohydration
• Frequently called voluntary dehydration
• Athletes may do this to:
– “Make weight” (e.g. wrestling, rowing, boxing, horse
racing)
– Improve muscle definition and physical appearance
(i.e. bodybuilding competition)
• Health consequences:
– Muscle fatigue, loss of concentration, poor exercise
performance, cardiac arrest, death
Hyponatremia
• EAH: Exercise-associated hyponatremia
– Abnormally low plasma sodium concentrations (< 135
mmol/L) before, during, or after exercise
– Usually occurs when excess water accumulates,
relative to Na, in the extracellular water compartments
of the body
– EAH occurs from over-consuming fluids (water or sport
drinks) in excess of total body fluid loss
– S&S: bloating, nausea, vomiting, headaches,
restlessness, undue fatigue, confusion, disorientation,
wheezy breathing, seizures, coma, death
Heat-Related Disorders
Exercise-Associated Muscle Cramps
• Exertional heat cramps
– Skeletal muscle spasms can occur after prolonged,
strenuous exercise in the heat when sweat losses are
high, urine volume is low, and sodium intake is
inadequate to replace the losses
– Cramping occurs in the legs, arms, or abdominal wall
– Treatment:
•
•
•
•
•
Stretch the muscle group at full length
Add Na to fluid (add 1/8 to ¼ tsp to 300-500 mL of fluid)
Eat salty snacks with water or try sipping broth
Consume 1-2 salt tablets with 300-500 mL of fluid
Add a higher amount of Na in the diet in the form on table salt
resolves the disorder
Exertional Heat Exhaustion & Heatstroke
• Fluid loss, which reduces blood flow from the
muscles to the skin, compromises the body’s
ability to dissipate the heat generated during
exercise and to adequately cool itself
– Core body temperature rises to >104°F (>40°C)
– Exertional heat exhaustion can lead to exertional
heatstroke
– S&S: excessive sweating (initially), headache, nausea,
dizziness, gradual impairment of consciousness,
difficulty concentrating, collapse, death
– Children & elderly are more susceptible
Rhabdomyolysis
• Rhabdomyolysis:
– Syndrome that occurs with novel, strenuous exercise
and is characterized by the breakdown of muscle fibres
and release of skeletal muscle contents into the blood
– Most commonly occurs in unaccustomed exercisers
who experience eccentric and concentric muscle
overuse or in exertional heat illness in trained athletes
when muscle tissue exceeds the critical temperature
threshold of cell membranes (hyperthermia)
– Dehydration can exacerbate symptoms (due to AKF)
Useful Hydration
Assessment Methods
for Athletes
Hydration Assessment Methods
• Urine colour
– Good field indicator of hydration status
– Urine should be “very pale yellow” or “pale yellow”
• Daily body weight changes
– Daily BW measurements (1st thing in the
AM, nude, after voiding) provide a simple
and useful tool to assess and monitor
daily fluid balance
– Females may experience larger
fluctuations due to their menstrual cycle
Hydration Assessment Methods
• Urine specific gravity & osmolality
– Urine specific gravity should be <1.020
– Urine osmolality should be <700 mosm/kg
• Plasma & serum osmolality
– Requires blood sampling (invasive)
– Plasma osmolality should be <290 mosm/kg
(euhydration)
• Acute changes in body weight
– Acute changes in BW before and after exercise can
be used to estimate sweat rate
Fluid & Electrolyte
Recommendations
for Exercise
Fluid Needs Before Exercise
• Athletes should begin exercise well hydrated
– Those at greatest risk of dehydration include:
1. Athletes in weight-class sports (“weigh-ins”)
2. People training/racing in environments to which they have not
yet acclimatized
• Pre-exercise fluid recommendation:
– Consume 5-7 mL/kg body weight 4 hr before exercise
– If urine volume is very small and the colour is dark, drink
another 3-5 mL/kg body weight about 2 hr before exercise
– Hot weather: may need to add an additional 250-500 mL
– Fluid bolus (300-400 mL) about 15-20 min before exercise
Hyperhydrating Agents
• Glycerol:
– Hyperhydrating agent
– Dosing: typically consumed in small amounts (1-1.5
g/kg BW) in combination with a large volume of fluid
(25-35 mL/kg BW) 2.5-4 hr before exercise
– Glycerol supplementation can result in a fluid
retention of 400-700 mL
• Whether glycerol improves performance in sport is
presently unclear
– Potential side effects: nausea, vomiting, GI upset
Hyperhydrating Agents
• Creatine:
– Unlike glycerol, creatine retains fluid predominantly
intracellularly
– Dosing: ingestion of ~20 g/day dissolved with 0.5 L
of water for 1 week
– Creatine supplementation can result in a fluid
retention of 400-800 mL
Fluid Needs During Exercise
• Goal of drinking fluids during exercise:
–
–
–
–
Maintain plasma volume & electrolytes
Prevent abnormal elevations in heart rate
Maintain appropriate core body temperature
Provide fuel to the working muscles
• Effectiveness of a fluid in replacing sweat
losses depends on:
– Exercise duration & intensity, volume & composition of
the fluid, environmental conditions, drinking frequency,
and the nutritional status of the individual prior to
exercise
Sweating Rates
• Factors affecting sweating rates:
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–
–
–
–
–
–
Type of sport
Gender
Intensity of exercise
Fitness status
Age
Environmental conditions: temperature, humidity, wind
Level of acclimatization to environmental conditions
• Sweating rates in athletes range from as little
as 0.2 L/hr to up to 2 L/hr
Figure 8.5
Fluid Replacement Strategies
During Exercise
• Fluid replacement strategies for athletes:
– Need to be adapted to the individual’s
sweating rate
– Should take into account the accessibility of
fluid and the feasibility of strategic, regular
fluid replacement during the exercise
session
– Thirst should be integrated into a daily or
weekly log
– Fluid replacement during exercise lasting
longer than 60 min should be common
practice (water, sport drink)
Fluid Needs After Exercise
• Goal of post-exercise rehydration:
– Replace water & electrolytes lost during exercise
– Prepare for a 2nd training session later in the day
• Replenishment process:
– Consume adequate water and food during the
recovery period to replace fluid and electrolytes
– If food is unavailable post-exercise, consume fluids
that contain electrolytes (esp. Na) & CHO
– Volume of fluid should be greater than the volume of
sweat lost (i.e. 150% of BW lost during exercise)
Alcohol
• Alcohol consumption & rehydration:
– Alcohol has a strong diuretic effect (increases urinary
volume output) and can interfere with recovery
– Delays the recovery process, esp. glycogen synthesis
– Should be avoided when timely
recovery between training sessions
is of key importance
Sport Drink and Fluid
Replacement Beverages
Optimal Sport Drink Formulas
• There is no general agreement among researchers
on the optimal formulation of a sport drink
• General recommendations:
–
–
–
–
–
–
Contains electrolytes & CHO to enhance fluid balance
CHO: 4-8% CHO from multiple sugars
Na: 500 to 700 mg (20-50 mmol/L)
Cool temperature
Pleasant flavour to increase consumption
Fluid intake should begin early during exercise and
occur frequently
8 Situations Where a Sports Drink
is a Better Choice than Water
1.
2.
3.
4.
5.
6.
7.
8.
During prolonged exercise lasting >60-90 min
During high-intensity or intermittent exercise
During exercise in the heat
During preseason training or two-a-days
In competition, during games, and during events
During phases of poor nutrition
During phases of compromised immune status
During exercise at altitude and in the cold
5 Situations Where Water is a
Better Choice than a Sports Drink
1. If exercise intensity is low and the session lasts <6090 min
2. If the goal of the fitness program (active individual) is
weight loss and the person engages in moderate
exercise lasting <60-90 min
3. If the goal of the training program (endurance athlete)
is optimization of fat metabolism
4. If the athlete is well fed and is training at low to
moderate intensity in a temperate or cool environment
5. During the off-season and on recovery days
Protein in Sport Drinks
• Effects of adding 2% protein to a sport drink:
– Has no effect on endurance performance (but also
doesn’t hurt it)
– Has been associated with decreased markers of
muscle damage or disruption (reduced plasma
creatine kinase, serum myoglobin, lactate
dehydrogenase) and subjective ratings of muscle
soreness
Caffeine in a Sport Drink
• Effects of adding caffeine to sport drinks:
– Boosts endurance performance
• Why?: increases glucose absorption, leading to increased
glucose oxidation in the muscle
– Caffeine is still regarded as a diuretic despite the
evidence that this is not so
– Word of caution: using caffeine-containing drinks in
heat to which an athlete is not acclimatized may
raise the risk for heat-related illness
Fluid Needs in Hot &
Cold Environments
Exercising in Hot Environments
• Exercising in the heat increases the risk of
dehydration & hyperthermia
• Hyperthermia: core body temperature exceeds
102-104° F (39-40° C)
– Exercise performance is severely impaired
– Cardiac output declines, perfusion pressure and leg
blood flow are reduced, leading to a rapid
suppression of oxygen delivery and uptake by muscle
– Brain blood flow and oxygen extraction also decline,
impairing mental performance
Exercising in Hot Environments
• Heat acclimatization: the process of adapting
to exercising in hot environments
– Nutrition strategies (esp. proper hydration) is an
important factor for adaptation
– Physical adaptations:
•
•
•
•
•
Lower body core temperature at rest
Decreased heart rate during exercise
Increased sweat rate and sweat sensitivity
Decreased sodium loss in sweat and urine
Plasma volume expansion
Table 8.7: Range of Days Required for
Different Adaptations to Occur
During Heat Acclimatization
Exercising in Cold Environments
• Exercising in cold environments stresses
the body’s thermoregulation mechanisms
– High altitude is an additional stress
– Respiratory water loss: cold air contains less water
than warmer air even if relative humidity is the same
(leading to a small but significant increase in
respiratory water loss
– Cold temperatures can impair the thirst response
– Other factors: layers of warm clothing, access to
washroom facilities outdoors
Fluid & Electrolyte Needs
for Children & Adolescents
Fluid Requirements of
Children & Adolescents
• Children overheat and become dehydrated
more quickly than adults
• Compared to adults, children:
– Are less efficient thermoregulators than adults, esp. when
exercising in warm environments
– Acclimatize more slowly
– Have a higher set point (change in rectal temperature at which
sweating starts)
– Have a lower sweating rate
– Produce more metabolic heat per kg of body weight during
exercise
– Have a greater physiological impairment from dehydration
Midterm Reminder
•
•
•
•
•
Date: Monday June 1, 2015
Time: 1:00-2:30pm
Location: Brescia, St. James Building, BR-201
Format: multiple choice
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