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8

Fluid and Electrolyte Balance

8

Fluid and Electrolyte Balance

chapter

Prof Jennifer Broxterman, RD, MScFN3373: Nutrition for Physical ActivityLecture 6

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: 45-

70% 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. Environmental conditions

2. Clothing

3. Exercise intensity

4. Level of physical conditioning

5. 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. Rate of fluid ingestion

2. Gastric emptying

3. Intestinal fluid absorption

4. 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 CHO-electrolyte 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:– 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. During prolonged exercise lasting >60-90 min

2. During high-intensity or intermittent exercise

3. During exercise in the heat

4. During preseason training or two-a-days

5. In competition, during games, and during events

6. During phases of poor nutrition

7. During phases of compromised immune status

8. 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 <60-90 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• What to Study:

– Lecture notes (lectures 1, 2, 3, 4, 5)– Textbook chapters (chapters 1, 2, 3, 4, 8)

• Worth: 25% of your overall grade• If you have a conflict, you MUST speak with

your academic advisor 1st before emailing me