Nutrition Chapter 10
Inverse relationship between post-race sodium concentration and ____
% change in body mass; Less weight loss = higher sodium concentrations
Bernard milieu interieur
- (the environment within) - represents the underlying principle of homeostasis first used by Harvard physiologist Walter Cannon who also coined the term "fight-or-flight"
ACSM recommendation for sodium in sports drinks
- 0.5 - 0.7 g of Na per L fluid consumed during exercise lasting over an hour
A person can tolerate ____ drop in deep body temperature but ___ increase
- 10 degree celsius drop - 5 degree celsius increase
How much sodium could be added to a rehydration drink to restore electrolyte balance? Potassium?
- 100 mmol/L of Na - 2-5 mmol/L of K
How much should someone drink after exercise in comparison to water lost?
- 125% - 150% -- extra water accounts for urine production
Prolonged exercise in heat causes sweat loss depletion of _____g of salt
- 13 - 17g of salt (2.3 - 3.4 g/L of sweat) --about 8g more than typically consumed daily ---Can be replaced by adding 1/3 teaspoon of salt to 1 L of water
- 4.3% dehydration of body mass = ___% decrease in walking endurance. Also decreased VO2max by ____%
- 18% - 22%
Average fluid loss in temperate climate for 90 minutes of soccer
- 2 L
How much does metabolic rate increase from sustained exercises
- 20-25x above resting level to 20 kcal/min -- could theoretically increase core temperature by 1 degree celsius every 5-7 minutes!
- 1.9% dehydration of body mass = ___% decrease in endurance performance and ___% decrease in VO2max
- 22% - 10%
How much does metabolic rate increase from shivering?
- 3 - 5x increase
Water loss by sweating in an acclimatized person peaks at about ___ L per hour during intense exercise in the heat and averages ___ L daily
- 3 L/hr - 12 L/day (26 lb)
Recommended fluid intake before exercise
- 400 - 600 mL (12 - 20 oz) about 20 minutes before exercise - this increases stomach volume, optimizing gastric emptying rate
Potassium loss at intense PA levels
- 5 - 18 mEq - Not significant --A glass of orange or tomato juice replaces all potassium, calcium, and magnesium loss from 3 L of sweat
Elite marathon runners experience fluid loss in excess of ___ L
- 5L; 6-10% of body mass
How long does it take to acclimate to stressful environments? How long to lose the acclimation?
- 8 - 14 to acclimate - 14 - 28 days to lose it
For each liter of sweat loss-induced dehydration, HR increases ____ beat/min
- 8 beat/min --- = 1.0 L/min decrease in cardiac output
Heat Index
- AKA Apparent Temperature - Made by US National Weather Service - Evaluates relative heat stress --exposure to full sunshine increases values by up to 15 degrees F
Hyponatremia
- AKA water intoxication - occurs when serum sodium concentration falls below 135 mEq/L (below 125 causes severe symptoms) - creates osmotic imbalance across blood-brain barrier causing rapid water influx into the brain
Men vs. Women
- Acclimate to similar degree - Women have more sweat glands but sweat less, therefore decreasing likelihood for dehydration - Women begin sweating at higher core temperatures than men - Women have equal heat tolerance to men despite decreased sweating - Women rely more on circulation to dissipate heat while men rely more on evaporation - Women cool off at a faster rate than men (possibly because body surface area-to-mass-ratio)
Heat acclimatization and sweat
- As acclimatization progresses, more blood shunts to cutaneous vessels to facilitate heat transfer from core to periphery - more effective cardiac output maintains blood pressure - lowered threshold for sweat (capacity nearly doubles with less dilution (less salt loss)) - These people require more water due to increased sweat loss
What is the main "workhorse" to control thermal balance?
- Circulatory system
Thermal balance and blood flow
- Heat conservation - -- shunting blood deep to cranial, thoracic, and abdominal cavities, optimizing insulation from fat and other areas of the body's shell - Heat loss -- peripheral vessels dilate to channel warm blood to cooler periphery
Cutaneous thermal receptors
- act as an "early warning system" that relays sensory information to the hypothalamus and cerebral cortex - the more numerous receptors usually exist near skin surface and play important role in initiating regulatory response to cold environments
Peripheral thermal receptors
- are responsive to rapid changes in heat and cold and exist predominantly as free nerve endings in the skin
Food intake in hot environments
- athletes voluntarily eat less calories in hotter environments with protein remaining the same but fat and carbs being less
Heat loss by radiation
- body is usually warmer than the environment and therefore the body emits electromagnetic heat waves into the environment - does not require molecular contact between objects - the body absorbs radiant heat energy when temperature in the environment exceeds skin temperature
Exercising in colder environments
- can create a 1 L daily fluid loss - Can increase urine production, further leading to fluid loss
Easy way to indicate extent of water loss/gain during/after exercise
- change in body weight
Mineral metabolism due to hot environmental exercise
- changes in chromium, copper, iron, magnesium, and zinc metabolism occur; extent unknown
Children and sweating
- children have more sweat glands but sweat less - compared to adults, children have lower Na, Cl, lactate, H+, and K concentrations in sweat - children take longer to acclimatize to heat --should exercise at lower intensity
Clothing and heat
- cotton and linen absorbs moisture - black clothes absorb light rays, adding to heat gain - Football players at high risk of heat stroke due to heavy equipment and body surface coverage
Heat loss by convection
- effectiveness depends on how rapidly air near the body exchanges once it warms - if no air movement, warmed air next to the skin remains in place and is counter-productive to heat loss, actually causing a zone of insulation - if cooler air continuously replaces the warmed air, heat loss occurs - air currents moving at 4mph cool twice as effectively as air moving at 1 mph`
Wet Bulb-Globe Temperature (WB-GT)
- evaluates environment for its potential thermal challenge - WB-GT = 0.1 x DBT + 0.7 x WBT + 0.2 x GT --DBT = Dry-bulb air --WBT = Wet-bulb air (70% of the index) -- GT = global temperature
Heat loss by evaporation
- evaporation of sweat provides major mechanisms of heat loss and defense against overheating - 1 L of evaporized sweat transfers 580 kcal of heat from the body - 2-4 million sweat (eccrine) glands secrete hypotonic saline solution (0.2 - 0.4% NaCl)
Fluid ingestion should occur how often for hourly losses up to 1000 mL water loss? Over 1000 mL?
- every 15 minutes - every 10 minutes
Voluntary replacement of water lost
- most people only replace 1/2 of water lost (<500 mL/hr) voluntarily
What exerts the greatest impact on the effectiveness of evaporative heat loss?
- relative humidity
Aldosterone
- released by adrenal cortex during repeated days of exercise in heat - sodium conserving hormone that increases the renal tubules' reabsorption of sodium and also decreases sodium in sweat (reducing osmolality), aiding in electrolyte conservation
Antidiuretic Hormone (Vasopressin)
- released by pituitary gland during heat exposure - increases water reabsorption from kidney tubules that causes urine to become more concentrated
Hypothalamus
- specialized neurons on the floor of the brain that regulates temperature (~37 degrees Celsius +- 1) - cannot turn off heat, can only start responses to protect body from heat gain or loss
Sodium content of typical sports drinks compared to normal plasma ranges
- sports drink = 10 - 25 mmol/L Na - plasma = 138 - 142 mmol/L Na
Heat loss by conduction
- transfers heat directly through solid, liquid, or gas from one molecule to another - the rate of conductive heat loss depends on existence of temperature gradient between the skin and surrounding surfaces and their thermal qualities
Diuretics for water loss
- trigger disproportionate reduction in plasma volume - impairs neuromuscular function - causes mineral loss and muscle weakness if excessive diarrhea and vomiting
Erythropoietin
-an illegal hormone that boosts production of red blood cells - illegal users experience an even greater risk of heat injury due to increased viscosity from increased hematocrit which magnifies as dehydration progresses
How much sweat is lost over 1 hour of moderate exercise?
0.5 - 1.5 L
Fluid loss of ___% body mass increases rectal temperature above that with the same exercise performed when fully hydrated
1%
5 Steps to reduce risk of overhydration and hyponatremia in prolonged exercise
1. 2-3 hr before exercise, drink 400 - 600 mL of fluid 2. Drink 150 - 300 mL of fluid about 30 min. before exercise 3. Drink no more than 1000 mL/h of water spread over 15 min intervals during or after race 4. Add (1/4 - 1/2 tsp of salt per 32 oz) to ingested fluid 5. Do not restrict dietary salt
3 accomplishments of mechanisms to balance body heat
1. Alter heat transfer to periphery or SHELL 2. Regulate evaporative cooling 3. Vary the rate of heat production
Body fluid loss coincides with these 5 changes in bodily function
1. Decreased plasma volume 2. Reduced skin blood flow for a given core temperature 3. Reduced stroke volume of the heart 4. Increased HR 5. General deterioration in circulatory and thermo-regulatory efficiency in exercise
4 major factors of concern in hot-weather exercise
1. Dehydration 2. Decreased plasma volume and resulting hemoconcentration 3. Impaired physical performance and thermo-regulatory capacity 4. Increased risk of heat injury (especially heat stroke)
3 Effects of Hyperhydration
1. Delays dehydration 2. Increases sweating during exercise 3. Diminishes the rise in core temperature
6 factors that determine differences in accomodation and acclimation
1. Genetics 2. Available resources 3. Age 4. Nature and duration of previous exposures 5. Number of similar previous experiences 6. Emotional and psychological response to environment
Mechanisms of heat loss
1. Radiation 2. Conduction 3. Convection 4. Evaporation of sweat
4 most prevalent factors associated with hyponatremia
1. Substantial pre-post race gain 2. Consumption of more than 3 L of fluid during race 3. Race time greater than 4 hours 4. Low BMI
3 factors that determine sweat evaporation from the skin
1. Surface area exposed to the environment 2. Temperature and relative humidity of ambient air 3. Convective air currents around the body
2 ways heat regulating mechanisms are activated
1. Temperature changes in blood perfusing the hypothalamus directly to stimulate thermoregulatory control 2. Thermal receptors in the skin provide input to modulate hypothalamic activity
8 environmental stressors that challenge the milieu interieur
1. heat 2. cold 3. hypoxia 4. noise 5. food 6. darkness 7. trauma 8. pathogens
what percent of cardiac output passes through the skin with extreme heat stress, greatly increasing thermal conductance of peripheral tissues?
15-25% - favors radiative heat loss
Heat loss from evaporative cooling under optimal conditions
18 kcal/min
Dehydration of ___% body mass impairs physical work capacity and physiologic function and predisposes to heat injury when exercising in hot environment
2%
Sweat sodium concentration loss from 1 L/hr sweat loss
20 - 100 mEq/L
Sweat rate during exercise in the heat in order to maintain thermal balance
3.5 L/hour
Fatal heat stroke occurs how much more frequently in obese adults than in healthy
3.5 x more frequently
Sweating reaches equilibrium directly related to exercise load after how long?
30 minutes
How much water vaporizes daily from the respiratory passages' mucous membranes?
300 mL
Insensible Persperation
350 mL of water seeps throught the skin each day
Ingesting fluid at ___degrees Celsius increases fluid consumption and improves endurance
4 degrees Celsius
Vapor pressure of moist skin
40 mm Hg - when humidity increases near this, decreases ability of evaporative heat loss because sweat does not evaporate
1 lb of weight loss from fluid represents ___ mL of dehydration
450 mL (15 fl oz)
What percent of collapsed individuals from ultraendurance events had symptoms of hyponatremia
9% --Body mass declined 2.5 kg
Claude Bernard
French scientist who discovered the body can protect against environmental stressors through diverse mechanisms
Dehydration
Inbalance in fluid dynamics when fluid intake does not replenish water loss from either hyper-hydrated or normally hydrated states
What promotes water movement into cells?
a reduced extracellular solute concentration
Sweat rate increases directly with what?
ambient temperature
What occurs at rest in hot weather?
cardiac output increases while superficial arterial and venous blood vessels dilate to divert warm blood to the body's shell--results in red face on hot day or during exercise
What decreases the effectiveness of conduction, convection, and radiation?
increased ambient temperature --they actually contribute to heat gain when ambient temperature is increased, thus forcing sweat evaporation and water vaporization from respiratory tract to be the only methods of heat loss
Acclimation/Acclimatization
intermediate adaptations to counteract environmental stress
Any degree of dehydration impairs:
the capacity of circulatory and temperature-regulating mechanisms to adjust to exercise demands
What antioxidant vitamins may reduce exercise-induced lipid peroxidation?
vitamins A, C, and E
Effects of heat rate on gastric emptying rate
"probably decrease, with mechanisms remaining unclear but possibly relating to dehydration"
Heat Cramps
- involuntary muscle spasms during or after PA - most likely occurs from imbalance in hydration and electrolytes - Prevention: 1. hydration with water + salt 2. increasing daily salt intake
Heat Exhaustion
- most common heat illness - occurs because of ineffective circulatory adjustments compounded by decreased plasma volume - blood pools in peripheral vessels, reducing central blood volume - symptoms include: weakness, dizziness, headache, nausea, low BP, weak and rapid pulse, and "goose bumps" - fluids should be given with 5% dextrose sugar in either 0.45% NaCl or 0.9% NaCl
Exertional Heat Stroke
- most serious heat illness - failure of heat-regulating mechanisms - sweat ceases, skin becomes hot and dry, body temperature rises over 41.5 degrees celsius - can be fatal! - can be treated with alcohol rubs, ice packs, immersion under ice water
5 factors that determine the physiologic strain imposed by heat
1. Body size and fatness 2. Level of training 3. Acclimatization 4. Adequacy of hydration 5. External factors
How much vitamin C above recommended daily levels may reduce heat stress during acclimatization
250 mg above RDA --too high of dose can reduce vitamin B12 absorption and are not recommended
Dehydration associated with ____% decrease in body weight slows gastric emptying rate
3%
A modest rise in core temperature reflects:
A favorable internal adjustment, creating optimal thermal environment for physiologic and metabolic functions
Hyperthermia
An increase in deep body temperature of 5 degrees Celsius
The requirement for which vitamin increases when living and working in hot environments
B vitamins
Heat Illness
includes heat cramps, heat exhaustion, and heat stroke
What can affect sodium loss?
level of acclimization
Genetic Adaptations
long term adaptations to counteract environmental stress --semi-permanent
The body's thermoregulatory mechanisms primarily protect against __________
overheating
Relative Humidity
percentage of water in the air at a particular temperature compared with total quantity of moisture the air could carry
As dehydration progresses and plasma volume decreases:
peripheral blood flow and sweating rate diminish and thermo-regulation becomes more difficult -- this contributes to increased HR, RPE, and core temperature
Heat acclimatization
physiologic adaptations that improve heat tolerance - occurs in first week of heat exposure (2-4 hr daily); completely acclimated in 10 days - Loss of acclimatization occurs within 2-3 weeks
What decreases as sweat loss increases?
plasma volume; can lead to circulatory failure
What is the most effective way to minimize or eliminate heat stress injuries?
prevention
Accomodation
short term adaptations to counteract environmental stress
When does core temperature rise?
when heat gain exceeds heat loss (vigorous exercise in warm environment)
What 2 populations require more attention for fluid intake?
women and elderly due to suppressed thirst drive