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