Sports Nutrition - HECO 336....

Fluids

The human body is approximately 60% water, thus water is the most essential of all the nutrients.  Most athletes recognize their additional need for fluids during exercise, but often forget that water is a critical part of their daily diet as well.
 
 
 

Hydration

The Water Pyramid
(guidelines for fluid intake and activity)
 
 
 
 

  Guidelines

Fluid Balance
Where does all that fluid go?

* Five liters of lost fluid is approximately 11 pounds of water loss or 7% weight loss from a 70 kg man.  Dehydration is about 3% body weight loss in fluids.
 
 

Mechanisms That Control Body Fluids
 

 Fluid homeostasis is maintained primarily by the kidney through hormonal regulation of electrolyte concentration [osmolality],
 
 
 

ELECTROLYTES
     Electrolytes are minerals that have an electrical charge.  Sodium (Na⁺), potassium (K⁺) and chloride (Cl^-) are the primary electrolytes involved in fluid balance.  Electrolytes with a positive charge are called cations (remember that the t in cation looks like a plus (+)).  Negative ions are anions.

Electrolytes, proteins and other substances which attract water effect fluid balance.  Fluid homeostasis is maintained primarily by the kidney through hormonal regulation of electrolyte concentration [osmolality],  one mole of a nonionic substance like glucose in one liter of water = 1 osmole (Osm); usually osmolality is given in milliosmoles (mOsm).  When a compound dissociates in water (separates into separate ions) then each osmole or milliosmole is multiplied by the ionic charge.  For example, one milliosmole of NaCl in one ml of water = 2 mOsm because of the dissociation of NaCl into Na⁺ + Cl^-.  Normal serum concentration is 280 mOsm.   Body water will shift from hypotonic areas (areas less than normal serum concentration) to hypertonic areas (areas more than normal serum concentration) to attain homeostasis.
 

Sodium is the major extracellular electrolyte thus controls fluid volume.
Normal plasma sodium levels ~ 140 mOsm
What is Salt Sensitivity and Could You be at Risk
 Stroke Quiz
- when blood pressure is higher than normal.  High blood pressure increases your risk of heart attack and stroke.  Hypertension is known as the silent killer since there are no symptoms until pressure becomes dangerously high.

Potassium is the major intracellular electrolyte. A normal plasma level of potassium is between 3.0 and 5.0 mOsm.
Diets High in Fruits and Vegetables Reduce your Risk of Hypertension
 

Other important electrolytes include:
    chloride          Cl^-
    calcium       Ca++
    magnesium     Mg++
    bicarbonate    HCO3^-
                    HPO4--
                    H2PO4--  2
 

Hormonal regulation

Antidiuretic Hormone (ADH) or vasopressin
Renin
Angiotensin
Aldosterone

 Osmoreceptors in the hypothalamus monitor electrolyte concentration, when concentration increases the hypothalamus signals the pituitary gland to release  antidiuretic hormone (ADH) also known as vasopressin which increases the reabsorption of water in the kidney.  Baroreceptors located throughout the body monitor blood volume, when volume drops the kidney releases renin which acts on angiotensinogen (produced in the liver) to form angiotensin I.   Angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE) (which is produced in the lung), agiotensin II acts on the adrenal gland to secrete aldosterone which increases the retention of sodium and the excretion of potassium.  This will ultimately increase water retention as well.  The hypothalamus also stimulates thirst mechanism but this significantly lags behind fluid status.
 
 

THERMAL REGULATION

 Normal functioning of temperature range is 97-104°F

The body aquires heat through a number of factors including: metabolism, hot air, high relative humidity, solar radiation and ground heat radiation/reflection.

Heat may be lost through four methods:
 conduction - direct transfer by physical contact
 radiation - heat radiates from the body into space
 convection - movement of air or water around the body transfers heat
 evaporation - heat lost when sweat is converted to a vapor = heat of vaporization = .54 kcal/g

All are included in the heat balance equation.

H  =   M ±  W  ±  C  ±  R  -E

H [heat bal] = M[rmr] ±W [work done] ± C [conduction & convection] ± R [radiation] - E [evaporation]

Sweating
The eccrine sweat glands have the greatest role in sweat production, loss and evaporative cooling.  As core temperature rises, warmed blood carries heat to the skin surface where the sweat glands produce sweat on the surface of the skin and air currents evaporate the water releasing heat, cooling the skin and the blood.  The cooler blood recirculates to the warmer muscle and internal organs picking up heat and continuing the process.  Trained and acclimatized athletes begin sweating earlier, and have sweat with a lower electrolyte content than untrained nonacclimatized individuals.  Thus regular exercisers are able to cool themselves more effectively than people who exercie sporadically and/or infrequently work out in the heat.
 

Circulatory Adjustments to Exercise and Volume Depletion
Cardiac Output = stroke volume  x  heart rate     Q = SV x HR
As exercise intensity increases HR increases.
As sweat loss increases without replacement SV decreases.
    What is the effect on cardiac output and performance?
 
 

Types of Heat Injury
Heat Cramps
Heat cramps are characterized by muscle cramps which occur during or after intense exercise.  Cramps typically occur in the exercised muscle and/or the abdomin due to a reduced level of electrolytes.  Cramps are relieved by fluid and electrolyte replacement.

Heat Exhaustion - body temp <104°F
This condition is the result of excessive fluid loss (usually from sweating) which causes a reduction in blood pressure and cardiac output.  Symptoms include headache, dizziness, low blood pressure, a weak and rapid pulse, and general body weakness.  Treatment includes stopping exercise, moving to a cool environment, replacing fluid and electrolytes - intravenous replacement may be preferred.

Heat Stroke - body temperature * 104°F (40°C)
Heat stroke is a life threatening condition which requires immediate medical attention!  Symptoms include dry, hot skin, the cessation of sweating, and a body temperature of 104° F or higher.  Core body temperature (rectal Temp.) is the most accurate for assessment.  Treatment includes whole body ice baths, ice packs and alcohol rubs etc.
 
 

FLUID REPLACEMENT 
 

Position Paper on Exercise and Fluids - American College of Sports Medicine 1996
 

Factors which affect fluid uptake:
ï Gastric Emptying
ï Intestinal Absorption

Drinking fluids before, during and after activity is the best way to maintain optimal hydration and performance and prevent heat injury.  A number of factors affect the rate of fluid absorption including volume consumed, temperature of the fluid, the presence of sugars and sodium and the type of physical activity being performed.
 
 

    Gastric Emptying
If tolerated during activity, larger volumes of fluid consumed actually leave the stomach at a faster rate.  For activity  involving running usually consuming  six to eight ounces of fluid every 15 -20 minutes is recommended.  Cold water also facilitates stomach emptying.  Studies vary in temperature recommendations.  It appears that beverages in the range of 60-70°F are preferred  when large volumes are consumed but the most enjoyable temperature is around 41°F.

The more dehydrated an athlete becomes the slower gastric emptying occurs, thus further delaying rehydration.

Intestinal Absorption
Generally water moves freely from the intestine into the vascular system so plain water is an excellent replacement beverage.  Some research show a slight improvement with small amounts of added glucose and sodium (glucose absorption tends to pull additional sodium which in turn pulls some additional water).  The optimal range of carbohydrates in fluids is between 4 and 8%, with glucose or glucose/sucrose combinations having and advantage over high levels of fructose which (at higher concentrations) may cause fluid retention in the gut.  AS an electrolyte replacer, sodium added to beverages at a rate of 125 mg/8oz fluid is best.
 
 

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 Last Modified: Jul 18, 2000