Saturday, May 18, 2013

Fluid Needs in Endurance Athletes

Fluid Needs in Endurance Athletes


It is good to keep drinking a lot of pure water during a long distance race, such as a marathon. T/F

An athlete does not need to drink fluids during intense endurance exercise as long as they drink enough in the 24 hours before exercise.  T/F

Sodium helps retain fluid during vigorous endurance exercise.  T/F

It is ok if I train in a semi-dehydrated state so that my body gets used to this and I don’t have to worry about slowing my pace to drink during races.  T/F

I don’t need to drink during endurance exercise in cold weather.  T/F

What is Important for Optimal Performance?

Water is the most important performance-enhancing nutrient when exercise and heat stress are combined

The question is, how much is really needed before and during exercise

AND how can the addition of electrolytes in water or sports drinks improve performance?

Intro to the Importance of Fluid Before and During Exercise

Excessive sweating=more serious fluid loss and a reduction in plasma volume

This leads to circulation failure within the body and core temperature increases to dangerous levels.

Intro to the Importance of Fluid Before and During Exercise

During near-maximal exercise in the heat, along with dehydration, relatively less blood diverts to peripheral areas (skin, muscle) for heat dissipation. 

Reduced peripheral blood flow reflects the body’s attempt to maintain cardiac output despite a decreased plasma volume caused by sweating.  (McArdle, Katch, & Katch, 2009)

Core Temperature During Exercise

Any loss of body weight > 3% seriously disturbs body temperature regulation and performance (Wein, 2011).

Hyperthermia –increased core temp. by 5°C or higher. 

Normal core temperature during exercise can reach up to 40°C (104°F).

It is dangerous when core temp. reaches 41°C, or 106° F (Vella & Kravitz, 2004).

Core Temperature During Exercise

Heat generated by the active muscles raises core temperature

Signs of heat related illness can result if one is exercising in the heat, which increase core temp. even further

Rises in core temperature even happen while swimming or running outside in cold weather! (McArdle, Katch, & Katch, 2009)

On a hot day, temperature receptors located in the skin send signals to the hypothalamus to cool the body by increasing the sweat rate (Vella & Kravitz, 2004).

Water Loss  in the Heat:  Dehydration

Dehydration=imbalance in fluid dynamics when fluid intake does not replenish water loss (McArdle, Katch, & Katch, 2009).

Any amount of dehydration impairs exercise performance.

A 2% loss of body weight results in decreased performance and signs of disorientation/confusion (Wein, 2011).

A loss of >3%  seriously disturbs temperature regulation!

The risk for dehydration increases even during intense exercise in the cold, so don’t let the cold weather fool you!

Water Loss  in the Heat:  Dehydration

Health risks associated with dehydration include:

  1)increased core body temperature

  2)increased cardiovascular strain

  3) increased glycogen use

   4) altered metabolic function and possibly altered CNS (Central Nervous System) 

Water Loss  in the Heat:  Dehydration

The risk of heat illness greatly increases when a person begins exercising in a dehydrated state (McArdle, Katch, & Katch, 2009).

Dehydration can slow the stomach emptying rate and cause stomach cramps and feelings of nausea.

Magnitude of Fluid Loss

During high-intensity exercise in the heat, a person can lose up to 3L of water/hour (McArdle, Katch, & Katch, 2009).

However, sweat rate can vary depending on:

  1)the environmental temperature


  3) type of clothing worn

  4) intensity

  5) fitness level

  6)acclimation to the environment (Vella  & Kravitz, 2004).

Body fluid loss results in these 5 negative factors on performance (McArdle, Katch, & Katch, 2009): 

Decreased plasma volume

Decreased blood flow to the skin

Decreased stroke volume (the amount of blood pumped out each heart beat)

Increased heart rate

General deterioration in the efficiency of circulation and thermoregulation during exercise

Water Replacement:  Rehydration

Properly scheduling fluid replacement maintains plasma volume, so circulation and sweating progress optimally (McArdle, Katch, & Katch, 2009)

A well-hydrated athlete always functions at a higher physiologic and performance level than a dehydrated athlete.

Achieving hyperhydration BEFORE exercising in a hot environment protects against heat stress because it:

  1) Delays dehydration

  2) Increases sweating during exercise to help cool the body

  3) Diminishes rise in core temperature

Pre-exercise hydration

It is recommended to consume plenty of fluids in the 24 hours before exercise (Vella & Kravitz, 2004).

In addition, the athlete should drink 16 oz of fluid about 20 minutes before the start of exercise (Vella & Kravitz, 2004).

The athlete still needs to consume fluids during exercise, especially in the heat (McArdle, Katch, & Katch, 2009).

During Exercise

Fluid requirements vary remarkably between athletes and between exercise situations. 

Fluid loss is effected by (AIS Sports Nutrition):

  1) Genetics-some people sweat more (Myself-this note was not in my presentation, lol, but because I sweat a lot, I noticed I have to drink more to stay hydrated than others it seems.)

  2) Body size-larger athletes vs. smaller athletes

      3) Fitness-more fit people sweat earlier in exercise and  in larger volumes

  4) Environment-sweat losses are higher in hot, humid conditions

  5) Exercise Intensity-sweat losses increase as exercise intensity increases

During Exercise

 Better to start drinking early and often during exercise (AIS Sports Nutrition).

Every 15-20 minutes, the athlete should consume between 200-300 mL (about .85-1.25 cups) (AIS Sports Nutrition).

Adequacy of Rehydration

Signs of dehydration include (Vella & Kravitz, 2004) :

  - dark urine with strong odor

  - muscle cramps

  - decreased sweat rate

  - fatigue (Vella & Kravitz, 2004)

The athlete drank enough fluids during and before exercise if they excrete a large volume of light colored, odorless urine (McArdle, Katch, & Katch, 2009)

Sodium Facilitates Rehydration

A moderate amount of sodium added to a rehydration beverage provides more complete rehydration.

Maintaining a relatively high plasma concentration of sodium helps (McArdle, Katch, & Katch, 2009):

  - Sustain thirst drive

  - Promote retention of fluids

  - More quickly restore lost plasma volume during  rehydration

Sodium Facilitates Rehydration

Restoring water and electrolyte balance in recovery can help by adding moderate to high amounts of sodium, such as 2300mg, to fluids or food (McArdle, Katch, & Katch, 2009).

Add 1/3 tsp. of table salt to 1L of water, especially if exercising for a prolonged period of time in the heat.


What About Potassium?

A tiny amount of potassium (36-90 mg) enhances water retention.

This amount may prevent any extra potassium loss that results from sodium retention by kidneys.

However, even at intense exercise levels, potassium lost in sweat is small and poses no immediate danger.

Can replace potassium lost through sweat by increasing the amount of potassium-rich foods (McArdle, Katch, & Katch, 2009).

Sodium Facilitates Rehydration

Fluid intake is increased when beverages are cool, flavored, and contain salt (AIS Sports Nutrition).

This makes sports drinks the perfect beverage to drink during exercise!  They have the right amount of sodium and potassium needed (AIS Sports Nutrition).

Pure water absorbed from the gut rapidly dilutes plasma sodium concentration, so it’s important to drink fluids with sodium in them to prevent a condition called Hyponatremia (McArdle, Katch, & Katch, 2009).

Hyponatremia:  Reduced Sodium Concentration in Body Fluids

Hyponatremia , or low blood sodium levels, causes symptoms similar to dehydration and is potentially life threatening.

The sodium concentration balance is disturbed which can cause swelling of the brain.

Can often occur in prolonged endurance events of 2 hours or more when large volumes of low sodium drinks (like water) are consumed and sweat losses are small (from dehydration).

Taking in sodium-containing fluids helps match fluid intake to sweat loss since sodium helps retain fluid and lowers the risk of hyponatremia (AIS Sports Nutrition).

To Reduce the Risk of Hyponatremia, an Individual Can Follow These 5 Steps:

1) 2-3 hours before exercise drink 14-22 oz. of fluid

2) Drink 5-10 oz. of fluid about 30 min. before exercise

3) Drink no more than 32 oz. of plain water spread over   15-minute intervals during or after exercise

4) Add a small amt. of sodium (about ¼-1/2 tsp. of salt  per 32 oz. to fluid.  Commercial sports drinks are also effective in providing water, carbohydrate fuel, and electrolytes.

5) Do not restrict dietary salt. 


         AIS Sports Nutrition.  (2009).  Fluid-who needs it?

  Australian Sports Commission.  Retrieved from

McArdle, W. D., Katch, F. I., &Katch, V. L.  (2009).  Sports and exercise nutrition.  (3rd ed.).  Baltimore, MD:  Lippincott Williams & Wilkins.

Vella, C., & Kravitz, L.  (2004).  Staying cool when your body is hot. 

  Retrieved from

Wein, Debra.  (2011).  Nutrition for ultra endurance events:  fluid and

  electrolytes guidelines.  Retrieved from