7/26/2012

Myths About Perspiration, Fluid Replacement Guidelines, Your Personal Hydration Program

We're on track for ta hot summer, and with everybody sweating more, many of you have asked about hydration and fluid replacement strategies.

This posting  reviews common myths about perspiration and provides hydration and fluid replacement guidelines to enhance performance and avoid heat related illnesses.  Information comes from authoritative sources including position stands of The American College of Sports Medicine and peer reviewed publications of the National Strength and Conditioning Association.



MYTHS ABOUT PERSPIRATION

#1 - The More I Sweat the More Calories I Burn.  FALSE.  Perspiration is part of our body's cooling system, it does not necessarily require burning calories or correlate with caloric expenditures.  Example:  stand outside on a very humid 90 degree day, and you will sweat profusely.  Run indoors in a very dry 65 degree environment, and you may hardly break a sweat.

#2 - I Can Sweat The Weight Off.  FALSE.  -Weight loss due to sweating indicates dehydration.  This weight is water that needs to be replaced, it is not the stored body fat that you really want to lose.  In sports like boxing, MMA and wrestling, participants may temporarily sweat off a few pounds to make weight - but will immediately begin rehydration before the exercise event.

YOUR BODY'S HEATING AND COOLING SYSTEM 
Whether from exercise, movement or shivering, muscle activity generates heat, which your blood circulates. Your body cools itself by increasing blood flow close to the skin and through evaporation of sweat. Research has shown sweat rates range from .5 to 2 liters per hour with marked differences between individuals.  The amount of sweat varies based on individual characteristics such as body weight, genetics, heat acclimatization, and conditioning, and environmental conditions such as heat, humidity, clothing and equipment. 

EUHYDRATION, DEHYDRATION, HYPERHYDRATION  
Euhydration means we're at our normal hydration level and weight, and this is the most desirable state.  For most people water is about 60% of body weight when we are euhydrated. Dehydration is easily measured by calculating lost body weight before and after exercise.  Ten to 14 days of training in heat will help you acclimatize and reduce risk of dehydration.

  • Lose 2% of body weight, aerobic exercise performance and cognitive abilities are degraded. 
  • Lose 3-5% of body weight, there is risk of heat related illness.  The rate of sweat production declines and can lead to hyperthermia - overheating of the body and brain.  Life-threatening exertional heatstroke occurs when body temperature exceeds 104 and internal organs begin to shut down.
Whether you're working out, running, walking, or even sitting outside in hot humid weather, every pound you lose is a sign that you've lost about a pint of water. Hyperhydration, drinking an excessive amount of water before an athletic event or exercise (more than euhydration), has not been found to improve athletic performance and is not recommended.

GENERAL FLUID AND ELECTROLYTE GUIDELINES
NSCA provides the following general guidelines for fluid and electrolyte replacement:
Before exercise event:  Drink 16 ounces of water two hours before;  drink 8 ounces sports drink 10-20 minutes before.
During exercise event:  Drink a sports drink that contains 30-60 grams of electrolytes and 120-240 grams of carbohydrates per hour to prevent fuel depletion.  Drink 8 ounces of fluids every 15-20 minutes.
After exercise event:  Drink 2-3 cups of fluid for every pound of body weight lost.
Competitive athletes, marathoners and triathletes will benefit from a more personalized hydration program tailored.

YOUR PERSONAL HYDRATION PROGRAM
 Developing your own individualized program is actually easy. ACSM'smost recent position stand on fluid replacement  recommends individualized programs because our sweat rates vary.

Weigh yourself before and after exercise to determine your rate of fluid loss due to sweating, and rehydrate accordingly.  If you lose a pound in a half hour, replace it with 16 ounces of fluid per half hour of exercise.  This may vary depending on weather, intensity and clothing, but over time you should be able to make adjustments.

Begin to prehydrate a few hours before your exercise event, so that your stomach contents are emptied, fluids are absorbed by your body, and urine flow returns to normal.  Rehydrate during the exercise event to replenish the fluids being lost. 

ELECTROLYTES, SODIUM AND HYPONATREMIA
Electrolyte and sodium depletion and replacement is more difficult to individually quantify and program because it requires blood testing.

  • Electrolyte and sodium depletion can cause muscle cramping.  
  • Profuse sweating over time can literally flush sodium out of your body;  if not replaced, a dangerous condition called hyponeutremia resuts.    
Most people don't need to be overly concerned with hyponatremia, but participants in extended exercises events, including marathons, triathalons and sports in hot humid weather should consider electrolyte replacement or salt tablets. The concern here is the potential for athletes to overhydrate with water.

Interestingly, ACSM has found that sodium replacement does not reduce cramping in triathletes, implying that muscle fatigue and energy replacement may be more important factors.