With the onset of warm weather comes the increased risk of exercise-related dehydration.  Sweating, a protective mechanism of your body, regulates your body temperature.  As your sweat evaporates, heat is dissipated.  This prevents your core body temperature from rising to dangerously high levels.  The rate at which your body sweats is dependent upon the intensity and duration of the exercise, environmental conditions, acclimatization, type and amount of clothing worn, individual biological differences, and your hydration status before exercise.

     Dehydration can result if fluid consumed does not match fluid lost from exercise.  When you are dehydrated your risk for heat-related illnesses increases and your exercise performance becomes impaired.  Physiologic function of the body starts to be compromised when 1%-2% of body weight is lost as a result of fluid loss.  Your risk for heat-related illnesses increases when 3% or more of your body weight is lost as a result of fluid loss.  If you are well-hydrated before exercise, it could take only one hour of exercise for this to occur, sooner if you are dehydrated from the start.

     An understanding of the signs and symptoms of dehydration is important in order for you to take appropriate action when they occur.

Signs and Symptoms of Dehydration

• Thirst
• Headache
• Nausea
• Vomiting
• Dizziness
• Cramps
• Weakness
• Chills

     You can reduce or prevent the likelihood of becoming dehydrated by following certain guidelines.  These include adhering to a fluid replacement schedule and incorporating steps to optimize your fluid replacement.

Fluid Replacement Schedule

Pre-Exercise

• Drink 17-20 fluid ounces of water or sports drink 2-3 hours before beginning exercise
• Ingest 7-10 fluid ounces of water or sports drink 10-20 minutes before exercise

During Exercise

• Ingest 7-10 fluid ounces every 10-20 minutes

Post-Exercise

• Drink approximately 16-20 fluid ounces of water or sports drink for every pound of weight lost from the activity within 2 hours after completing your exercise session

     If you find it difficult to adequately rehydrate, there are some tactics you can follow to optimize fluid replacement.

Steps to Optimize Fluid Replacement

• Carry water bottle with you while you exercise
• Choose a water bottle with measurement markings to keep track of the amount of liquid consumed
• Choose drinks that are palatable, such as flavored water or sports drink

Characteristics of Fluid to be Consumed

• Palatable
• Cool.  The ideal temperature range of the drink is 50-59 degrees Fahrenheit (10-15 degrees Celsius)
• Post-exercise drink should include a carbohydrate source, especially if your session lasts more that 45 minutes, to replenish glycogen (energy) stores 
  
• Contains electrolytes, such as sodium chloride (0.3 - 0.7 g/L), to offset loss from sweat and to reduce your risk for conditions associated with electrolyte imbalance (muscle cramps) - this is particularly important if exercise lasts longer than 4 hours and during the initial days of warm weather when your body hasn't yet been acclimated.

Drinks to Avoid

• Caffeinated and alcoholic beverages which can increase fluid loss through increased urine output
• Carbonated beverages which may lead to a "full" sensation causing you to decrease your fluid intake
• Drinks with carbohydrate concentrations that are greater than 8% are not recommended to drink during exercise because they interfere with stomach emptying and intestinal absorption
  

Resources

Journal of Athletic Training, 2000:35(2):212-224; "National Athletic Trainers' Association Position Statement: Fluid Replacement for Athletes," Casa D.J. et al.

Track Shack - Sports Nutrition Developed by the Gatorade Sports Science Institute www.trackshack.com