As the number of heat-related injuries has been increasing and global warming may increase the frequency and intensity of heat waves, one must be extremely cautious to exercise in a hot environment.  A hot and humid environment puts stress on the body’s temperature regulation and fluid homeostasis. High heat and humidity reduce the body's ability to lose heat by radiation, convention and/or evaporation.  Temperature regulation is not only influenced by the environment (heat/cold) but also by dehydration.  Dehydration occurs when the amount of water leaving the body is greater than the amount being taken in.  When sweating becomes the primary means of heat dissipation, sweat loss must be matched by fluid consumption to avoid dehydration. This is difficult, because the stimulus to drink is not initiated until an individual has incurred a water deficit of approximately 2% of body mass. 

Heat cramp, heat exhaustion and heat stroke are only a few of the many heat related illnesses. Heat cramps, first described by Talbot, is muscle cramping in persons performing physical work in hot and humid environments.This type of heat cramp should not be confused by exercise associated muscle cramps.  Heat exhaustion is often described as a condition in which an athlete collapses during or after exercise in the heat. This condition is generally believed as dehydration-induced heat retention.One condition that is linked to elevated core temperature is heat stroke.  Heat stroke is a life threatening illness which is clinically defined as a body temperature greater than 40.6C.

 

Exercise related heat illnesses can be prevented by following several guidelines. The 2003 IOC Consensus Conference concluded the followings with regards to hydration: 1) Sufficient fluid should be consumed during exercise to limit dehydration to less than about 2% of body mass. 2) Sodium should be replenished when sweat losses are high, especially if exercise lasts more than 2 hours.  3) Athletes should not drink so much that they gain weight over exercise.  4) Rehydration should include replacement of water and salts lost in sweat during recovery from exercise.

 

Whole-body precooling, clothing, heat acclimatization and rehydration are all different methods that one can use to avoid hydration in the heat. The goal of whole-body precooling is to reduce the heat content of the body by cooling the periphery before exercise.  This will increase the margin for metabolic heat production and increase the time to reach the critical limiting temperature when the intensity for a given exercise can no longer be sustained. The cooling effect can be achieved by different techniques such as cold air cooling, cold water immersion and the use of water-cooled garment.Exposing a maximal surface area of skin for evaporation is also important in the hydration process. The type and amount of clothing worn can have a major impact on heat dissipation during exercise.  When removal of clothing during exercise is not possible, one should incorporate frequent rest and cool down breaks along with the use of intermittent clothing. Regular exercise in a hot and humid environment results in a series of physiological adjustments.  These adjustments, called acclimatization, are designed to reduce the negative effects associated with exercise in the heat.  Examples of primary adaptations of athletes who exercise in hot environment are;  increased plasma volume, earlier onset of sweating, higher sweat rate, reduced salt loss in sweat, a reduced skin blood flow, and increased synthesis of heat shock proteins.

 

Heat acclimatization increases the requirement for fluid replacement, because of during exercise a heat acclimatized athlete starts to sweat earlier. Replacement of the fluid or rehydration is also necessary because of the loss of carbohydrates and electrolytes. Plain water ingestion after exercise results in a fall in plasma osmolality and sodium concentration, which in turn stimulates urine production and reduces the urge to drink. In contrast, the addition of sodium chloride to plain water increases intake while reducing urine output. Therefore, beverage formulations for the replacement of fluids and minerals lost through sweat are shown to replenish the cells.

 

Based on the Vital Role of Sodium in Exercise interviews conducted by the Gatorade Sports Science Institute, it is essential to replenish any sodium lost during exercise. Sodium loss can be determined by genes, diet, heat acclimatization and exercise training. For example, white film on the shirt or around the eyes can be a sign that an athlete's sweat has a great amount of sodium. If sodium loss is not replenished it may lead to early fatigue, heat cramps and sodium deficit. Adequate salt intake helps athletes distribute fluid more properly in the body. Therefore, not over drinking water before, during and after exercise is very important to achieve the right sodium balance. Hydrating, nourishing, adding salty snacks, eating the right amount of calories, rehydration and keeping track of the amount of fluid lost during exercise are key factors to lower heat related stress and achieve better performance.

 

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1. Murray B., Bergergon M., Kundrat S., Kenney L., Stachenfeld N. Gatorade Sports Science Institute: The Vital Role of Sodium in Exrecise- Part III Sodium Challenges

2. American College of Sports Medicine. New Release. Heat and hydration: important concerns for outdoor activities. 2008

3. Wendt D., Loon L., and Lichtenbelt W.: Thermoregulation during Exercise in the Heat: Strategies for Maintaining health and Performance. Sports Med 2007: 37 (8) 669-682

4. Murray B., Bergergon M., Kundrat S., Kenney L., Stachenfeld N. Gatorade Sports Science Institute: The Vital Role of Sodium in Exercise- Part II Sodium and exercise

5. Shirreffs S., Casa D., and Carter R. Fluid need for training and competition in athletics. Journal of Sports Sciences, 2007; 25: S83-S91

6. Shirreffs S., Aragon-Vargas L., Mhairi Keil, Love T., and Phillips S. Rehydration after exercise in the heat: A comparison of 4 commonly used drinks. International Journal of Sport Nutrition and Exercise Metabolism, 2007, 17, 224-258