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Stay Cool: Hot Weather Adaptation

 

In extremely hot climates or due to an internal infection, core body temperatures can rise to dangerous levels. Hyperthermia, a life-threatening disorder, typically starts in humans when body temperatures rise to 105-107° F. (40.6-41.7° C.). Athletes lose heat to the surrounding environment by 4 pathways [1, 2]:
  1. Radiation of Heat Waves - 60%
  2. Evaporation of Sweat - 22%
  3. Convection of Air Currents - 15%
  4. Conduction to Objects - 3%
There are LIMITS to how much heat the body can withstand during exercise. But there are ways by which adaptation to heat stress may be improved.

Simple radiation is responsible for most of the loss during normal temperature range, but in hot dry climates evaporative cooling and sweating is more significant. Adapting to hot environments is complex. The effect of heat on human bodies varies with the relative humidity of the air. HIGH TEMPERATURES + HIGH HUMIDITY makes it very difficult to lose excess body heat. This is due to the fact that when the moisture content of air goes up, it becomes increasingly more difficult for sweat to evaporate. The sweat stays on our skin and we feel clammy. As a result, we do not get the cooling effect of rapid evaporation.

In DRY HOT weather, humidity is low and sweat evaporates readily. The higher the desert temperatures, the more significant of a cooling effect we get from evaporation. This relationship between relative humidity and air temperature is quantified at http://anthro.palomar.edu/adapt/adapt_2.htm. When the apparent temperature is above 60°F.-60% humidity, dehydration, hyponatremia, heat exhaustion and cramps are likely for humans. Much above 60°-60%, life threatening heat stroke may result [1].

Fluids (water) are absorbed in the stomach cavity first, then a few minutes later in the small intestines. Carbohydrates rely on sodium molecules to cross gut linings, so some sodium is absorbed with carbohydrates. However, sodium and chloride and potassium are absorbed at a high rate by the colon. Fluid loss and body core temperature elevation drive thirst to increase fluids and this is good to a point. But loading too much water volume without electrolytes may create a low serum sodium or hyponatremia, an event-ending medical emergency.

The ability to physiologically acclimatize to hot conditions requires 14-21 days training in the heat. “The salt concentration of sweat progressively decreases while the volume of sweat increases. Urine volume also reduces. loss through radiation. Vasodilation also causes flushing, or reddening, of the skin since more blood is close to the surface.” [2]

Tips for adaptation to the heat:

  1. Distance Training @ Aerobic Pace 14- 21 consecutive days
  2. Train body to refuel, rehydrate, and process electrolytes during #1
  3. Train at a reduced pace to compensate for overheating 
  4. Dilute rehydration solutions slightly increase electrolytes
  5. Increase fluid volume cautiously; resist drinking above 1 liter per hour [3] 
  6. Keep head, trunk, & quads WET to increase evaporative heat loss

The human body will adapt within 21 days aerobic heat stress exposure. (DO NOT wear extra clothing or plastic sweat suits to raise body core temperatures in preparation for a hyperthermic event). If body core temperatures exceed evaporative cooling rate, fluid loss and electrolyte loss will present as dehydration, tachycardia, excessive cramping, and the athlete will be forced to reduce pace or stop exercising. When body core temperatures exceed the evaporative cooling rate, drinking cold fluids and emersion in cold water are the fastest remedies to reduce body core temperatures. Keep in mind, maintaining a slower pace (lower gears, easy cadence) in the heat may mean a better finish place over the course of a hyperthermic endurance event. It is best to be able to complete adaptive training in the heat with race event light colored clothing, exposing as much skin as possible. DO NOT wear skull caps in the heat, unless a crew supplies an ice packs often over the top of the head.

References

[1] Source: U.C. Berkeley Wellness Letter, August 1996
http://anthro.palomar.edu/adapt/adapt_2.htm

[2] Adapting to Climate Extremes
http://anthro.palomar.edu/adapt/adapt_2.htm

[3] When drinking above 34 fluid ounces per hour, the risk of dilutional hyponatremia increases (medical emergency)

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