Body temperature is raised by environmental conditions and exercising muscle. Cooling is accomplished primarily by the evaporation of sweat. The most important barrier against effective cooling is humidity. Humidity is not your friend The rate of sweating is higher in humid conditions but the cooling is less. The reason is that because the air is already very saturated with water, sweat can't evaporate. Sweat that beads up and rolls off doesn't function in the cooling process. However, this "futile sweat" does deplete the body of vital water and salt.
As dehydration progresses cooling becomes more difficult. Performance drops and heat injury becomes a real threat.
Deaths have occurred when the air temperature was less than 75 degrees F (24 degrees C) but the relative humidity was above 95%.
There are three stages to heat illness; heat cramps, heat exhaustion, and heat stroke -- listed in order of increasing severity. Often the border between them is blurred into a continuous spectrum.
Heat cramps are due to muscle spasms and often occur in the arms, legs, or abdomen. They are thought to be caused by dehydration and loss of salt and other electrolytes.
Heat exhaustion is due to more profound loss of water and electrolytes. It is characterized by generalized weakness, headache, dizziness, low blood pressure, elevated pulse, and temperature elevation as high as 104 degrees F (40 degrees C). Both can usually be treated by moving out of the sun, drinking fluids, and eating salty food.
Heatstroke is a life threatening condition and represents severe dehydration, high body temperature, and a shut-down of the cooling mechanisms. The patient may be delirious or comatose, and half of the victims have stopped sweating. The pulse is rapid and weak, the blood pressure is low and body temperature is greater than 105°̊ree;F (40.6°̊ree;C) and may reach as high as 110°̊ree;F (43°̊ree;C). The oral temperature is notoriously inaccurate in these circumstances. Damage to the brain, heart, lungs, kidneys and other organs may occur.

Sometimes despite the best medical care, death is the end result. As with most diseases, preventing one is always better than getting one The environmental conditions that lead to dehydration and heat illness are out of your control, but there are many things that you can do to help prevent getting sick Clothing Your choice of clothing can influence your cooling efficiency. Light colored clothing reflects light and so is cooler than dark colored clothing. The traditional black cycling shorts are not good for exercise in hot climates -- white is a better choice. Loose, lightweight material allows for better air circulation and facilitates evaporation of sweat. Clothing that is dry slows down evaporation of sweat, but once wet, cooling continues. Thus, changing into dry clothes during transitions is not a good idea.
Some medications interfere with cooling Certain drugs may cause dehydration or interfere with sweating. Antihistamines and some blood pressure medications decrease sweating. Caffeine and alcohol are diuretics and thus cause your body to lose water. For those under a physician's care it is best to check with your doctor about medication . How do you know if you are drinking enough? A good sign of hydration is the output of large volumes of clear, dilute urine. Drinking approximately 400 - 600 ml (13 - 20 ounces) of cold water or an electrolyte solution can help delay the process of dehydration. many people underestimate the magnitude of their fluid loss.
The maximum rate of fluid absorption by the gastrointestinal (GI) tract during exercise is approximately 800 ml per hour (27 fluid ounces/hr). The rate of fluid loss through sweating may average as high as 1.5 - 2 liters per hour (50 - 68 fluid ounces per hour). Thus often, despite the best fluid intake, dehydration will occur. Drinking 150 - 250 ml (5 - 8 ounces) every 10 - 15 minutes is probably the best way to attempt to stay hydrated while racing. For some people, drinking a lot causes discomfort and a feeling of being "bloated". Thus guzzling a liter once per hour will likely cause problems. Also realize that the more dehydrated you get the harder it is for your GI system to absorb what you drink. Dehydration also causes a variety of GI symptoms (nausea, cramping, and diarrhea). You must determine and plan you hydration strategy ahead of time.




Why is Salt important Sweat contains between 2.25 to 3.4 grams of sodium chloride per liter. A sweat rate of 1 liter per hour would thus cause a salt loss of 27- 40 grams Failing to replace salt can result in hyponatremia (low salt concentration in the blood). From recommended to ingest an average of 1 gram of sodium per hour Overheating causes more sweat production. The net fluid lost per hour is greater.
When faced with unusual circumstances be conservative and cautious. Know your body There is large variability between individuals with respect to net water loss while exercising in the heat. This depends upon sweat rate, rate of fluid ingestion, rate of gastric emptying, type of fluid ingested, percentage body fat, and many other variables. Because of this there is no simple answer for which fluid to drink, how much, and how often. So how do you know what is right for you?
You should become familiar with what you need to do to stay hydrated under a variety of conditions. Keep a log about your experiences. Change only one variable at a time to develop a plan that works. Remember heat can kill.

About the Author
Head coach/founder Chikwang Dojo. Founder of (P.D.C.0 Prograssive Defense Concepts. Pro-Fighter and trainer...