Dehydration , Heat Injury & Hyponatremia

Thermal regulation- 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%.

Recognizing symptoms of heat injury 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 athlete 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.

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.


Is a low concentration of sodium in the blood and occurs when athletes sweat profusely and then drink copious amounts of pure water to replenish fluids, without adequately replacing electrolytes. It occurs more frequently during endurance events that last longer than several hours, where athletes attempting to rehydrate over several hours of exertion may end up compounding the problem without realizing it. The simplest way to prevent getting hyponatremia is to include diluted electrolyte-containing fluids such as Gatorade. Sweat contains roughly 3 grams of salt per liter, and the rate of perspiration during a long, hot climb or race can average .5-1 liter per hour. If you are climbing all weekend to the tune of 10-15 hour days, you can easily lose too much salt. Try to replenish sodium at a rate of about 1 gram per hour.

For hikes lasting less than 1 hour, water alone is adequate. For longer hikes, there are many commercially available sports drinks. The most important features are taste, carbohydrate and sodium content.
Taste is important, because if you don't like it you won't drink it. Carbohydrate content in the range of 4 - 8% is best for endurance. Levels above 10% are poorly absorbed and can cause diarrhea. Most sports drinks have a sodium content in the range of 10 - 20 mmol/liter (Gatorade is 23 mmol/liter = 1.3 grams of salt per liter). Higher levels are better for salt replacement, but tend to be less palatable.

If you have ever wondered why nuts, soups, trail mix, Doritos, pizza, Mexican food (chips and salsa) or other salty foods taste so delicious during or following long endurance outings, it is because your body needs a certain level of sodium to replace that lost through sweating, and it will tell you in no uncertain terms exactly what it needs. If you or a training or climbing team member experience nausea, muscle cramps, slurred speech, confusion, disorientation, or inappropriate behavior, and yet your urine is clear from your abundant hydration efforts, you may need to get some salt rather than still more water. Severe hyponatremia is a true medical emergency and can result in seizure, coma or even death.