We lose fluid in two ways, sensibly (meaning we are aware of it) and insensibly (we aren’t). Normal sensible losses include elimination (peeing), and sweating. In dry climates, our sweat might evaporate so quickly that we aren’t aware of how much we are sweating. Insensibly, we lose fluid through breathing, and also through our skin (in addition to sweat glands). Normal respiration (when at rest) can lose about two liters of water a day by evaporation from the lungs, and skin “respires” too, losing about ½ liter per day.
On a normal day, we lose about four liters of fluid which we usually replace with consuming our drinks of choice and with meals. This amount can be dramatically increased by illness (vomiting or diarrhea).
The most common cause of increased fluid loss is through exertion. When we are active, we increase the amount of fluid we lose, and depending upon the humidity and the altitude, we can lose even more. The lower the humidity, and the higher the altitude, the more fluid is lost through respiration. During heavy exertion, we can lose between one to three liters of fluid per hour which can quickly put us in a deficit that is hard to make up if we aren’t drinking constantly.
The symptoms of mild dehydration include headache, dry lips and mouth, decreased urine output, fatigue, and even dizziness. Thirst may or may not be present. The effects of these symptoms are decreased coordination, and impairment of judgment. Dehydration can worsen symptoms of other conditions including diabetes, and increase the likelihood of other heat-related problems such as heat exhaustion and heat stroke.
When engaging in outdoor activity, especially in a very cold or hot environment, and more so at altitude, don’t wait until you are thirsty to drink. Try to drink a liter an hour, and to eat frequent snacks to replace needed electrolytes.
Learn more about dehydration: http://www.youtube.com/watch?v=q3bjUu_ONjc#t=214
