Dehydration Symptoms

 Dehydration is defined as sweat, urine or respiratory water loss. It results from the voluntary restriction of intake of water. Thirst is not a good indicator for when to replace water. This has been seen in marathon runners who are accustomed to running long distances and have conditioned themselves thusly so. When endorphins take over, the thirst mechanism is disregarded. So being thirsty is not a good indicator for how dehydrated one might be. Dehydration also decreases work performance.

It is assumed that the total body water (TBW) in the idealized 70-kg person is 42 L (about 60% of body mass), and that two thirds of this water (28 L) is intracellular and one third is extracellular (3.5 L of plasma and 10.5 L of interstitial fluid). By calculation, the intravascular or PV is equivalent to one twelfth of the total body water (42 L TBW/12 = 3.5 L PV). It is further assumed that there is no salt loss. Therefore TBW at the thirst threshold, compared with euhydration, is calculated by:

Normal TBW x normal plasma = thirst TBW x thirst plasma osm

42 L x 280 mOsm/kg = ? L x 295 mOsm/kg

11,760 mOsm/295 mOsm/kg = 39.9 L

That is, TBW at the thirst threshold is 39.9 L.

Therefore the actual water loss to reach threshold is 42 L - 39.9 L = 2.1 L, which represents nearly 3% of body weight ([2.1 kg/70 kg] x 100). By exercising in the heat for only about 2 hours, one can incur an additional water loss of 2.1 L (= 4.2 L total), which then results in a water deficit equivalent to 6% of body mass. This 6% TBW deficit decrements performance and increases risk of heat illness." (Page 260, 261 Wilderness Medicine, Paul Auerbach, M.D.)

You can lose 2% of your body weight before thirst is initiated, which may get worse as the severity of dehydration increases. With 4%-6% body water loss, you are anorexic, impatient and you have symptoms of headache. With 10% loss, you have dizziness and cyanosis. You also become light-headed and have syncopal episodes. With 12% loss, you have difficulty swallowing and you require assistance in re-hydration. 15%-25% water loss is lethal.

Dehydration produces a decrease in ability to stay alert. Your power can decrease 6% and time performance by 12% in the heat. Sweat rates can be as high as 1-1/2 liters per hour, or roughly 15 liters a day. Water requirements are not only imperative in the heat, but they are imperative in the cold. They are also higher than most people believe. At high altitudes, water requirements can be very similar to those in the desert. Some very rough estimates for water replacements in the cold: Approximately 2 quarts per day, severe cold with heavy exercise, up to 2 gallons. Altitudes above 10,000 feet, definitely more than 2 gallons. Dr. Murray Hamlet postulates that one should drink a little all the time, a lot at meals and at bedtime. You want to be able to urinate pale yellow. If you can't remember the last time you urinated, you are significantly dehydrated. Drink by your watch in hot environments. Forced drinking in the absence of thirst saves lives in the heat.

There can also be a cumulative effect of dehydration and heat exhaustion which can occur over days, and is not recognized, especially in athletics. This is commonly seen during the month of August when football players train. The cumulative effect is usually not taken into consideration.