What is dehydration?

Causes and Symptoms

The average adult’s total body weight is approximately 60 percent water. Daily water requirements vary based on age, gender, level of physical activity, and climate. Dehydration, loss of 3 to 5 percent or more of body weight, is often accompanied by the loss of essential electrolytes such as sodium, potassium, and chloride. Conditions that deplete body water faster than it is absorbed include fever-induced sweating, diarrhea, vomiting, acidosis, anorexia nervosa, bulimia, diabetes mellitus and insipidus, poor nutrition, obesity, and lack of acclimatization to heat stress. People exercising in hot, humid environments provide an excellent example of how dehydration develops and progresses. Symptoms of dehydration may include dry mouth, lips, and skin; decreased salivation; dizziness; dark-colored urine; weakness; constipation; and confusion.

Heat gain is higher and evaporative heat loss is lower during physical exertion for children than adults, predisposing children to more rapid and severe dehydration. Both child and adult bodies attempt to reduce the buildup of metabolic heat through blood flow adjustments and sweat gland secretion. Flushed, red skin indicates that peripheral blood vessels have dilated, carrying blood and internal heat to the body surface for cooling. Once the heat is carried to the periphery by the bloodstream, dissipation occurs mainly by sweat evaporation. Large quantities of sweat may roll off the skin in a high humidity environment, but cooling only occurs when the sweat evaporates. Children exhibit a higher number of sweat glands per unit of body surface area than do adults, with each immature sweat gland producing about 40 percent as much sweat as an adult sweat gland. Children also gain heat from the environment faster than do adults because of their larger body surface area to body weight ratio; they dehydrate quicker as a result of lower overall fluid storage capacity. A large portion of the fluid released as sweat comes from the circulating blood plasma, making fluid consumption to rebuild blood plasma volume and to replenish lost water weight very important. Children acclimatize to a heat stress environment such as a sauna more slowly than do adults. They generally need at least six exposures before adjusting, whereas adults need only about three acclimation bouts.

The effects of dehydration are of particular concern in infants and young children, since their electrolyte balance can become precarious.

Treatment and Therapy

Rapid restoration of fluid volume and electrolyte balance are primary treatment goals that may require intravenous infusion if sufficient fluid cannot be ingested orally.

“Prehydrating” the body by consuming liberal amounts of fluid before anticipated heat stress and “trickle hydrating” while losing body fluid are critical. Cool fluids of about 40 degrees Fahrenheit (about 5 degrees Celsius) empty from the gastrointestinal tract and supply the dehydrated cells more quickly than warmer or colder temperature fluid. Studies of fluid absorption indicate that excessive sugar in electrolyte drinks slows water movement into the bloodstream. Children have been shown voluntarily to drink nearly twice as much when flavored fluids, as compared to plain water, are allowed.

Monitoring body weight before and after dehydration episodes and drinking enough water to regain lost weight is important. Nearly all body weight lost during exercise is attributable to water loss, not fat loss. Consuming 1 pint (473 milliliters) of fluid will replenish 1 pound (9.45 kilograms) of water weight loss. People should drink back all lost water weight even though they may not feel thirsty, as the human thirst mechanism is not a good indicator of actual need. Checking the urine is also recommended, as dark yellow urine indicates that more water consumption is needed and clear, nearly colorless urine indicates that adequate rehydration has been achieved. Wearing light-colored, loose-fitting clothing in the heat is recommended, as rubberized or tight-weave clothing interferes with sweat evaporation and body cooling.

Other suggestions for countering dehydration include getting into good physical condition and acclimatizing to the heat. Conditioning increases the body’s metabolic efficiency, so that fewer of the calories burned accumulate as heat; enhances blood plasma volume to enable a larger sweat reserve; and reduces fat weight that insulates the body and retards heat dissipation. Eating a carbohydrate-enriched diet will retain water in muscle cells at a rate of nearly 3 grams of water per 1 gram of stored glycogen, whereas stored fat retains minimal water.

Perspective and Prospects

Many episodes of dehydration can be prevented from developing into heat cramps, heat exhaustion, and heatstroke during sporting events by adhering to the aforementioned guidelines. Heat cramps, especially muscle spasms in the calves and stomach, may occur during intense sweating, with the accompanying loss of electrolytes. Mineral loss, however, is always of secondary importance to fluid loss because water provides the medium in which all cellular processes occur.

Heat exhaustion occurs when increased sweating and peripheral blood flow reduce venous return of blood to the heart, resulting in cool and clammy skin, lower-than-normal blood pressure, and a rapid but weak heart rate. Less blood is pumped to the brain, causing weakness, faintness, dizziness, headaches, and a grayish look to the face. Treatment includes lying down in a shaded, breezy place, drinking cool fluids, removing excess clothing, and replenishing electrolytes. Heatstroke occurs when the brain can no longer maintain thermal balance, as evidenced by the cessation of sweating, hot (sometimes white to gray) skin, rapid and full pulse, and a rise in body temperature over 104 degrees Fahrenheit leading to disorientation and unconsciousness. Heatstroke is rare but requires immediate medical attention to reduce body temperature. The body temperature should be lowered quickly by placing cool cloths or ice packs to the groin, neck, and under the arms. Cool sheets may be placed over and under the patient. The patient should not be allowed to shiver, which increases the body temperature. Caretakers should be alert for seizures and the possible need to perform cardiopulmonary resuscitation (CPR). The most effective treatment is prevention through proper hydration.

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