A Balance of Fluid Loss and Intake

Physical activity creates heat, and this heat must be dissipated for the athlete to continue performing the activity. Failure to dissipate heat will eventually lead to heatstroke and, potentially, death. One of the main mechanisms for dissipating heat is sweat production; sweat cools the body down when it evaporates off the skin. The inability to produce sufficient sweat will cause the body to overheat. Since athletes have a finite storage capacity for water, and a tremendous ability to produce sweat, fluids must be consumed during physical activity to maintain the sweat rate.

Athletes working intensely in the heat can lose 2.5 liters of sweat per hour. Sweat contains electrolytes (mainly sodium chloride but also potassium, calcium, and magnesium), with a sodium concentration that ranges from 20 to 80 millimoles per liter, depending on common sodium consumption in the diet, the sweat rate, acclimatization to the heat (better acclimatization results in lower sodium loss), and content and amount of rehydration beverages.⁴

Because the electrolyte concentration of sweat is different from that of plasma and intracellular water, there are concerns that an electrolyte imbalance will develop with intense physical activity. Of greatest concern is the potential sodium imbalance that could occur. The loss of a single liter of sweat containing 50 millimoles per liter of sodium translates into a loss of nearly 3 grams of sodium chloride. Athletes who lose 2.5 liters of sweat per hour will lose almost 15 grams of sodium in 2 hours, a level that could easily exceed normal daily sodium intakes.⁵

Temperature regulation represents the balance between heat produced or received (heat-in) and heat removed (heat-out). When the body's temperature regulation system is working correctly, heat-in and heat-out are in perfect balance, and body temperature is maintained. Both internal and external factors can contribute to body heat. Radiant heat from the sun contributes to body temperature, as does the heat created from burning fuel (carbohydrate, protein, or fat). Somehow, athletes must find a way to dissipate from the body the same amount of heat that has been added to the body to maintain a constant body temperature.

The two primary systems for dissipating, or losing, heat involve (1) moving more blood to the skin to allow heat dissipation through radiation and (2) increasing the rate of sweat production. These two systems account for about 85 percent of heat removal when a person is at rest. Heat losses through conduction (the natural transmission of heat from a hotter body to the cooler air environment) and convection (heat transfer from tissue to the blood and through the skin) account for the remaining 15 percent of heat-out. During exercise, however, virtually all heat loss occurs via evaporation (sweat).

Both of these systems rely on maintenance of an adequate blood volume. A lower blood volume results in a reduced movement of blood to the skin, and sweat production is also reduced. Working muscles demand more blood flow to deliver nutrients and to remove the by-products of burned fuel. However, at the same time there is a need to shift blood away from the muscles and toward the skin to increase the sweat rate. With low blood volume, one or both of these systems fail, with a resultant decrease in athletic performance. In fact, the maintenance of blood volume is rightly considered by many to be the primary indicator of whether an athlete's performance can be maintained at a high rate.

Energy metabolism is only about 20 to 40 percent efficient, meaning that only 20 to 40 percent of food energy can be converted to the mechanical energy of muscular work. The remaining 60 to 80 percent of the food energy that is burned is lost as heat. However, when the rate of energy burn goes up, as happens during physical activity, the amount of heat added to the system is dramatically increased, so the heat-out systems must be "turned up."In fact, heavy exercise can produce 20 times the amount of heat produced at rest. Without an efficient means of heat removal, body temperature will rise quickly. The upper limit for human survival is about 110 degrees Fahrenheit (43.3 degrees Celsius), or only 11.4 degrees Fahrenheit (or 6.3 degrees Celsius) higher than normal body temperature. Body temperature has the potential to rise approximately 1 degree Fahrenheit every 5 minutes. It is conceivable, therefore, that an underhydrated athlete could be at risk for heatstroke and death less than 1 hour after the initiation of exercise.

Athletes doing very mild exercise that burns 300 kilocalories of energy during 30 minutes would use approximately 75 kilocalories for muscular work, and 225 kilocalories would be lost as heat.⁶ This excess heat must be dissipated to maintain normal body temperature. Athletes working twice as intensely would create 450 kilocalories of excess heat that would need to be dissipated over the same 30 minutes to maintain body temperature. It is estimated that 1 milliliter of sweat can dissipate .5 kilocalories,⁷ so over this 30-minute period the athlete would lose approximately 900 milliliters (almost 1 liter) of sweat. In 1 hour of high-intensity activity, approximately 1.8 liters of water would be lost. On sunny and hot days when the heat of the sun is added to the heat generated from muscular work, the athlete must produce more sweat to remove more heat. The fluid requirement is compounded when exercising intensely on a hot and humid day Sweat doesn't evaporate off the skin as easily when it is humid, so even more sweat must be produced. In these conditions, a person can easily lose between 1 and 2 liters of fluid (via sweat) per hour.

Well-trained athletes exercising in a hot and humid environment may lose more than 3 liters of fluid per hour. To protect athletes from placing themselves at increased heat-stress risk, the heat index was developed. This index simultaneously considers environmental temperature and relative humidity to establish exercise risk.

Factors Affecting Fluid Loss

Factors Affecting Fluid Intake

Gastric Emptying and Fluid Delivery to Working Muscles