Studies have demonstrated that athletes experience a degree of voluntary dehydration, even in the presence of available fluids, that lowers blood volume and negatively affects performance. Given the tremendous amount of heat that must be dissipated during exercise through sweat evaporation, athletes must pursue strategies that will sustain the hydration state. Failure to do so will result in poor performance and may also lead to heat illness.
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. The two primary systems for dissipating, or losing, heat while at rest are (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 the heat lost when a person is at rest, but during exercise virtually all heat loss occurs from the evaporation of sweat.
Working muscles demand more blood flow to deliver nutrients and to remove the metabolic by-products of burned fuel, but at the very 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.
Heavy exercise can produce heat that is 20 times greater than the heat produced at rest. Without an efficient means of removing this excess heat, 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. With the potential for body temperature to rise at the rate of about 1 degree Fahrenheit every 5 minutes, it is conceivable that underhydrated athletes could be at heatstroke risk only 57 minutes after the initiation of exercise.
Athletes working hard for 30 minutes would create 450 kilocalories of excess heat that need to be dissipated to maintain body temperature. Since 1 milliliter of sweat can dissipate approximately .5 calories, athletes would lose about 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 produced from muscular work, athletes would need to produce even more sweat to remove more heat. Sweat doesn’t evaporate off the skin as easily when it is humid, so even more sweat must be produced in hot and humid weather. Well-trained athletes exercising in a hot and humid environment may lose more than 3 liters of fluid per hour.
No level of low body water is acceptable for achieving optimal athletic performance and endurance, so athletes should have a strategy for maintaining optimal body water during exercise. The problem is that athletes often rely on thirst as the marker of when to drink. Since the thirst sensation occurs only after a loss of 1 to 2 liters of body water, thirst is an inappropriate indicator of when to drink. Instead, the athlete should strategize on how to never get thirsty. Ideally, this strategy should involve helping athletes determine how much fluid is lost during typical bouts of physical activity and developing a fixed fluid-consumption schedule from that information (typically 3 to 8 ounces every 10 to 15 minutes of a sodium-containing 6 to 8 percent carbohydrate solution.)