Because sweat has a lower osmolality than does plasma (i.e., sweat is hypotonic), profuse sweating increases plasma osmolality. Whether or not this increased plasma osmolality affects body temperature or cooling capacity in an exercising individual is, as yet, unclear, but a sufficient change in osmolality and volume does stimulate the kidneys to excrete sodium and reduce urine output by producing more concentrated urine.
Several factors affect the rate at which an athlete can produce sweat. Higher ambient temperatures result in a greater potential for sweat production. Higher humidity is also responsible for higher sweat production, but because the vapor pressure gradient and skin is low, the cooling potential (i.e., the rate of evaporation off the skin) is lower in humid environments. The same problem also exists with clothing that traps sweat against the skin (i.e., does not breathe). This type of clothing results in a reduced cooling efficiency that forces a greater sweat rate. (Sweat-soaked clothing doesn’t mean an athlete is effectively controlling body temperature, it just means he or she is losing water.) Some new materials designed for athletes actually wick sweat away from the skin to improve evaporative efficiency. Athletes with large body surface areas may also have an enhanced sweat production capacity and, therefore, an enhanced evaporative heat loss. But these athletes may also gain more heat from the environment through radiation and convection in hot weather. The conditioning or training state of an athlete makes a difference. Well-conditioned athletes have a higher sweat volume potential that results in an enhanced cooling potential. However, this higher sweat rate requires a greater during-exercise fluid consumption to avoid higher heat-stress risk.
An athlete’s state of fluid balance also plays a factor. The better the hydration state, the greater the sweat potential. As athletes become progressively dehydrated, the sweat rate is reduced, and body temperature rises. This is a problem because fluid consumption during activity is rarely greater than 2 cups (480 milliliters) per hour, or only 30 to 40 percent of the amount of fluid lost in sweat, an amount that will inevitably lead to the athlete’s becoming dehydrated. Consider that marathoners competing in a cool temperature of 50 to 54 degrees Fahrenheit (10 to 12 degrees Celsius) lose between 1 and 5 percent of total body mass. Marathoners competing in warm weather lose about 8 percent of total body mass, or between 12 and 15 percent of total body water.
This is an excerpt from Advanced Sports Nutrition.