Thermoregulation, Exercise, and Thirst
Thermoregulation is the manner in which the body is able to maintain a consistent internal temperature, notwithstanding significant fluctuations in external temperatures caused by the environment. Thermoregulation is a primarily involuntary function, with the controls centered in the hypothalamus, the region of the brain that controls many other important systems, including the production of hormones, the chemical signals generated throughout the body in the endocrine system, as well as the function of the heart. Humans have evolved to function best at an internal temperature that can be maintained at approximately 98.6°F (37°C).
Exercise may be broadly defined as any exertion of the musculoskeletal system that goes beyond the involuntary functions of basic human metabolism, such as eating, breathing, or sleeping; exercise levels will naturally place a correspondingly greater impact on the body's ability to regulate temperature. The impact of exercise on the thermoregulatory system will also vary subject to the presence or absence of environmental conditions such as heat or humidity.
Thirst is a universal human experience. All humans, when their fluid levels are low, will crave water or other fluids. The thirst mechanism is also progressive in its signal from the involuntary system to the human senses, a sensation that cannot be shut out or deactivated through any means. It is the timing of the activation of the thirst mechanism that is of interest. Unlike other mammals, the human thirst mechanism does not activate until the body supply of water is depleted by approximately 15 oz (500 ml). Although a person cannot survive for more than three or four days without water, it is ironic that a person may survive as long as 30 days without food, even though the hunger craving is a more powerful one than that of thirst.
There is a crucial component of the osmoregulation of the body, the control of the body's levels of water and mineral salts, particularly those of sodium and potassium. Osmoregulation, thermoregulation, and the maintenance of the body's blood glucose levels, are the three main aspects of homeostasis, or the balance achieved by the body in its involuntary operating functions.
The thirst mechanism must also be considered in contrast to the daily fluid requirements of the body. Of the water consumed daily in various forms, a sedentary person will eliminate 45 oz (1,500 ml) as urine, 15 oz (500 ml) through evaporation and perspiration, 10 oz (300 ml) through the lungs, and 6 oz (200 ml) through the digestive and other gastrointestinal processes. An athlete engaged in a demanding workout or who is active in a warm weather environment may lose between 1 qt (1 l) and 4 qt (4 l) of fluid through perspiration in less than 90 minutes of activity. The fluid losses in such circumstances outpace the thirst mechanism, putting athletic performance and athlete health in jeopardy. It is entirely possible for an athlete to begin to sustain the adverse effects of dehydration, which include impaired cardiovascular function, impaired muscle function, and loss of coordination and motor control, before the thirst mechanism has been signaled.
The gap between the triggering of the thirst mechanism and fluid levels is also a factor at the beginning of the activity. If an athlete relies on the thirst sensation to determine hydration, the athlete will often begin the activity in a mildly dehydrated state.
The warm weather activation of the anti-diuretic hormone (ADH) is also an important factor when assessing the function of the thirst mechanism. ADH is released when the body senses, through the hypo-thalamus, that it is becoming dehydrated and that the blood volume has been reduced through additional perspiration. ADH is the signal conveyed to the kidneys to produce less urine, and to direct greater amounts of water into the blood. This process also may be triggered in advance of any thirst experienced by the athlete.
Because the thirst mechanism is an unreliable indicator of the body's true thermoregulatory and osmoregulatory condition during exercise, an athlete must develop a fluid replacement/hydration strategy that permits optimal function irrespective of the thirst sensation. The consumption of water in the period prior to, during, and subsequent to performance is essential. When the athlete will be involved in significant exercise for periods greater than one hour, the consumption of sport drinks that will assist in maintaining the sodium level, a key component in how the body maintains fluid levels, is important. In warm weather circumstances, the consumption of water to address dehydration, absent proper sodium levels, can lead to a state where the water will not be absorbed into the body as the involuntary systems strive to maintain a desired sodium balance with the sodium remaining. This condition is hyponatremia, a state of water intoxication.
Urine color is a useful general indicator of hydration. When urine is a light yellow color, it signals a proper fluid level; when urine is dark yellow, it is evidence of dehydration, as the urine is too concentrated, a result of the body is secreting greater amounts of ADH to limit fluid outflow.