Fluid requirements

Daily fluid requirements for the general population are difficult to estimate because of the large variations in fluid losses due to physical activity. Many textbooks estimate the fluid requirements of sedentary individuals at 2 L/day. This minimum requirement (2 L equals 8 glasses/day) can be met by a variety of sources, including milk, soft drinks, fruit juices, sports drinks, water, fruits, soups, etc. Physically active individuals have daily fluid requirements well above 2 L/day. Some athletes and workers have fluid requirements above 10 L/day. These high fluid requirements are caused by the enormous volume of sweat lost during exercise, which can occasionally exceed 3 L/hour in well-trained and acclimatized athletes. This rapid fluid loss is often not matched by an equivalent volume of fluid intake, resulting in dehydration.

Fluid is periodically excreted from the body by the kidneys (urine), gastrointestinal tract (faeces) and sweat glands, and constantly - from the respiratory tract and through the skin. The total volume of fluid loss per day is determined by environmental conditions, the size (and surface area) of the individual, the intensity of his metabolism and the volume of excreted fluid. Insensible water loss through the skin is relatively constant, and insensible losses through the respiratory tract depend on the ambient temperature, relative humidity and the volume of ventilation of the lungs. Passing through the respiratory tract, inhaled air is humidified, its relative humidity reaches 100% (vapor pressure is equal to 47 mm Hg). In the case of inhalation of warm and humid air, insensible fluid loss is slightly reduced, since the inhaled air already contains water vapor. In athletes and workers, insensible fluid loss through the respiratory tract is greater due to the general increase in the intensity of breathing, which accompanies physical exertion. The air inhaled during cold exercise contains relatively little water vapor, so as it passes through the respiratory tract it is heated and humidified, causing additional moisture loss. For this reason, it is important to remember that even in cold weather work conditions, fluid loss through the sweat glands and respiratory tract can be quite high.

Losses with urine in athletes and workers are less than in sedentary people, and even less in warm weather, since the body tries to conserve fluid. Physical activity leads to a reduction in urination, since the kidneys try to conserve water and sodium to compensate for losses with sweat.

Even without exercise, daily fluid losses average at least 2-3 liters. If athletes train or compete in high temperatures, their daily fluid requirements are high. For example, an athlete who trains 2 hours each day can easily lose an additional 4 liters of fluid, which increases the daily fluid requirement to 6-7 liters. Many people are active for more than 2 hours each day, thereby increasing their fluid requirements. Such losses create stress on the fluid regulation system, so thirst becomes an inadequate indicator of fluid intake and dehydration results.

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