in turn depends on such factors as exercise intensity, exercise duration, environmental conditions, state of training and heat acclimatization, gender, and age. Selected studies are used to illustrate the influence of these different factors rather than to review the literature. Finally, the prediction of sweat losses under a variety of conditions is discussed, as well as the calculation of water requirements under these circumstances.

Total body water constitutes about 70 percent of lean body mass and is most simply divided into two major compartments: (a) intracellular water, which represents 50 percent of body weight or 35 liters in a 70-kg man, and (b) extracellular water, which represents 20 percent of body weight or 14 liters. The latter compartment is subdivided into plasma volume (5 percent body weight) and interstitial fluid volume (15 percent body weight). Intracellular water is not readily measured. It is calculated from measurements of total body water and extracellular fluid volume.

Table 5-1 gives normal values for daily water intake and output in a healthy adult. However, these values are subject to marked variation. For example, respiratory water loss can range from 200 ml per day when breathing humidified air to 1500 ml per day when exercising at high altitude. Water loss from cutaneous evaporation could range from 500 ml per day at rest in a cool environment to 10 liters per day during exercise in the heat. Fecal losses could range from 100 ml per day when on a mixed diet to 32