Physical activity and environmental exposure will increase water losses and therefore increase daily fluid needs. Physically active persons are often more likely to be outdoors and exposed to ambient environmental conditions (e.g., hot weather). Because dehydration will reduce physical exercise capabilities and increase heat strain (e.g., body temperature), it is important that active populations adequately replace their fluid losses (IOM, 1994; Sawka and Coyle, 1999).

As previously discussed, this increased water loss is essentially equal to sweat losses, as increased respiratory water losses are essentially offset by increased production of metabolic water. Water balance studies (Table 4-5) indicate that going from minimal activity to sedentary activity levels in temperate environments increased daily water requirements from approximately 2.5 to 3.2 L/day, respectively. Water turnover studies (Table 4-6) indicate that individuals with more strenuous levels of activity (> 60 minutes per day of activity) compared with individuals engaging in relatively sedentary activity (i.e., less than 60 minutes per day of activity) in temperate environments have increased daily total water requirements of approximately 3.0 to 4.5 L/day in men (Fusch et al., 1998; Leiper et al., 1996). Higher levels of physical activity further increase water requirements; for example, very active fire fighters had daily water requirements of about 7 L/day (Ruby et al., 2002).

Data from NHANES III (Table 4-15; Appendix H) indicate that individuals reporting leisure time activity five or more times per week had higher median daily water intakes by ≈ 0.5 L/day (e.g., 19 to 30 years: men 3.16 to 3.78 L/day, women 2.60 to 2.93 L/day). If persons perform physical activity in hot weather, then daily water requirements will be markedly increased. For active populations living in tropic or desert weather, daily sweat losses are often an additional 2 to 7 L/day (IOM, 1993, 1994; Molnar, 1947). Several analyses of water losses in hot weather (Figures 4-16, 4-17, and 4-18) support that active individuals who are continually exposed to hot weather can often have daily water requirements of 6 to 8 L/day or more.

Figure 4-20 provides approximate daily fluid requirements based on modeling for adults (assuming approximately 1.0 L for minimal needs for urine, respiratory, gastrointestinal, and insensible losses [Table 4-2]) wearing light-weight clothing while exposed to a vari-