and increased respiratory water losses (see Table 4-3). Cold-induced diuresis (CID) is well studied and is a “normal” physiological response to body cooling. Urine specific gravities decrease with CID; however, they cluster around 1.009 (Bass and Henschel, 1956). CID induces an isoosmotic hemoconcentration, and there is little relationship between the magnitude of diuresis and hemoconcentration (Bass and Henschel, 1956; Young et al., 1987). The reduction in body water with contracting vascular volume is probably of no concern as long as the body remains cool.

Dehydration does not modify thermoregulation during cold exposure as evidenced by body heat balance (O’Brien et al., 1998) or peripheral vascular responses (O’Brien and Montain, 2003). However, if the dehydrated person were to subsequently exercise and produce body heat while wearing highly insulating clothing, then heat stress will be encountered. (The effects of dehydration and heat stress on thermoregulation and physical work performance have been discussed earlier in this chapter.)

Caffeine is one of three methylxanthines found in foods; it is naturally present in coffee, teas, and chocolate, is added to colas and other beverages (IOM, 2001a), and is a component of many medications (Passmore et al., 1987). It is estimated that 20 to 30 percent of Americans consume more than 600 mg of caffeine daily (Neuhauser-Berthold et al., 1997). The other two methylxanthines, theobromine (found in chocolate) and theophylline (found in tea), demonstrate some, but not all, of the pharmacological effects of caffeine (Dorfman and Jarvick, 1970).

It has long been thought that consumption of caffeinated beverages, because of the diuretic effect of caffeine on reabsorption of water in the kidney, can lead to a total body water deficit. However, available data are inconsistent. As early as 1928 it was reported that caffeine-containing beverages did not significantly increase 24-hour urinary output (Eddy and Downs, 1928). Caffeine-containing beverages did not increase 24-hour urine volume in healthy, free-living men when compared with other types of beverages (e.g., water, energy-containing beverages, or theobromine-containing beverages) (Dorfman and Jarvik, 1970; Grandjean et al., 2000).

Conversely, in a study in which 12 individuals who normally consumed caffeinated beverages were required to abstain from all