This reflex increase in heart rate, however, is not sufficient to compensate for the decrease in stroke volume (Rowell et al., 1966); consequently, maximal cardiac output is reduced.
Several studies have shown that cardiovascular performance is compromised following thermal or exercise-induced hypohydration ≤(2 percent body weight loss) (Armstrong et al., 1985; Costill et al., 1976; Pitts et al., 1944; Saltin, 1964). Cardiac output is reduced by almost 2 liters per minute with decreased blood volume (Fortney et al., 1983; Nadel et al., 1980). This reduction in cardiac output can almost entirely account for decreases in as a result of hypohydration (Rowell et al., 1966; Saltin, 1964). Significant reductions in physical work capacity have been seen in wrestlers after hypohydration-caused weight loss (Herbert and Ribisl, 1972), as well as in runners after diuretic-induced weight loss (Armstrong et al., 1986).
The acute heat stress in hot climates that causes and is caused by dehydration has been associated with several factors. It can be precipitated by an increase in resting and submaximal exercise metabolic rates (Consolazio et al., 1961, 1963; Dimri et al., 1980; Fink et al., 1975), increases in plasma or muscle lactate levels (Dill et al., 1930; Dimri et al., 1980; Fink et al., 1975; Nadel, 1983; Robinson et al., 1941; Young et al., 1985), and glycogenolysis during submaximal exercise.
Early studies that investigated dehydration and exercise in heat and humidity found differences in sweat rate and endurance, with women sweating less than men for a given thermal stress (Fox et al., 1969; Wyndham, 1965). These studies were initially interpreted as evidence that women were not as capable as men in coping with heat stress. More recent studies comparing the effects of exercising in heat and humidity in men and women continue to find differences in sweat rate. Gender differences in response to thermal stress (body core temperature, acclimatization, etc.) however, appear to result from differences in aerobic power, due to disparities in body weight-to-mass ratio or level of physical fitness (Armstrong et al., 1990; Avellini et al., 1980; Dill et al., 1977; Grucza et al., 1985; Havenith and van Middendorp, 1990; O'Toole, 1989; Paolone et al., 1978; White et al., 1992; Chapter 5, this volume).
Avellini et al. (1980) compared acclimation to work in humid heat in an environmental chamber in men and women with similar aerobic capacities and surface-area-to-mass ratios. The women were tested both pre-and postovulation. Prior to acclimation, the women sweated less than the men, their endurance was greater, and their rectal temperature and heart rate did not increase to the level seen in men. After acclimation, rectal temperature and heart rates were similar, although there was an increased difference in sweat