will increase in both situations. The potassium concentration in sweat is approximately 4 to 5 mmol (0.2 g)/L. This concentration can increase up to 14 mmol (0.5 g)/L upon thermal exposure (Fukumoto et al., 1988). The sweat potassium concentration in heat-acclimatized individuals exposed to heat stress (40°C [104°F]) was approximately 5.4 mmol (0.2 g)/L, and total sweat loss was approximately 11 L/day, yielding a total potassium sweat loss of approximately 60 mmol (2.3 g)/day while consuming a diet providing 3.8 g (97 mmol)/day (Malhotra et al., 1976). When total estimated losses (sweat + urine) were summed (estimating sweat volume of 8 L/ day), the maximum loss was equivalent to 116 mmol (4.5 g)/day, which exceeded the potassium intake. Average sweat potassium daily losses of three men who were exposed to 37.8°C (100°F) heat for 7.5 hours per day for 16 days fell from 79 mmol (3.1 g) measured on day 2 to 14 mmol (0.55 g) by day 11. Hence, there was a decline in sweat loss over time, demonstrating that acclimation occurred and suggesting that potassium balance might be achieved over a short period of time.

The effects of 90 minutes of heat exposure (46°C [117°F]) without exercise during low (0.78 to 1.2 g [20 to 30 mmol]/day) and high (7.8 g [200 mmol]/day) sodium intakes on a number of parameters, including plasma potassium concentrations, were studied in eight healthy volunteers (20 to 28 years of age) (Follenius et al., 1979). There were significant changes in plasma potassium concentrations whether the subjects were on a low or high sodium diet. Plasma potassium concentrations ranged from about 3.97 to 4.15 mmol/L.

Seven healthy men, 18 to 23 years of age, were exposed to 40°C (104°F) heat in a controlled heat chamber and to exercise (Fukumoto et al., 1988). Sweat potassium concentration was 11.3 ± 3.1 mmol/L during the running exercise compared with 14.2 ± 4.6 mmol/L during thermal exposure (40°C). Conversely, sweat sodium losses were greater during the running exercise (123.1 ± 33.6 mmol/ L) compared with the heat exposure (84.3 ± 31.5 mmol/L).

Approximately 1.2 g (32 mmol)/day of potassium losses from sweat were observed during 6 hours of intermittent treadmill activity in a 40°C environment in 12 unacclimatized men (Armstrong et al., 1985). No significant changes in serum potassium concentrations in 10 experienced male marathon runners were seen after they completed three 20-mile runs under three different fluid replacement treatments (water, electrolyte-glucose solution, or a caffeine solution [5 mg of caffeine/kg of body weight]) (Wells et al., 1985). Pre-exercise serum potassium concentrations were about 4.4