Sweat mineral-element responses during 7 h of exercise-heat stress.

CONTEXT

Uncertainty exists regarding the effect of sustained sweating on sweat mineral-element composition.

PURPOSE

To determine the effect of multiple hours of exercise-heat stress on sweat mineral concentrations.

METHODS

Seven heat-acclimated subjects (6 males, 1 female) completed 5 x 60 min of treadmill exercise (1.56 m/s, 2% grade) with 20 min rest between exercise periods in 2 weather conditions (27 degrees C, 40% relative humidity, 1 m/s and 35 degrees C, 30%, 1 m/s). Sweat was collected from a sweat-collection pouch attached to the upper back during exercise bouts 1, 3, and 5. Mineral elements were determined by using inductively coupled plasma-emission spectrography.

RESULTS

At 27 degrees C, sweat sodium (863 [563] microg/mL; mean [SD]), potassium (222 [48] microg/mL), calcium (16 [7]) microg/mL), magnesium (1265 [566] ng/mL), and copper (80 [56] ng/mL) remained similar to baseline over 7 h of exercise-heat stress, whereas sweat zinc declined 42-45% after the initial hour of exercise-heat stress (Ex1 = 655 [362], Ex3 = 382 [168], Ex5 = 355 [288] microg/mL, P < 0.05). Similar outcomes were observed for sweat zinc at 35 degrees C when sweat rates were higher. Sweat rate had no effect on sweat trace-element composition.

CONCLUSIONS

Sweat sodium, potassium, and calcium losses during multiple hours of sustained sweating can be predicted from initial sweat composition. Estimates of sweat zinc losses, however, will be overestimated if sweat zinc conservation is not accounted for in sweat zinc-loss estimates.