During exercise in the heat, sweat output often exceeds water intake, resulting in a body water deficit (hypohydration) and electrolyte losses. Prolonged strenuous exercise can result in marked changes in chromium, copper, iron, magnesium, and zinc metabolism. Evidence of these changes can persist for several days after the exercise is discontinued. Some of the observed changes in plasma electrolyte – mineral concentrations may be attributed in part to an acute-phase response, which occurs as a result of tissue trauma or stress.
Reductions in plasma mineral concentrations may also in part reflect an increased loss of these minerals from the body via urine and sweat. The increased rate of electrolyte and mineral loss that occurs in sweat with exercise is amplified by the simultaneous exposure to hot temperatures.
Hypohydration increases heat storage and reduces a person’s ability to tolerate heat strain. The increased heat storage is mediated by a lower sweating rate (evaporative heat loss) and reduced skin blood flow (dry heat loss) for a given core temperature. Heat-acclimated persons need to pay particular attention to fluid replacement because heat acclimation increases sweat losses, and hypohydration negates the thermoregulatory advantages conferred by acclimation. It has been suggested that hyperhydration (increased total body water) may reduce physiologic strain during exercise heat stress, but data supporting that notion are not robust.
Because daily water losses can be substantial, persons need to emphasize drinking during exercise as well as at meals.
- Am J Clin Nutr. 2000 Aug;72(2 Suppl):564S-72S, Sawka MN1, Montain
- Carl L. Keen, SJ.Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations.