SWEAT AND BLOOD RESPONSES OCCUR AT DIFFERENT TIMES DURING HEAT ACCLIMATION

Estevez, E., Del Coso, J., & Mora-Rodriguez, R. (2006). Regional sweating responses during 9 days of dry-heat acclimation. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 2043.

"It is well established that sweat rate increases with active dry-heat acclimation together with an earlier onset of sweating and decreased sodium chloride losses in sweat (1). It has been suggested that the reductions in sweat sodium maintain intravascular sodium and the osmotic drive to defend plasma volume during dehydration (2). If so, a good relationship should be found between the reduction in sweat sodium concentration and the maintenance of blood volume with acclimation to the heat."

This study investigated the relationship between reduced losses of [Na+] in sweat and maintenance of blood volume during nine days of acclimation to the heat. Active non-acclimated males (N = 9) pedaled for 90 min at 63% VO2max in a hot-dry environment (35ºC). Whole body and local sweat rates were measured on days 1, 3, 6 and 9. Plasma volume changes were measured from hematocrit and hemoglobin concentrations. Sweat patches from the upper back were collected after 60 and 90 minutes of exercise to measure osmolality (freezing point) and [Na+] (flame photometry).

After only 3 days of acclimation sweat sodium concentration decreased and remained lower on days 6 and 9. Sweat osmolality followed the same pattern. In addition, sweat osmolality on days 6 and 9 decreased significantly from the 60 to the 90 min collection time. Blood volume was better maintained during the 90 minutes of dehydration only on day 9 in comparison to day 1. The correlation between total sodium losses (sweat volume * sweat concentration) and blood volume maintenance was not statistically significant (r= -0.03).

Implication. Three days of acclimation are enough to achieve almost complete reduction of the losses of sodium in sweat. However, plasma volume maintenance during exercise was not evident until day 9 of acclimation. The retention of sodium (decrease total sweat losses) is not enough to defend blood volume during dehydration. Other factors (e.g., osmotic pressure due to the colloid effects of proteins) are more relevant for the physiological response to heat acclimation.

Return to Table of Contents for this issue.