Exercise in the heat: effect of fluid ingestion on blood-brain barrier permeability.

INTRODUCTION This study examined changes in serum S100beta concentration, a peripheral marker of BBB permeability, in response to exercise in the heat with and without fluid ingestion. METHODS Eight physically active males completed up to 90 min of intermittent exercise at a power output corresponding to 55% VO2peak in a warm environment (35 degrees C, 56% rh). Trials were completed with (F trial) and without (NF trial) the replacement of sweat losses. During the fluid trial, an aliquot of plain water was ingested at 15-min intervals to match the volume of sweat lost during the previous period of exercise. RESULTS Exercise time was 80.7 +/- 13.0 min in the NF trial and 85.1 +/- 9.5 min in the F trial (P = 0.107). Fluid ingestion resulted in a smaller rise in core temperature (P = 0.050) and heart rate (P = 0.027) during the latter stages of exercise. Serum S100beta concentrations were 0.08 +/- 0.02 microg.L at rest, increasing to 0.20 +/- 0.06 microg.L at the end of exercise in the NF trial, with this response attenuated by the ingestion of fluid (0.13 +/- 0.03 microg.L; P = 0.046). Both serum sodium concentration (P < 0.001) and serum osmolality (P = 0.003) were significantly lower at the end of exercise in the F trial than in the NF trial. CONCLUSION The results of this study demonstrate that water ingestion can limit exercise-induced increases in serum S100beta, consistent with the preservation of BBB integrity. It is possible that this response was mediated through the maintenance of lower extracellular osmolality late in exercise, thus potentially limiting the osmotically driven movement of fluid across the BBB.