The physiological effects of ingesting high sodium drinks before, during, and after exercise in the heat

This thesis investigated whether highly concentrated sodium solutions ([HS] 126-164 mmol.L-1 NaCl) could provide viable strategies before during and after exercise in the heat to improve cardiovascular and thermoregulatory functioning and exercise performance. To do this it also examined the gustatory responses to HS drinks before, during and after exercise. All studies compared HS with a low sodium control ([LS] 10-27 mmol.L-1). Chapter 4 found that during 3 h recovery from dehydration, ingestion of 120 % body mass losses of HS restored fluid balance to a greater extent (121 vs. 84 %) than LS. Chapter 7 was the first to investigate the effects of ingesting HS during exercise in the heat and in an untrained population. HS attenuated the decline in stroke volume [SVDrift] and increase in heart rate [HRDrift], but did not affect rectal temperature [TRec], cardiac output, or oxygen uptake during the second of two consecutive 45 min bouts at 55% . In Chapters 8 and 9 untrained participants ingested either HS or LS during 30-45 min pre-exercise rest. HS reduced HRDrift and SVDrift but did not affect TRec during 45-60 min exercise at 10% of the difference between and gas exchange threshold [∆]. HS also increased both time to exhaustion and exercise toleranceduring subsequent exercise bouts at 60-70% ∆. Chapters 5 and 6 found that taste perceptions act as physiological regulators, in this case, one reflecting the priority to restore hyperosmolality over hypovolemia. Exercise-induced dehydration increased the palatability of water, and decreased the palatability HS, when measured before, immediately after and during 3 h recovery. The changes were highly correlated with physiological indicators of fluid balance. The ingestion of highly concentrated sodium solutions can be both an efficient and acceptable means to improve hydration, reduce cardiovascular stress, and improve exercise performance in the heat. Whilst highly effective, caution should apply since the unpleasant taste evoked by these solutions persists for at least three hours post exercise.