The factors that govern thirst in exercising individuals are not completely understood, but may be particularly important in the light of recommendations that active individuals should drink according to the dictates of thirst rather than using any drinking strategy based on known or predicted sweat losses. It is clear that both blood volume and plasma osmolality, which is determined to a large degree by the plasma sodium concentration, are important factors. These two parameters, however, may change independently of each other during exercise because of differences in the electrolyte content of sweat. Again, the primary electrolyte lost is sodium, but in contrast to the plasma sodium concentration which is normally closely controlled at about 140 mmol/l, the sodium concentration of sweat typically ranges from about 10-80 mmol/l in any population sample. A few healthy individuals may be found with even higher sweat sodium concentrations, but a very high sweat sodium concentration is normally an indication of cystic fibrosis (CF).
The normal response to sweating is a reduction in the plasma volume and an increase in the circulating sodium concentration. When sweat has a very high sodium content and when large sweat losses are incurred, it might be expected that the reduction in blood volume and the increase in sodium concentration would both be less than expected. Brown et al set out to investigate this and to assess the effects on the thirst response. They recruited groups of healthy subjects with high-sweat sodium concentration (91 +/- 17 mmol/l), whom they called salty sweaters (SS), Controls with average sweat sodium concentration (44 +/- 10 mmol/l), and physically active CF patients with very high sweat sodium concentration (133 +/- 6 mmol/l) cycled in the heat without drinking until they had lost 3% of their starting body mass. The increase in serum osmolality was less (P < 0.05) in CF (6.1 +/- 4.3 mosmol/kg) and SS (8.4 +/- 3.0 mosmol/kg) than in Control (14.8 +/- 3.5 mosmol/kg). The relative reduction in plasma volume was greater (P < 0.05) in CF (19.3 +/- 4.5%) and SS (18.8 +/- 3.1%) than in the Control subjects (-14.3 +/- 2.3%).
Subjective ratings of thirst were measured during the exercise at different levels of dehydration, but were not different among the three groups. However, when subjects were allowed unrestricted access to drinks in the post-exercise period, the ad libitum fluid replacement was substantially less (by 40%) in the CF subjects than in SS and Control. It seems from these results that thirst is appropriately maintained during exercise in the heat as a linear function of the amount of water lost in sweat, with relative contributions from hyperosmotic and hypovolemic stimuli dependent upon the magnitude of salt lost in sweat. The authors concluded that individuals with CF exhibit lower ad libitum fluid restoration following dehydration, which may reflect physiological cues directed at preservation of salt balance over volume restoration.
[separator margin=”both”]
Abstract [button text=”View at PubMed” url=”http://www.ncbi.nlm.nih.gov/pubmed/21813870″ target=”_blank” color=”blue”][/button]
[blockquote]Sweat Na(+) concentration ([Na(+)]) varies greatly among individuals and is particularly high in cystic fibrosis (CF). The purpose of this study was to determine whether excess sweat [Na(+)] differentially impacts thirst drive and other physiological responses during progressive dehydration via exercise in the heat. Healthy subjects with high-sweat [Na(+)] (SS) (91.0 ± 17.3 mmol/l), Controls with average sweat [Na(+)] (43.7 ± 9.9 mmol/l), and physically active CF patients with very high sweat [Na(+)] (132.6 ± 6.4 mmol/l) cycled in the heat without drinking until 3% dehydration. Serum osmolality increased less (P < 0.05) in CF (6.1 ± 4.3 mosmol/kgH(2)O) and SS (8.4 ± 3.0 mosmol/kgH(2)O) compared with Control (14.8 ± 3.5 mosmol/kgH(2)O). Relative change in plasma volume was greater (P < 0.05) in CF (-19.3 ± 4.5%) and SS (-18.8 ± 3.1%) compared with Control (-14.3 ± 2.3%). Thirst during exercise and changes in plasma levels of vasopressin, angiotensin II, and aldosterone relative to percent dehydration were not different among groups. However, ad libitum fluid replacement was 40% less, and serum NaCl concentration was lower for CF compared with SS and Control during recovery. Despite large variability in sweat electrolyte loss, thirst appears to be appropriately maintained during exercise in the heat as a linear function of dehydration, with relative contributions from hyperosmotic and hypovolemic stimuli dependent upon the magnitude of salt lost in sweat. CF exhibit lower ad libitum fluid restoration following dehydration, which may reflect physiological cues directed at preservation of salt balance over volume restoration.[/blockquote]