Thirst sensitivity to post-exercise fluid replacement needs and controlled drinking

Abstract

Purpose: Thirst was evaluated as a dependent variable, to see if perceived thirst (TP) can clearly distinguish among several levels of acute dehydration, and if so, how it responds over time to the ingestion of a predetermined volume of water post exercise. TP reliability was also evaluated. Methods: in a repeated-measures design, eight physically active students (24.5±3.6 years, mean±standard deviation), reported to the laboratory after an overnight fast (10 hours or longer), on four non-consecutive days. They exercised intermittently in a controlled climate chamber at 32±3°C db and 65±6% r.h. to a randomly assigned dehydration equivalent to 0, 1, 2 and 3% of body mass (BM). Following exercise, subjects ingested a fixed volume of water equivalent to 1.20% BM in 30 minutes; urine output, TP and plasma volume changes were measured every 30 minutes over 3 hours. Results: Baseline characteristics were not different among conditions (p>0.05). TP was not different before taking a shower from 30 minutes later after showering (p = 0.86), but it was clearly different among conditions after exercise (TP = 2.50 ± 0.45, 4.44 ± 0.72, 6.38 ± 0.82, and 8.63 ± 0.18 for 0, 1, 2, and 3% BM, p = 0.001). TP was already the same for all conditions 30 min after drinking, (1.1±0.3, 1.1±0.3, 2.6±1.4, and 3.3±2.3 for 0, 1, 2 and 3% BM, respectively, p>0.05); it remained so for 3h. There was a clear association between TP and net fluid balance (rpart = -0.62, p < 0.0001). Conclusion: this subjective scale of thirst perception is able to detect dehydration equivalent to 2% BM or greater. The measure is reliable, and it shows a clear, significant association with net fluid balance. It is, however, disproportionately reduced in dehydrated subjects after acute ingestion of water. Under these conditions, we deem thirst to be insufficient as it responds inappropriately to water intake.