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To reduce the adverse consequences of exertion-related and acute intentional dehydration research has focused on monitoring hydration status. This investigation: 1) compared sensitivity of urine specific gravity (U_sg), urine osmolality (U_osm) and a criterion measurement of hydration, plasma osmolality (P_osm), at progressive stages of acute hypertonic dehydration and 2) using a medical decision model, determined whether U_sg or U_osm accurately reflected hydra-tion status compared to P_osm among 51 subjects tested throughout the day. Incremental changes in P_osm were observed as subjects dehydrated by 5% of body weight and rehydrated while U_sg and U_osm showed delayed dehydration-related changes. Using the medical decision model, sensitivity and specificity were not significant at selected cut-offs for U_sg and U_osm. At the most accurate cut-off values, 1.015 and 1.020 for U_sg and 700 m_osm/kg and 800 m_osm/kg for U_osm, only 65% of the athletes were correctly classified using U_sg and 63% using U_osm. P_osm, U_sg, and U_osm appear sensitive to incremental changes in acute hypertonic dehydration, however, the misclassified outcomes for U_sg and U_osm raise concerns. Research focused on elucidating the factors affecting accurate assessment of hydration status appears warranted.

This study investigated the relationship between changes in upon-waking body mass (BM) and changes in urine specific gravity (U_sg) and urine color (U_col) from 1 day to the next. Throughout the 5-day investigation, healthy adolescent Singaporean athletes (n = 66) had their upon-waking, bladder-voided BM measured. A small aliquot of the first bladder void each day was collected and analyzed for U_sg and U_col, the latter by both an investigator (IU_col) and individual participants (SU_col). Results revealed a significant inverse relationship between changes in BM and changes in U_sg (p = .003) and U_col (p = .001). On average, U_sg and U_col changed by ~0.003 units and ~1 color (across a 9-unit scale), respectively, with every 1% change in BM from 1 day to the next. There was a stronger relationship between U_sg and IU_col (r = .82, p < .001) than between U_sg and SU_col (r = .60, p < .001). These results suggest that the degree of fluid deficit may be predicted from the U_sg measurements among moderately hypohydrated athletes. In addition, training athletes to interpret and use the U_col chart is recommended.

Hypohydration is known to impair performance and increases the risk of heat injury. Therefore, the consumption of appropriate fluid volumes before, during, and after tennis play is important to maintain physiological homeostasis and performance. Tennis is a sport that typically has points lasting fewer than ten seconds, with short-to-moderate rest periods between each work bout. This sequence is repeated over hours. Most fuid and hydration research has focused on continuous aerobic exercise, which provides vastly different physiological strain compared with tennis practice and competition. Consequently, practical recommendations on maintaining hydration status for aerobic continuous exercise may not be appropriate for tennis athletes. Tennis players can sweat more than 2.5 L·h⁻¹ and replace fluids at a slower rate during competition than in practice. In warm and hot environments, electrolyte-enhanced fluid should be consumed at greater than >200 mL per changeover and ideally closer to 400 mL per changeover. Tennis scientists, coaches, and players need to individualize hydration protocols to arrive at the optimal hydration strategy.