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First morning urine (FMU) assessment would be a practical and convenient solution for clinically acceptable detection of underhydration prior to competition/training, and for the general public. Thus, we thus sought to determine the diagnostic accuracy of FMU as a valid indicator of recent (previous 24 hr, 5 days average) hydration practices. For 5 consecutive days and one final morning, 67 healthy women (n = 38) and men (n = 29; age: 20 [1] years, body mass index: 25.9 [5.5]) completed 24-hr diet logs for total water intake (from beverages and foods, absolute and relative to body mass), 24-hr urine and FMU collection (last morning only) for osmolality (Osm), specific gravity (SG), and color (Col), and morning blood sampling for plasma osmolality and copeptin. Correlations determined significance and relationship strength among FMU and all other variables. Area under the receiver operating characteristic curves, sensitivity, specificity, and positive likelihood ratios were employed using previously reported values to indicate underhydration (total water intake < 30 ml/kg, osmolality > 500, and >800 mOsm/kg, specific gravity > 1.017, and copeptin > 6.93 pmol/L). FMU_Osm and FMU_SG were significantly correlated (p < .05) to all variables except the previous 5-day plasma osmolality. FMU_Col was only significantly correlated with other color time intervals and total water intake per gram. FMU_Osm held greatest utility (area under the receiver operating characteristic curve, sensitivity, and specificity >80%) overall, with the best outcome being FMU_Osm indicating a previous 24-hr osmolality threshold of 500 mOsm/kg (FMU_Osm criterion >710 mOsm/kg and positive likelihood ratio = 5.9). With less effort and cost restriction, FMU is a viable metric to assess underhydration.

Athletes and researchers could benefit from a simple and universally accepted technique to determine whether humans are well-hydrated, euhydrated, or hypohydrated. Two laboratory studies (A, B) and one field study (C) were conducted to determine if urine color ( ${\text{U}}^{\text{col}}$ ) indicates hydration status accurately and to clarify the interchangeability of ${\text{U}}_{\mathrm{col}}$ , urine osmolality ( ${\text{U}}^{\text{osm}}$ ), and urine specific gravity ( ${\text{U}}^{\text{sg}}$ ) in research. ${\text{U}}_{\mathrm{col}}$ , ${\text{U}}_{\mathrm{osm}}$ , and ${\text{U}}_{\mathrm{sg}}$ were not significantly correlated with plasma osmolality, plasma sodium, or hemato-crit. This suggested that these hematologic measurements are not as sensitive to mild hypohydration (between days) as the selected urinary indices are. When the data from A, B, and C were combined, ${\text{U}}_{\mathrm{col}}$ was strongly correlated with ${\text{U}}_{\mathrm{hg}}$ and U„_sm. It was concluded that (a) ${\text{U}}_{\mathrm{col}}$ may be used in athletic/industrial settings or field studies, where close estimates of ${\text{U}}_{\mathrm{sg}}$ or ${\text{U}}_{\mathrm{osm}}$ are acceptable, but should not be utilized in laboratories where greater precision and accuracy are required, and (b) ${\text{U}}_{\mathrm{osm}}$ and ${\text{U}}_{\mathrm{sg}}$ may be used interchangeably to determine hydration status.