Novel hydration assessment techniques employing thirst and a water intake challenge in healthy men

Exploring novel hydration indices is important because no human biomarker has been shown to be incontrovertibly valid in all life situations. The present investigation was designed to identify inexpensive, nontechnical methods to use when self-assessing hydration status. This investigation evaluated the validity and efficacy of 2 novel techniques (i.e., thirst sensation and urine volume) to assess hydration state of 29 active men (mean +/- SD; age, 23 +/- 4 years; body mass, 76.02 +/- 11.94 kg) at rest. Eight combinations of 4 water challenges (4.8, 9.3, 11.0, or 14 mL.kg(-1)) and 2 hydration states (mildly hypohydrated (HY), -2.0%; euhydrated (EU), -0.2% body mass) were employed. First, thirst was linearly related to body water loss, and ratings of thirst distinguished HY from EU (p < 0.001) subsequent to 19 h of controlled food and fluid intake. Second, measurements of urine volume 60 min after consuming a water bolus (11.0 or 14 mL.kg(-1)) were strongly and inversely correlated with entering hydration state, assessed by urine specific gravity (r(2) = 0.76, p < 0.0001) and urine osmolality (r(2) = 0.77, p < 0.0001). We concluded that healthy men can employ simple measurements of morning thirst sensation and urine volume to identify the presence of mild hypohydration and to guide fluid replacement. These 2 techniques are relevant because HY (-2% body mass) is the approximate threshold for the onset of thirst, reduced endurance exercise performance, and decrements of working memory and mood.