Plasma and serum volume remain unchanged following a 12-h fast from food and drink despite changes in blood and urinary hydration markers.

Background/objectives: The effect of mild dehydration on plasma and serum volume has not been well established. Furthermore, the ability of urinary and blood biomarkers to monitor small hydration changes have not been solidified. There were two objectives of this research: 1. Determine if mild dehydration affects plasma and serum volume; 2. Determine if mild dehydration can be detected better by urinary or blood biomarkers.

Subjects/methods: 47 subjects were recruited; 10 subjects were removed from the study and 37 subjects (27% male) completed the study. This was a crossover study design such that each subject underwent all protocols in a counterbalanced order. Protocols consisted of 12-h dehydration, 12-h hydration, and control.

Results: Neither plasma volume (p = 0.914), plasma volume status (p = 0.649), nor serum volume (p = 0.273) were different among protocols. Body mass (p < 0.001) was lower following the dehydration protocol. Urine color (p < 0.001), urine osmolality (p < 0.001), urine specific gravity (p < 0.001), serum osmolality (p < 0.001), and plasma osmolality (p < 0.001) were all lower following the hydration protocol. Hematocrit (p = 0.842) and hemoglobin concentration (p = 0.558) were not different among protocols.

Conclusions: Dehydration did not affect plasma or serum volume. Therefore, a 12-h fast from food and water as done in this study will not likely affect laboratory test results of biomarker concentration. All 3 urinary measures were able to detect changes in hydration status, whereas only 2 blood measures were able to detect changes in hydration status. This may indicate that urinary measures are best at detecting small changes in hydration status.