Comparison between blood and urinary fluid balance indices during dehydrating exercise and the subsequent hypohydration when fluid is not restored

Blood serum osmolality (S _OSM) is the gold standard to assess body fluid balance. Urine specific gravity (U _SG) is also a body fluid balance index but it is not invasive. However, U _SG capability to detect the minimal level of dehydration that affects athletic performance (i.e., 2 %) remains untested. We collected urine and blood samples in eighteen euhydrated trained athletes in the morning and that evening while dehydrating by 1, 2, and 3 % of body mass by cycling (60 % \( \dot{V}{\text{O}}_{{ 2 {\text{peak}}}} \)) in the heat (32 °C, 46 % rh, 2.5 m s⁻¹ air flow). At 9:00 pm, subjects left the laboratory and went to bed after ingesting 0.7 ± 0.2 L of a sports drink. The next morning, subjects awoke 3 % hypohydrated, and blood and urine samples were collected and test terminated. We found that 2 % dehydration increased S _OSM and U _SG above exercise-baseline values (P < 0.05). The next morning, S _OSM and U _SG remained elevated compared to the first morning while euhydrated (287 ± 5 vs. 282 ± 3 mOsmol kg⁻¹ H₂O and 1.028 ± 0.003 vs. 1.017 ± 0.005, respectively, P < 0.05). However, when comparing 3 % dehydration (end of exercise) to 3 % hypohydration (next morning), U _SG increased (1.025 ± 0.003 to 1.028 ± 0.003; P < 0.05) while S _OSM decreased (295 ± 5 to 287 ± 5 mOsmol kg⁻¹ H₂O; P < 0.05). In summary, during exercise-induced dehydration, U _SG is as sensitive as S _OSM to detect low levels of dehydration (i.e., 2 %). Both indices maintain the ability to detect a 3 % overnight hypohydration although S _OSM approaches euhydration values, while U _SG remains a superior index to detect hypohydration.