Abstract
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.
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Acknowledgments
The authors wish to thank the participants for their invaluable contribution to the study. Nassim Hamouti and Juan Del Coso were supported by a predoctoral fellowship from the Castilla-La Mancha government in Spain. The assistance of Andrea Ávila and Emma Estevez is greatly appreciated.
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The authors of this study declare that they have no financial, professional or other personal interest of any nature in any product, service and/or company that could be construed as influencing the position presented in this manuscript.
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The authors of this study declare that the experiments comply with the current laws of the country in which they were performed. The study was approved by the local Hospital Research Ethics Committee and conducted in accordance with the guidelines of the revised Declaration of Helsinki.
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Communicated by Narihiko Kondo.
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Hamouti, N., Del Coso, J. & Mora-Rodriguez, R. Comparison between blood and urinary fluid balance indices during dehydrating exercise and the subsequent hypohydration when fluid is not restored. Eur J Appl Physiol 113, 611–620 (2013). https://doi.org/10.1007/s00421-012-2467-9
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DOI: https://doi.org/10.1007/s00421-012-2467-9