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Effects of athletes’ muscle mass on urinary markers of hydration status

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To determine if athletes’ muscle mass affects the usefulness of urine specific gravity (U sg) as a hydration index. Nine rugby players and nine endurance runners differing in the amount of muscle mass (42 ± 6 vs. 32 ± 3 kg, respectively; P = 0.0002) were recruited. At waking during six consecutive days, urine was collected for U sg analysis, urine osmolality (U osm), electrolytes (\( {\mathop U\nolimits_{[{\text{Na}}^{ + } ]} }\), \( {\mathop U\nolimits_{[{\text{K}}^{ + } ]} }\) and \( {\mathop U\nolimits_{[{\text{Cl}}^{ - } ]} }\)) and protein metabolites (U [Creatinine], U [Urea] and U [Uric acid]) concentrations. In addition, fasting blood serum osmolality (S osm) was measured on the sixth day. As averaged during 6 days, U sg (1.021 ± 0.002 vs. 1.016 ± 0.001), U osm (702 ± 56 vs. 554 ± 41 mOsmol kg−1 H2O), U [Urea] (405 ± 36 vs. 302 ± 23 mmol L−1) and U [Uric acid] (2.7 ± 0.3 vs. 1.7 ± 0.2 mmol L−1) were higher in rugby players than runners (P < 0.05). However, urine electrolyte concentrations were not different between groups. A higher percentage of rugby players than runners (56 vs. 11%; P = 0.03) could be cataloged as hypohydrated by U sg (i.e., >1.020) despite S osm being below 290 mOsmol kg−1 H2O in all participants. A positive correlation was found between muscle mass and urine protein metabolites (r = 0.47; P = 0.04) and between urine protein metabolites and U sg (r = 0.92; P < 0.0001). In summary, U sg specificity to detect hypohydration was reduced in athletes with large muscle mass. Our data suggest that athletes with large muscle mass (i.e., rugby players) are prone to be incorrectly classified as hypohydrated based on U sg.

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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. Andrea Ávila was supported by a Latin-American grant from the Gatorade Sport Science Institute.

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. The assistance of 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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Correspondence to Ricardo Mora-Rodriguez.

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Communicated by Nigel Taylor.

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Hamouti, N., Coso, J.D., Ávila, A. et al. Effects of athletes’ muscle mass on urinary markers of hydration status. Eur J Appl Physiol 109, 213–219 (2010).

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