European Journal of Applied Physiology

, Volume 113, Issue 3, pp 611–620

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

  • Nassim Hamouti
  • Juan Del Coso
  • Ricardo Mora-Rodriguez
Original Article

DOI: 10.1007/s00421-012-2467-9

Cite this article as:
Hamouti, N., Del Coso, J. & Mora-Rodriguez, R. Eur J Appl Physiol (2013) 113: 611. doi:10.1007/s00421-012-2467-9

Abstract

Blood serum osmolality (SOSM) is the gold standard to assess body fluid balance. Urine specific gravity (USG) is also a body fluid balance index but it is not invasive. However, USG 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 SOSM and USG above exercise-baseline values (P < 0.05). The next morning, SOSM and USG 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), USG increased (1.025 ± 0.003 to 1.028 ± 0.003; P < 0.05) while SOSM decreased (295 ± 5 to 287 ± 5 mOsmol kg−1 H2O; P < 0.05). In summary, during exercise-induced dehydration, USG is as sensitive as SOSM to detect low levels of dehydration (i.e., 2 %). Both indices maintain the ability to detect a 3 % overnight hypohydration although SOSM approaches euhydration values, while USG remains a superior index to detect hypohydration.

Keywords

Urine specific gravityBlood serum osmolalityBody fluid shiftHydration statusRenal water reabsorption

Copyright information

© Springer-Verlag 2012

Authors and Affiliations

  • Nassim Hamouti
    • 1
  • Juan Del Coso
    • 1
  • Ricardo Mora-Rodriguez
    • 1
  1. 1.Exercise Physiology LaboratoryUniversity of Castilla-La ManchaToledoSpain