Potential impact of a 500-mL water bolus and body mass on plasma osmolality dilution.
A methodological discrepancy exists in the hydration assessment literature regarding the establishment of euhydration, as some investigations utilize a pre-hydration technique, while others do not (overnight fluid/food fast). However, the degree that plasma osmolality (P (osm)) dilutes when using the pre-hydration method and how body mass/composition might influence the results is not known. Thirty subjects (22 M, 8 F; 20 ± 2 years (mean ± SD); 1.8 ± 0.1 m; 75.8 ± 13.5 kg) had P (osm) measured after an 8-h food and fluid fast (overnight fast) and 90 min after a 500-mL (4-9 mL/kg) water bolus (pre-hydration). From pre- to post-bolus, participants' P (osm) declined from 297 ± 3.5 to 295 ± 3.8 mmol/kg (p < 0.05; ∆ -1.7 ± 3.5 mmol/kg). One-third of the sample diluted to more than -3 mmol/kg. The effect of body mass on P (osm) dilution was investigated by comparing dilution in the ten lightest (62.8 ± 3.4 kg) and heaviest (92.0 ± 9.8 kg) participants; however, the change between the light (∆ -1.9 ± 3.8 mmol/kg) versus heavy groups (∆ -1.1 ± 3.0 mmol/kg) was not different (p > 0.05). The correlation between body mass or total body water and change in P (osm) was weak (p > 0.05), as was the correlation between relative fluid intake based on mass and change in P (osm) (p > 0.05). The two methodologies appear to produce similar P (osm) values when measured in most individuals. However, the potential for significant dilution (>3 mmol/kg) should be considered when choosing the pre-hydration methodology.
Thermal and Mountain Medicine Division, U.S. Army Research Institute of Environmental Medicine (USARIEM), Natick, MA 01760, USA. Kurt.Sollanek@us.army.mil
Kurt J
KJ
Sollanek
Robert W
RW
Kenefick
Samuel N
SN
Cheuvront
Robert S
RS
Axtell
D000293:Adolescent; D000328:Adult; D001769:Blood / Q000378:metabolism*; D001774:Blood Chemical Analysis; D001835:Body Weight / Q000187:drug effects* / Q000502:physiology; D003681:Dehydration / Q000097:blood / Q000517:prevention & control; D004326:Drinking / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009994:Osmolar Concentration; D014867:Water / Q000494:pharmacology*; D014882:Water-Electrolyte Balance / Q000187:drug effects / Q000502:physiology; D055815:Young Adult
D014867:Water
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