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.