Relationship Between Plasma Osmolality And Urine Specific Gravity In Recreational Runners

PURPOSE: Urine specific gravity (USG) is a method frequently used in field settings but may demonstrate latency when compared to measurements derived from plasma. Thus, the purpose of this investigation was to assess the relationship between measurements of urine specific gravity and plasma osmolality before and after a 60 min run. METHODS: 19 recreationally trained runners (female, n = 6; age, 27 ± 7 y) participated in a single 60 min outdoor run during the months of August or September. The 60 min run was conducted on a 1.6 km concrete loop. Pre- and Post-run measures of body mass (BM), urine specific gravity (USG), and plasma osmolality (Posm) were collected. Participants were provided a sports drink and ad libitum fluid ingestion was recorded. Pearson correlations were used to assess associations between pre-run USG and Posm and between post-run USG and Posm. Paired samples t-tests were used to assess the difference in Pre-run and Post-run values for USG, Posm, and BM. A significance level of α = 0.05 was used for all statistical tests. RESULTS: Pre-run measures of USG, BM, and Posm were 1.015 ± 0.008, 74.84 ± 12.21 kg, and 288 ± 4 mosm/kg, respectively. Post-run measures of USG, BM, and Posm were 1.011 ± 0.007, 73.86 ± 12.41 kg, 295 ± 4 mosm/kg, respectively. Pre-run USG (r = 0.226, p = 0.352) and Post-run USG (r = 0.020, p = 0.935) were not significantly associated with Pre-run or Post-run Posm, respectively. Post-run BM (Δ = -0.98 ± 0.83 kg, p < 0.001) and USG (Δ = -0.003 ± 0.006, p = 0.038) were significantly lower than pre-run. Whereas Post-run measures of Posm (Δ = 7 ± 3 mosm/kg, p < 0.001) were significantly greater than Pre-run measures. CONCLUSIONS: A significant overall decrease in mean BM supports that the participants dehydrated during the run. USG and Posm were not significantly associated before or after a 60 min run. Both USG and Posm demonstrated differences between pre-run and post-run, however, the changes were dichotomous such that the USG changes suggest a greater level of hydration after the run, while the Posm changes suggest a greater level of dehydration after the run. Thus, our findings support that Posm and BM measurements may be more accurate in determining changes in hydration than USG following exercise. Supported by: This material is based upon work supported by the National Science Foundation under Cooperative Agreement No. 2234491.