Influence Of Lean Body Mass On Urine Hydration Indices In Young Adults

PURPOSE: Monitoring and interpreting hydration status via urine indices may not be a one-size-fits-all endeavor. Of note, higher lean body mass (LBM) has been associated with increased urine urea and creatinine, thereby contributing to individual differences in urine markers. The purpose of this study is to investigate the relationships between urine specific gravity (USG) and urine color (Ucol) with LBM (via bioelectrical impedance - BIA) and total body mass (BM) in young adults. METHODS: A total of 48 (n = 48; male n = 5) young adults (age range 18-28 y) were included in this project. All participants provided a morning urine sample - after an overnight fast - during multiple visits to the laboratory (22 females provided 3 samples, 21 females provided 2 samples, 5 males provided 2 samples; total samples n = 118). During each visit, participants had their body composition assessed via a standing, foot-to-foot bioelectrical impedance scale. USG was assessed using a digital refractometer, and Ucol was measured against a paper urine color chart (color range 1 - 8). Means ± standard deviations are reported for all continuous variables, and Pearson's correlations were run to investigate relationships. For categorical data, medians and ranges are reported, and Spearman's Rho was utilized to investigate relationships involved with ordinal data. RESULTS: USG (1.0203 ± 0.0069) was weakly and negatively correlated (r = -0.23) with LBM (50.4 ± 9.0 kg), BM (70.5 ± 15.7 kg; r = -0.24), and body fat% (31.6 ± 10.4 %; r = -0.12). USG had a moderately-strong, positive correlation (rho = 0.69) with Ucol (M = 4, range 1-7 au). Additionally, Ucol was weakly and negatively correlated with LBM (rho = -0.07), BM (rho = -0.14), and body fat% (rho = -0.16). CONCLUSIONS: Urine indices related to hydration were not meaningfully associated with LBM, BM, nor BF%. Alternatively, these data weakly support the opposite line of thinking - as LBM increased, USG and Ucol decreased.