Evaluation Of Dietary Moisture, Dietary Protein, And Body Composition In Prediciting Urine Specific Gravity

Urine specific gravity (USG) is often used as a single point to assess individual hydration, particularly in sports. Despite this, other possible variables that may affect USG include skeletal muscle mass (SMM), lean body mass (LBM), and protein intake, and failing to account for these variables could lead to misleading conclusions about hydration status. PURPOSE: The present study evaluated the extent to which the aforementioned variables, along with dietary moisture intake, associated with USG measured under two conditions (morning and afternoon). METHODS: Participants (n = 43; 13 female, 30 male) had their body composition assessed utilizing bioelectrical impedance analysis (BIA) and gave two separate urine samples. The first sample was from a morning first void in a fasted state. The second sample was given at least two days later, after 12:00 pm, and participants were not fasted. Dietary intakes of moisture and protein were assessed by utilizing a 24-hour dietary recall tool from the National Cancer Institute. Bivariate Pearson correlation analysis was used to evaluate associations between USG and measured variables, including intakes of protein and moisture (g/day) and absolute amounts (kg) of LBM, SMM, and total body water (TBW). RESULTS: On average, participants had 60.5 ± 13.1 kg of LBM, 34.2 ± 7.9 kg of SMM, and 44.2 ± 9.6 kg of TBW. Average USG for the morning, fasted sample was 1.018 ± 0.006 while it was 1.010 ± 0.008 for the non-fasted afternoon sample. LBM, SMM, and TBW correlated positively (p < .05) with afternoon USG. Correlations were as follows: LBM: r = 0.37, p = .015; SMM: r = 0.38, p = .012; TBW: r = 0.37, p = .015. There were no significant correlations between body composition data and morning, fasted USG. Dietary protein correlated negatively with fasted USG (r = -0.39, p = .027) and positively with non-fasted USG (r = 0.31, p = .046). Interestingly, there was no correlation between USG and dietary moisture. CONCLUSION: The positive correlation between SMM and USG is possibly due to creatinine excretion being elevated with greater muscle mass. This indicates that USG—at least spot afternoon samples—may be higher in individuals with large amounts of muscle mass. The opposing directions of correlations between dietary protein and USG based on time of day could be spurious but may still warrant further investigation.