Relationship Between Changes in Upon-Waking Urinary Indices of Hydration Status and Body Mass in Adolescent Singaporean Athletes

This study investigated the relationship between changes in upon-waking body mass (BM) and changes in urine specific gravity (U_sg) and urine color (U_col) from 1 day to the next. Throughout the 5-day investigation, healthy adolescent Singaporean athletes (n = 66) had their upon-waking, bladder-voided BM measured. A small aliquot of the first bladder void each day was collected and analyzed for U_sg and U_col, the latter by both an investigator (IU_col) and individual participants (SU_col). Results revealed a significant inverse relationship between changes in BM and changes in U_sg (p = .003) and U_col (p = .001). On average, U_sg and U_col changed by ∼0.003 units and ∼1 color (across a 9-unit scale), respectively, with every 1% change in BM from 1 day to the next. There was a stronger relationship between U_sg and IU_col (r = .82, p < .001) than between U_sg and SU_col (r = .60, p < .001). These results suggest that the degree of fluid deficit may be predicted from the U_sg measurements among moderately hypohydrated athletes. In addition, training athletes to interpret and use the U_col chart is recommended.