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.