Sensitivity Of Urine Specific Gravity As A Hydration Index During And After Dehydrating Exercise

During exercise, serum osmolality (Sosm) detects low levels of dehydration while urine specific gravity (Usg) is not sensitive below 3% dehydration. This finding belongs to studies where subjects ingested a bolus of water prior to the dehydrating exercise which may have diluted the urine and thus influence Usg sensitivity. PURPOSE: To determine if Usg is sensitive to detect small % of dehydration during exercise without pre-exercise fluid ingestion. Additionally, we wanted to determine if Usg can accurately identify the hypohydration that results from prolonged exercise in the heat. METHODS: Physically fit male athletes (N = 18; VO2max: 58 ± 10 mL-1·kg-1·min-1) pedaled at 60% of VO2max in the heat (32 ± 1 °C; 46% rh; 2.5 m·s-1 wind speed) until they lost 3% of their initial body weight. Serum osmolality (Sosm), urine specific gravity (Usg) and osmolality (Uosm) were measured every 30 min of exercise. Subjects remained dehydrated overnight and blood and urine were collected the following morning in a 3% hypohydrated state. RESULTS: Table 1 shows the mean ± SD of urinary and blood measurement. * Different from baseline euhydration. † Difference between 3% dehydration and 3% hypohydration (p < 0.05). CONCLUSIONS: During exercise, Usg and Uosm were as sensitive as Sosm to detect small levels of dehydration (i.e., 0.5% dehydration). In addition, the relationship between Usg and Sosm during dehydration was positive and highly significant (r = 0.60; p < 0.0001). The following morning, despite subjects being hypohydrated by 3%, Sosm returned towards euhydration levels while Usg remained at high levels. This suggests that Usg may be a more accurate index of hypohydration than Sosm. Supported by GSSITable