Does Skeletal Muscle Mass Predict Urine and Saliva Measures of Hydration Status? A Cross-Sectional Analysis

Available evidence indicates that fat-free mass and muscle mass positively associate with baseline urine specific gravity, particularly in physically active individuals and athletes. Relatively little, however, is known about the relationships between skeletal muscle mass and other hydration biomarkers in more diverse samples. This investigation sought to evaluate the associations between skeletal muscle mass and hydration status measured via urine specific gravity, urine osmolality, and saliva osmolality.

Eighty-three adults (37 men, 46 women; 36.1 ± 13.2 yr, 170.6 ± 9.4 cm, 90.0 ± 17.9 kg; 27.9 ± 11.8% body fat) visited the laboratory once to complete body composition testing and provide spot urine and saliva samples. Urine and saliva were analyzed using a digital pocket refractometer, benchtop osmometer, and portable handheld saliva osmometer.

Urine osmolality and specific gravity were highly associated with one another (ρ = 0.98, p < 0.001; n = 81). In contrast, saliva osmolality showed no significant associations with urine specific gravity (ρ = 0.04, p = 0.726; n = 82) or urine osmolality (ρ = 0.03, p = 0.795; n = 81). Furthermore, urine specific gravity, urine osmolality, and saliva osmolality all showcased no statistically significant relationships with skeletal muscle mass, in either unadjusted or adjusted (age, past 4h fluid intake, and time of day) analyses.

The insignificant associations between muscle mass and hydration measures are possibly due to the diverse physical activity of participants, variations in sampling time of day, and limited control over pre-visit nutritional intake. In comparison to homogenous athlete samples, there may be less of a relationship between muscle mass and hydration measures in diverse populations, particularly when relatively unrestricted pre-testing controls are imposed.