The Relationship Between Urine Specific Gravity and Fat-Free Mass in Military Personnel

Military fluid replacement guidelines highlight the importance of hydration assessment prior to training and combat operations. Urine specific gravity (USG) is a commonly used assessment method to estimate hypohydration, typically based on a threshold of ≥1.020. However, recent evidence has suggested that this USG threshold may be influenced by body composition, primarily fat-free mass (FFM), thus a more liberal USG threshold (i.e., ≥1.025) may be warranted. The purpose of this exploratory study was to determine if there is a relationship between USG and FFM, a proxy for skeletal muscle, and compare FFM between those with and without elevated USG (≥1.020 and ≥1.025).

A sample of 346 military personnel provided urine samples for analysis via refractometer and had FFM estimated via bioelectrical impedance analysis (n = 165) or dual-energy x-ray absorptiometry (n = 181). Correlations (r) and linear regression were used to describe the relationship between FFM and USG. An ANCOVA with body composition device as a covariate determined the difference in FFM based on hydration status. Significance was set a priori at p < .05.

Regardless of USG threshold, male military personnel were proportionally more likely to be considered hypohydrated. There was a significant positive, albeit weak, correlation between USG and FFM (r = 0.268, p < .001). Based on linear regression, 7.2% of the variance in USG was explained by FFM (p < .001). Military personnel with USG ≥1.020 or ≥1.025 had 5.0 kg (p < .001) and 7.6 kg (p < .001) more FFM than those considered euhydrated, respectively. No sex differences were identified.

These results suggest that USG is partially influenced by FFM and indicate that the liberal USG threshold for determining hypohydration may be the most appropriate for military personnel and other populations with higher FFM (e.g., strength and power-based athletes).