The Relationship Between Urine Specific Gravity And Fat-free Mass In Military Personnel

Urine specific gravity (USG) measurement via refractometer is a hallmark hydration assessment method in both laboratory and field-based settings. Urine specific gravity is a reliable and cost-effective way to estimate the prevalence of hypohydration, typically based on the threshold of ≥1.020. However, recent evidence has suggested that this common USG threshold may be influenced by body composition, primarily fat-free mass (FFM), thus leading to misclassification of hypohydration in those that are, in fact, hydrated. PURPOSE: To determine if there is a relationship between USG and FFM and to compare FFM between those with and without elevated USG based on two common cutoffs (≥ 1.020 and ≥ 1.025). METHODS: Cross-sectional data from 162 military personnel comprising 357 observations were analyzed. Military personnel provided a urine sample for analysis via refractometer (MISCO Palm AbbeTM PA201X-093, MISCO, Solon, OH) and had FFM measured via bioelectrical impedance (n = 208) and dual-energy x-ray absorptiometry (n = 149). Simple linear regression was used to determine the relationship between USG and FFM. Fat-free mass between military personnel based on measured hydration status using two USG cutoffs (≥ 1.020 and ≥ 1.025) was determined using univariate ANOVA with body composition device as a covariate. All statistical tests with a p < .05 were considered significant. RESULTS: There was a significant positive, albeit weak, correlation between USG and FFM (r = 0.143, p = .007). Based on the linear regression, 2.1% of the variance in USG was explained by FFM (p = .007). When controlling for body composition device, military personnel with USG ≥ 1.020 had significantly higher FFM compared to those with USG < 1.020 (mean difference ± SE [95%CI] = 4.63 ± 1.35 kg [1.98, 7.28], p < .001). Similarly, those with a USG ≥ 1.025 had higher FFM than those with USG < 1.025 (6.70 ± 1.81 kg [3.13, 10.26], p < .001). CONCLUSIONS: These results confirm that USG, at least partially, is influenced by FFM and indicate that higher USG thresholds for determining hypohydration, such as ≥1.025, may be warranted in military personnel and other populations with higher FFM (e.g., strength and power-based athletes). Although higher risk situations, such as combat, may require hydration assessment based on a lower USG threshold (< 1.020).