The Influence Of Fat-free Mass On Hydration Status In Male And Female Military Personnel

For military personnel to meet mission requirements, physical fitness, proper nutrition, and hydration are essential. Urine specific gravity (USG) is commonly used to assess hydration status where, traditionally, values ≥1.02 indicate dehydration. However, USG analysis may be confounded by higher levels of fat-free mass (FFM) in servicemembers, therefore misclassifying hydration status. Additionally, since males often have higher FFM, the relationship between USG and FFM should be investigated separately in males and females. PURPOSE: To determine the relationship between FFM and USG among male and female servicemembers. METHODS: Cross-sectional data from 89 military personnel (43 males, 46 females) comprising 220 observations (107 male, 113 female) were analyzed. Participants reported to the lab after an 8-hour fast. A urine sample was provided to assess USG using a handheld digital refractometer (MISCO Palm AbbeTM PA201X-093, MISCO, Solon, OH). Bioelectrical impedance analysis (InBody 770, Biospace CO., Cerritos, CA, USA) was conducted to estimate FFM. Variables were not normally distributed, therefore nonparametric tests were used. Descriptive data are reported as median (25-75th percentile). The Spearman’s rank correlation coefficient (ρ) assessed the strength of the relationship between FFM and USG. The Mann Whitney U compared FFM between military personnel meeting and exceeding the traditional USG cut-off (<1.02). Significance was set a priori as p < .05. RESULTS: In both male and female servicemembers, FFM had a significant, weak association with USG (ρ = .260, p = .007 and ρ = .239, p = .011, respectively). Median FFM for males was not significantly different for those meeting (63.4 kg [59.9, 69.9]) or exceeding (66.2 kg [57.9, 74.9]) a USG of 1.02 (U = 1535, z = 0.713, p = .476). Findings were similar among females meeting (48.3 kg [43.0, 52.4]) or exceeding (48.4 kg [43.3, 54.4]) the traditional USG cutoff (U = 1259, z = 0.660, p = .509). CONCLUSION: Results indicate a weak association between FFM and USG among male and female servicemembers. Although nonsignificant, males with a USG >1.02 had an additional 2.8 kg FFM compared to males with a USG <1.02. Females only differed by 0.1 kg. Further research is needed to determine if there is a threshold of absolute FFM that may lead to a naturally higher USG value.