Fat-Free Mass Is Positively Associated With Urine Specific Gravity in Athletes and Active Adults: A Quantitative Review

Background: Urine specific gravity (USG) is a commonly used assessment method to estimate the prevalence of hypohydration, typically based on a common threshold of ≥ 1.020. Some research has shown that USG can vary based on body size and composition, but the evidence to date is limited. This review examined whether an association between fat-free mass (FFM), a component of body composition, and USG could be detected among published articles that have reported these variables in athletes and physically active individuals. Methods: By searching Google Scholar, the authors identified a large number of published articles (n = 161) reporting anthropometrics and USG. Only articles reporting data on adult samples of athletes or physically active individuals were included. Given differences in body composition and FFM between men and women, articles that did not report data separately for each sex were excluded. Spearman's rank correlation coefficient (ρ) was used to assess the association between variables. Results: In men, FFM showed a significant, weak-sized positive association with USG (ρ = 0.36, p < 0.001). Among women, FFM showed a significant, moderate-sized positive association with USG (ρ = 0.57, p=0.006). When results were combined independent of sex, the association between FFM and USG remained significant (ρ = 0.38, p < 0.001). Conclusion: Athletes with larger amounts of FFM are more likely to have elevated USG. Protein and muscle metabolites such as creatinine, urea, and uric acid likely play some role in the observed relationships. If USG continues to be utilized in sport settings, more research is needed to determine if development of FFM-specific USG thresholds may be more appropriate for determining hydration status.