Relationship between Hydration Status and Muscle Catabolism in the Aged Population: A Cross-Sectional Study

Background: The physiopathology of sarcopenia is still not completely understood. Aim: To assess the relationship between dehydration and skeletal muscle catabolism, muscle mass, and sarcopenia in an aged population. Methods: Observational cross-sectional study of community-dwelling subjects aged 70 years and older. Dehydration was assessed by plasma osmolarity; bioimpedance analysis (BIA) was used to assess body composition and water content; sarcopenia was established according to the EWGSOP-2 criteria; and 3-methyl-histidine (3MH) was used as an indicator of muscle catabolism. Results: 190 participants were recruited (77.4 years; 51.6% women). In total, 22.6% and 20.5% presented plasma osmolarity of 295–300 mOsm/L and >300 mOsm/L, respectively. Age was correlated with plasma osmolarity (rs = 0.439; p < 0.001). Plasma osmolarity was correlated with 3MH (rs = 0.360; p < 0.001) and showed an effect on 3MH levels, with an adjusted (by age, sex, and number of medications) beta of 0.283 (p < 0.001). BIA water content indicators showed no correlation with 3MH. Lower in sarcopenic compared to non-sarcopenic subjects were the intracellular water percentage (60.3 vs. 61.2%; p = 0.004) and intracellular water/free-fat mass ratio (44.3 vs. 45.0; p = 0.004). Conclusions: Dehydration is a highly prevalent clinical condition in aged populations, increases with age, and is associated with muscle catabolism but not sarcopenia.