Water Distribution in the Body as a Source of Information on Markers of Skeletal Muscle Ageing

Background/Objectives: Body water, including its distribution between the intracellular (ICW) and extracellular (ECW) compartments, plays an important role in skeletal muscle physiology. The aim of this study was to assess whether body water distribution indices, namely ECW/ICW and ECW/TBW, differ according to age and sex, and whether they are associated with skeletal muscle index (SMI) after adjustment for BMI, age, and sex. Methods: The study included 188 healthy individuals: 71 older women (71.6 ± 6.4 years), 37 older men (72.7 ± 4.7 years), 40 female students (22.5 ± 0.9 years), and 40 male students (22.8 ± 1.8 years). Body composition was assessed using a Tanita MC-780 bioimpedance analyser. The ECW/ICW and ECW/TBW ratios were calculated. Nonparametric analyses, Kendall’s rank correlations, and linear regression models adjusted for BMI, age, and sex were used. Results: Older adults had significantly higher ECW/ICW and ECW/TBW values than younger participants, and women had higher values than men within the same age groups. Both indices correlated positively with age and BMI, and negatively with SMI. In multivariate models, ECW/TBW remained negatively associated with SMI after adjustment. BMI and male sex correlated positively with SMI, whereas age was not a significant predictor. Conclusions: Body water distribution indices (ECW/ICW, ECW/TBW) reflect age- and sex-related differences in water distribution and correlate with SMI independently of BMI, age, and sex. They may serve as complementary markers of skeletal muscle ageing.