Changes in the fluid volume balance between intra- and extracellular water in a sample of Japanese adults aged 15-88 years old: a cross-sectional study

The fluid volume balance between intracellular water (ICW) and extracellular water (ECW) gradually changes with age and various medical conditions. Comprehension of these physiological changes would aid in clinical decision-making related to body fluid assessments. A total of 1,992 individuals (753 men and 1,239 women) aged ≥15 years included in this study had their body composition measurements performed at training gyms in 2014. We developed a regression formula to assess the association of age with the ratio of ECW to ICW in these subjects. The mean ages of male and female subjects were 51.2 ± 15.2 and 57.4 ± 15.2 years, and their mean body mass indices were 23.4 ± 3.3 and 21.1 ± 2.8 kg/m 2 , respectively. The total fluid volumes of male and female subjects were 39.6 ± 4.9 and 27.7 ± 3.0 L, whereas the percent body fat mass/kg of body weight were 19% and 26%, respectively. The ECW/ICW ratio increased with age due to the steeper decrease in the ICW content than in the ECW content, especially after the age of 70 years. The regression formulas used for calculating the age-adjusted ECW/ICW ratio were as follows: [0.5857 + 7.4334 × 10 -6 × (age) 2 ] in men and [0.6062 + 5.5775 × 10 -6 × (age) 2 ] in women. In conclusion, the fluid imbalance between ICW and ECW contents is driven by decreased cell volume associated with aging and muscle attenuation. Therefore, our proposed formula may serve as a useful assessment tool for the calculation of body fluid composition.