Body water compartments with human aging using fat-free mass as the reference standard

Area of Science:

• Gerontology
• Human Physiology
• Body Composition Analysis

Background:

• Traditional body water measurements often use weight, which can be misleading due to changes in body fat.
• Previous studies suggest age and sex influence total body water (TBW), extracellular water (ECW), and intracellular water (ICW).

Purpose of the Study:

• To investigate the aging trends of body water compartments using fat-free mass (FFB) as a reference standard.
• To compare body water distribution and aging trends across sexes and species using FFB.

Main Methods:

• Studied 45 healthy men and women (aged 16-39 and 59-89 years).
• Measured total body water (TBW) and extracellular water (ECW); calculated intracellular water (ICW = TBW - ECW).
• Determined fat-free mass (FFB) using inert gas uptake for body fat measurement.

Main Results:

• TBW and ICW were lower in older adults and women when expressed per unit of weight, height, or surface area.
• However, TBW/FFB remained constant with age, and ICW/FFB showed only a slight, statistically insignificant decrease.
• ECW/FFB increased slightly with age, indicating a higher proportion of extracellular water within the fat-free body.

Conclusions:

• Fat-free mass (FFB) is a more stable and useful reference standard for body water compartments than body weight.
• The stability of ICW and FFB with aging in healthy individuals does not support the hypothesis of significant cellular mass loss in aging mammals.
• Body water distribution and aging trends are similar across sexes and even in male rats when normalized to FFB.