Hydration of the fat-free body mass in children and adults: implications for body composition assessment.
Level 4 - case-series / case-control
Cross-sectional observational study comparing physiological parameters across age cohorts.
PubMed 8338157 · doi:10.1152/ajpendo.1993.265.1.E88
What was done
The authors evaluated the water content of fat-free mass (W/FFM) in 121 healthy White participants: 28 prepubescent children (ages 5 to 10), 31 young adults (ages 22 to 39), and 62 older adults (ages 65 to 84). Total body water was determined by 2H2O dilution, and fat-free mass was estimated using a multicomponent model based on body density, total body water, and regional bone mineral density. Differences in percent fat between the multicomponent approach and the Siri two-component model were compared against W/FFM.
What was found
Prepubescent children (72.7 +/- 1.6%) and older adults (72.5 +/- 1.4%) had significantly higher mean W/FFM than young adults (70.8 +/- 1.2%, P < 0.01). Discrepancies in percent fat between the multicomponent and two-component models ranged from -10% to 6% fat and were significantly correlated with W/FFM (r = -0.62, P < 0.0001).
Why it matters
Standard two-component body composition models assume constant fat-free mass hydration, but hydration varies across the lifespan. These findings show that failing to account for higher hydration in children and older adults causes substantial errors in percent fat estimation.
Limits
The study was cross-sectional and restricted to healthy White participants, limiting generalizability to other racial, ethnic, or clinical populations. Subgroup sample sizes for children (n = 28) and young adults (n = 31) were relatively small.
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- supports Lean muscle mass is highly hydrated tissue, whereas fat mass contains very little water.