The trajectories of bioelectrical impedance analysis-derived raw variables (phase angle and impedance ratio) in healthy Italian children and adolescents: a retrospective observational study.

Phase angle increased similarly in both sexes from 6 to 11 years, after which the slope was ten times steeper in boys and three times steeper in girls.

• A breakpoint in PhA trajectories was detected around 12 years of age in both sexes.
• In the second decade of life, the rate of PhA increase diverged substantially between sexes.
• The study enrolled 302 boys and 278 girls aged 6-16 years.
• PhA was measured at 50 kHz for the whole body.

A breakpoint in impedance ratio (IR) trajectories was also detected, but was statistically significant in boys only.

• IR was calculated as the ratio of impedance Z at 250 kHz to Z at 5 kHz.
• The breakpoint for IR was observed around 11-12 years of age.
• Similar breakpoint patterns were observed for both PhA and IR.
• The IR breakpoint was significant in boys but not in girls.

Independent predictors of phase angle differed between sexes, with body weight, height, FFM, and %BF predicting PhA in boys, while age, body weight, height, and FFM predicted PhA in girls.

• Fat-free mass (FFM) was an independent predictor in both boys and girls.
• Percentage body fat (%BF) was an independent predictor in boys but not in girls.
• Age was an independent predictor in girls but not in boys.
• Similar findings for independent predictors were observed for IR as for PhA.

Segmented linear regression identified discontinuities (breakpoints) in the relationships between PhA or IR and age around 11-12 years in both sexes.

• Segmented linear regression was applied to assess whether relationships between PhA or IR and age showed discontinuities with time.
• Breakpoints were observed particularly in boys around 12 years of age.
• Changes in both variables were associated with age.
• The methodology allowed identification of distinct linear phases in the trajectories of BIA-derived raw variables.

Changes in BIA-derived raw variables PhA and IR were associated with variability in fat-free mass across the first two decades of life.

• FFM and %BF were estimated by BIA.
• Anthropometry was measured with standardized procedures.
• The study characterized trajectories of PhA and IR as proxy indicators of body cell mass and muscle structure.
• The sample comprised healthy Italian children and adolescents aged 6-16 years (302 boys and 278 girls).