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

## TL;DR

This study tracks how two bioelectrical impedance variables change with age in healthy Italian children and adolescents, finding significant shifts around age 12.

## Contribution

The study reveals age-related breakpoints in bioelectrical impedance variables, particularly in boys around 12 years old.

## Key findings

- Phase angle increases similarly in both sexes from 6 to 11 years, then more steeply in boys and moderately in girls after 12 years.
- A significant breakpoint in impedance ratio was observed in boys around 12 years of age.
- Changes in both variables are associated with age and fat-free mass variations.

## Abstract

There is a growing interest for bioelectrical impedance analysis (BIA)-derived raw variables such as phase angle (PhA) and impedance-Z ratio, since they are considered proxy indicators of body cell mass and muscle structure. So far, limited information is available on the trajectories of PhA and IR during the first two decades of life. Anthropometry was measured with standardized procedures. For the whole body, PhA was measured at 50 kHz, while IR was calculated as the ratio of Z at 250 kHz/Z to that 5 kHz. Fat-free mass (FFM) and percentage body fat (%BF) were estimated by BIA. Segmented linear regression was applied to assess whether the relationships between PhA or IR and age showed discontinuities with time. Healthy children and adolescents were enrolled (302 boys and 278 girls aged 6–16 yrs). Independent predictors of PhA were body weight, height, FFM and %BF in boys and age, body weight, height and FFM in girls. Similar findings were observed for IR. PhA increased similarly in both sexes from 6 to 11 yrs. In the second decade the slope was ten times steeper in boys and three times in girls, with a breakpoint around 12 yrs in both sexes. A breakpoint was also detected for IR which was significant in boys only. This study provides a comprehensive description of the trajectories of PhA and IR in healthy children and adolescents. Changes of both variables were associated with age, with breakpoints observed around 11–12 yrs of age, followed by more marked variations with time.
What is known:• In children and adolescents, BIA-derived raw variables, such as phase angle and impedance ratio, have been related to muscle strength and cardiorespiratory fitness and were considered as a marker of outcome in hospitalized paediatric patients.• A limited number of papers described the trajectory of these variables in paediatric population.
What is new:• There are very clear age-related differences in BIA-derived variables over the first two decades of life, with a variability related to variation in fat-free mass.• For both variables, breakpoints were observed particularly in boys around 12 yrs of age.

What is known:

• In children and adolescents, BIA-derived raw variables, such as phase angle and impedance ratio, have been related to muscle strength and cardiorespiratory fitness and were considered as a marker of outcome in hospitalized paediatric patients.

• A limited number of papers described the trajectory of these variables in paediatric population.

What is new:

• There are very clear age-related differences in BIA-derived variables over the first two decades of life, with a variability related to variation in fat-free mass.

• For both variables, breakpoints were observed particularly in boys around 12 yrs of age.

The online version contains supplementary material available at 10.1007/s00431-026-06748-2.