Body composition and fat distribution change dramatically during adolescence. Data
based on longitudinal studies to describe these changes are limited. The aim of this
study was to describe age-related changes in fat free–mass index (FFMI) and fat mass
index (FMI), which are components of BMI, and waist circumference (WC) in participants
of Project HeartBeat!, a longitudinal study of children.
Anthropometric measurements and body composition data were obtained in a mixed longitudinal
study of 678 children (49.1% female, 20.1% black), initially aged 8, 11, and 14 years,
every 4 months for 4 years (1991–1995). Trajectories of change from ages 8 to 18 years
were measured for FFMI, FMI, and WC. Because of the small number of observations for
black participants, trajectories for this group were limited to ages 8.5–15 years.
Body mass index, FFMI, and WC increased steadily with age for all race–gender cohorts.
However, in nonblack girls, FFMI remained constant after about age 16 years. For black
boys and girls, FFMI was similar at age 8.5 years but increased more steeply for black
boys by age 15 years. In girls, FMI showed an upward trend until shortly after age
14 years, when it remained constant. In boys, FMI increased between age 8 years and
age 10 years, and then decreased.
The extent to which each component of BMI contributes to the changes in BMI depends
on the gender, race, and age of the individual. Healthcare providers need to be aware
that children who show upward deviation of BMI or BMI percentiles may have increases
in their lean body mass rather than in adiposity.