Group-Based Fat Mass Index Trajectories in Boys and Girls from Elementary to High School

There is a pressing need to understand developmental patterns of adiposity as well as factors that influence it. Group-based trajectories, usually based on body mass index (BMI), have been used in adults and children to investigate adiposity trajectories. The objectives of this study were to identify sex-specific groups with similar longitudinal trajectories for fat mass index (FMI) and to compare sex-specific longitudinal trajectory groups on demographic and behavioral characteristics.

This study employed a longitudinal design to examine patterns of age-related change in FMI-assessed adiposity in children transitioning from elementary to high school. FMI, moderate-to-vigorous physical activity (MVPA), sedentary behavior, and diet quality were measured at 5^th, 6^th,7^th, 9^th, and 11^th grades when the children were 10-16 years of age. Sex-specific group-based trajectory analyses were conducted for FMI, and repeated ANOVAs were used to compare behaviors between trajectories.

Both boys' (n = 320) and girls' (n = 389) group-based trajectory analysis resulted in three groups, each with a low FMI and stable trajectory. In girls, two trajectories increased from 10 to 16 years of age, while in boys, one trajectory increased, and one decreased over time. In girls only, the groups differed by race and parent education. There were also group differences in MVPA for boys and in diet quality for girls.

An accurate assessment of adiposity, FMI, was used to establish the trajectories in this study. The characteristics of the groups provide valuable information about when and for whom to target obesity prevention efforts.