Piecewise Linear Mixed Effects Model to Track the Temporal Changes in Early Childhood Body Mass Index (BMI) Trajectories

• Activity Number: 84
• Type: Contributed
• Date/Time: Sunday, August 4, 2013 : 4:00 PM to 5:50 PM
• Sponsor: Section on Statistics in Epidemiology
• Author(s): Md Jobayer Hossain*+ and Samuel Gidding and H Timothy Bunnell and Sandra Hassink and Timothy T. Wysocki
• Companies: Nemours Biomedical Research A.I. DuPont Children Hospital and A I duPont Children HOSPITAL and Nemours Biomedical Research and A I Dupont Children Hospital and Nemours Biomedical Research A.I. DuPont Children Hospital
• Keywords: Tracking ; temporal change ; BMIz ; piece-wise ; linear mixed effect model ; individual level variability

Abstract

Tracking of early childhood BMI trajectories of individual children may identify the pattern of temporal change in BMI associated with obesity at this and later ages as well as its correlates and comorbidities. We tracked individual and population level changes in BMIz of 3365 children from birth to 5 yrs of age. Weight-for-length (age< 2 yrs) and BMI (age =2 yrs) at clinic visits and standardized scores (BMIz) of these two measures were calculated. Data were unbalanced over time. Mean BMIz over 5 yrs showed a cubic polynomial trend. A piecewise linear mixed effects model with knots at 8 and 27 months yielded the best fit with an increased, decreased, and then sharply increased mean trend in the first, second and third segments, respectively. Approximately 46%, 44%, and 35% of children had a trend contrary to population mean trend at the first to third segments, respectively. There was substantial individual to individual variability in BMIz at birth and in the rates of change. Males had a steeper slope and a longer time of BMIz increase in the first segment, while females had a steeper slope after 2 yrs. Hispanics maintained a significantly higher level of BMIz at all ages.