Tracking and variability in childhood levels of BMI: The Bogalusa Heart Study

OBJECTIVE: Although the tracking of BMI levels from childhood to adulthood has been examined, there is little information on the within-person variability of BMI. METHODS: We used longitudinal data from 11,591 schoolchildren, 3096 of whom were reexamined as adults, to explore the tracking and variability of BMI levels. We focus on changes in age-adjusted levels of BMI. RESULTS: There was strong tracking of BMI levels. The correlation of adjusted BMI levels was r=0.88 and 78% of children with severe obesity at one examination had severe obesity at the next examination (mean interval, 2.7 years). Further, an increase in adjusted BMI from +5 kg/m(2) (above the median) to +10 increased the risk for adult BMI ≥ 40 by 2.7-fold. However, BMI levels among children and adolescents were variable. Over a 9- to 15-month interval, the SD of adjusted BMI change was 0.9 kg/m(2) and 0.7% of children had an absolute change ≥ 3.5. This variability was associated with the interval between examinations and with the initial BMI. CONCLUSIONS: Despite the high degree of tracking of BMI, annual changes of 3.5 kg/m(2) or more are plausible. Knowledge of this variability is important when following a child over time.