Purpose: The objective of this study was to test the relationship of change in body mass index (BMI) percentile score group (from sixth to eighth grade) with change in cardiovascular fitness (CVF), baseline BMIz-score, and CVF.
Methods: Children (3998 (92%)) in the HEALTHY trial provided complete data at the beginning of sixth and end of eighth grades. Height and weight were assessed according to standardized protocol. CVF was measured using the 20-m shuttle run. Changes in BMI percentile were categorized into five groups: increased a BMI category, stayed obese, stayed overweight, stayed at a healthy weight, and decreased a BMI category. Data were analyzed separately by sex, controlling for race, parental education, change in pubertal stage, and baseline BMIz-score and CVF.
Results: Youth (male and female) who lowered their BMI group or remained in the healthy or overweight groups had significantly larger increases in CVF than those in the stayed obese or increased a BMI category groups. However, these relationships accounted for a small percentage of variance (i.e., weak relationship). Staying obese was associated with the highest baseline BMIz-score, with the second highest among those who decreased a BMI category. BMI category change accounted for the most variance in baseline BMIz-score.
Conclusions: Changes in BMI categories were substantially more strongly related to sixth-grade values of BMIz-score than to CVF changes. Because preexisting adiposity may inhibit adiposity change, changes in CVF and adiposity should be attempted before middle school.