Tracking of body mass index from childhood to adolescence: a 6-y follow-up study in China.

BACKGROUND Although extensive descriptive research shows that childhood obesity predisposes a person to adult obesity, little is understood about the dynamics of weight during childhood and the predictors of weight tracking. OBJECTIVE Our objective was to examine tracking patterns of body mass index (BMI) as well as their predictors between childhood and adolescence. DESIGN A cohort of 975 Chinese children aged 6-13 y was followed for 6 y (1991-1997). Tracking of BMI was defined as an individual maintaining a certain status (overweight or underweight) or relative position (relative BMI quartile) over time. Relative BMI related BMI to age- and sex-specific BMI cutoffs. RESULTS After 6 y, approximately 40% of the subjects had maintained their relative positions, but 30% had moved into a lower or higher quartile. The BMIs of thin and fat children were more likely to track: 51% and 46% remained in the bottom and upper quartiles, respectively. Nearly one-third of the underweight children remained underweight in 1997. Overweight children were 2.8 times as likely as all other children to become overweight adolescents; underweight children were 3.6 times as likely to remain underweight as adolescents. Parental obesity and underweight, individuals' initial BMIs, dietary fat intake, and family income helped predict tracking and changes in BMI. CONCLUSION In a society undergoing enormous changes in diet and activity, BMI tracking is still very important between childhood and adolescence in China.