Association Between Age and Obesity Over Time.

Background And Objectives: A decline in the prevalence of obesity among 2- to 5-year-olds in the United States was recently reported. This decline may be due to changes in the population composition of children over time or may be a consequence of changes in how strongly individual- or family-level factors are linked to childhood obesity. We applied regression decomposition techniques to identify the sources of the decline.

Methods: We used data from the 2003-2004 and 2011-2012 NHANES restricted to 2- to 5-year-old children and Blinder-Oaxaca regression decomposition techniques to partition the decline in early childhood obesity into 2 components: changes resulting from (1) how demographic, economic, and health characteristics of children have changed over this period (ie, changes in population composition) and (2) changes in how these demographic, economic, and health factors are associated with obesity (ie, changes in associations).

Results: The obesity rate was lower in 2011-2012 than it was in 2003-2004 mainly because obesity was strongly and positively associated with age in 2003-2004 (ie, older children were more likely to be obese than younger children) but not in 2011-2012 (ie, older children were not more likely to be obese than younger children).

Conclusions: If the weaker association between age and obesity we observed for this cohort of 2- to 5-year-old children in 2011-2012 persists for subsequent cohorts of young children, the obesity rate for young children will remain at or near the lower rate seen in 2011-2012.